Parkinson Disease
Clinical Snapshot
Parkinson disease (PD) is a chronic, progressive neurodegenerative disease caused by loss of dopaminergic neurons in the substantia nigra pars compacta, producing a striatal dopamine deficiency that underlies its cardinal motor features — resting tremor, bradykinesia, and rigidity, with postural instability emerging later in the disease course.[1] Nonmotor features — cognitive impairment, depression, psychosis, and autonomic dysfunction such as orthostatic hypotension and bladder dysfunction — often predate the motor diagnosis by years and become the more disabling problem as disease progresses. No treatment has been definitively shown to modify the underlying neurodegenerative process; every currently available therapy is symptomatic, replenishing or mimicking dopamine within the brain, though treatment meaningfully reduces morbidity and improves function.
Recognize the Condition
Cardinal motor features
Classic PD presents with an asymmetric or unilateral-onset resting tremor (a pill-rolling tremor of the hand is typical), bradykinesia (slowness and a progressive decrement in the amplitude of repetitive movements, e.g., on finger tapping), and rigidity (cogwheel or lead-pipe resistance to passive movement). Postural instability and gait impairment (shuffling steps, reduced arm swing, freezing of gait) typically emerge only later in the disease course — early, prominent postural instability is itself a red flag for an alternate diagnosis rather than an expected early finding (see Red Flags & Refer).
Prodromal and early nonmotor features
Nonmotor symptoms often precede the motor diagnosis, sometimes by more than a decade, and should raise suspicion when clustered together: hyposmia (reduced sense of smell), constipation, REM sleep behaviour disorder (dream-enactment behaviour during sleep), fatigue, depression, and subtler changes such as micrographia (progressively smaller handwriting) or a tremulous sensation in one arm without an obviously visible tremor.[1]
Risk Factors
- Increasing age — the strongest known risk factor.
- Male sex.
- Family history of Parkinson disease, or a known pathogenic mutation (e.g., LRRK2, GBA, SNCA, and PARK2/PINK1 in particular for younger-onset disease).
- Environmental pesticide or herbicide exposure.
- Prior significant head trauma.
- Rural living and well-water use have been proposed as contributing exposures.
- Cigarette smoking and caffeine intake have both shown an inverse association with PD risk in observational data, though neither is a recommended preventive intervention.
Red Flags & Refer
Refer or escalate urgently for any of the following:
- Signs of parkinsonism-hyperpyrexia syndrome — a neuroleptic-malignant-syndrome-like reaction with fever, severe rigidity, autonomic instability, and altered consciousness, usually triggered by abrupt reduction or discontinuation of dopaminergic therapy (hyponatremia is an additional risk factor) — this is a medical emergency requiring immediate resumption of dopaminergic therapy and supportive care, not simply symptomatic treatment.[1]
- Acute or significantly worsening psychosis (hallucinations or delusions) that is distressing to the patient or unsafe, or accompanied by agitation.
- Severe orthostatic hypotension causing falls, syncope, or significant functional impairment.
- Features suggesting an atypical parkinsonism rather than idiopathic PD — early or prominent postural instability and falls within the first year, poor or absent response to an adequate levodopa trial, unusually rapid progression, early significant autonomic dysfunction, impaired eye movements, or dementia/hallucinations preceding motor symptoms by more than a year — refer for specialist reassessment of the diagnosis rather than continuing to escalate PD therapy.
- An impulse control disorder causing real financial, relational, legal, or safety harm — needs prompt dose reduction or a medication change, not silent continuation.[2]
- New, significant swallowing difficulty or aspiration risk.
- Motor fluctuations or dyskinesia that remain disabling despite optimized oral/transdermal therapy — refer for device-aided therapy assessment.
Assessment & Diagnosis
Differentiating idiopathic Parkinson disease from its mimics
| Feature | Idiopathic Parkinson disease | Drug-induced parkinsonism | Atypical parkinsonism (Parkinson-plus) | Essential tremor |
|---|---|---|---|---|
| Onset and symmetry | Asymmetric or unilateral onset | Usually symmetric, subacute onset after starting the offending drug | Often symmetric or early bilateral | Symmetric, bilateral from onset |
| Tremor character | Rest tremor, classically pill-rolling, suppressed with action | Rest and/or postural tremor can occur | Tremor often less prominent or absent; may be irregular | Action/postural tremor, absent at rest, worsens on reaching or with a sustained posture |
| Rigidity and bradykinesia | Present; responds well and durably to levodopa | Present; mirrors idiopathic PD but reverses on stopping the drug | Present; poor or absent levodopa response | Absent |
| Postural instability / falls | A late feature | Uncommon | Early and prominent (within the first 1-2 years) — a hallmark red flag | Not a feature |
| Other distinguishing clues | Hyposmia, REM sleep behaviour disorder, sustained and robust levodopa response | Clear temporal link to an antipsychotic or a central dopamine-blocking antiemetic (metoclopramide, prochlorperazine); improves within weeks to months of stopping it | Early severe autonomic failure suggests multiple system atrophy; early falls and impaired vertical gaze suggest progressive supranuclear palsy; early, prominent hallucinations with fluctuating cognition suggest dementia with Lewy bodies | Often familial; frequently improves transiently with alcohol; no rigidity, bradykinesia, or gait impairment |
| Response to a levodopa trial | Robust and sustained | Not indicated — stop the offending drug instead of trialing levodopa | Minimal, transient, or absent | Not applicable — not a dopaminergic disorder |
Diagnostic criteria
PD remains a clinical diagnosis. Parkinsonism is defined as bradykinesia plus rest tremor and/or rigidity. Once parkinsonism is confirmed, the Movement Disorder Society (MDS) clinical diagnostic criteria are used to determine whether idiopathic PD, rather than an alternate cause, is the most likely diagnosis — weighing supportive criteria (e.g., a clear and dramatic response to dopaminergic therapy, levodopa-induced dyskinesia, rest tremor) against absolute exclusion criteria and red flags that must be counterbalanced by supportive features before a PD diagnosis is made. The presence of dementia, even early in the disease course, is no longer an exclusion criterion.[3]
History
- Ask specifically about early, easily-missed complaints: fatigue, hyposmia, constipation, REM sleep behaviour disorder, general slowness, deteriorating handwriting, or a tremulous sensation in one arm without a visible tremor.
- Take a complete medication history targeting dopamine-blocking drugs — first- and second-generation antipsychotics and central dopamine-blocking antiemetics (metoclopramide, prochlorperazine) are the most common drug causes of parkinsonism, and screening for these is a medication-review task pharmacists are well positioned to lead.
- Ask about family history and age at onset — younger-onset disease raises the relevance of a genetic cause and of excluding Wilson disease.
Examination
Look specifically for asymmetric or unilateral onset of rest tremor, bradykinesia (decrement in amplitude on repetitive finger tapping), and cogwheel rigidity. Early, prominent postural instability, impaired eye movements, or a symmetric presentation from onset are not expected in early PD and should prompt reconsideration of the diagnosis.
Investigations
- Routine neuroimaging is not indicated for a classic, straightforward presentation — reserve MRI for young-onset disease or when atypical features are present, primarily to exclude a structural or vascular cause.
- In young patients, exclude Wilson disease with a serum ceruloplasmin, a slit-lamp examination for Kayser-Fleischer rings, and a 24-hour urine copper.
- A diagnostic trial of at least 1 month of properly dosed dopaminergic therapy can help confirm the diagnosis in an uncertain case — remember that levodopa may take several weeks to reach its full effect once fully titrated, so an inadequate trial duration or dose should not be misread as a poor response.[1]
No scoring tool is used to diagnose PD or to guide routine drug therapy decisions in practice — the Unified Parkinson Disease Rating Scale (UPDRS/MDS-UPDRS) is a comprehensive, clinician-administered, multi-domain rating instrument used mainly in research and specialist follow-up, not a simple point-sum a pharmacist would calculate at the counter, so no interactive calculator is provided for it here.
Treatment Targets
- Improve and maintain functional independence and quality of life throughout the disease course.
- Minimize acute and long-term medication adverse effects, including delaying the onset of motor fluctuations and dyskinesia where achievable.
- Achieve consistent, predictable symptom control across the day, minimizing off-time.
- Identify and treat nonmotor symptoms proactively, since these often become more disabling than the motor features as disease progresses.
- Prevent falls and injury from postural instability and orthostatic hypotension.
- Avoid any abrupt interruption of dopaminergic therapy at every care transition — surgery, hospitalization, NPO status, and medication reconciliation are all high-risk moments for an inadvertent missed or stopped dose.
- Support a realistic, positive outlook for the patient and caregiver, given the chronic and progressive nature of PD.
Choose Initial Treatment
Two treatment tracks by disease phase
Early / mild disease
Motor symptoms are new or mild; the main decision is when, and with which agent, to start symptomatic therapy. Levodopa remains effective for years and is not withheld to preserve it for later.
Motor fluctuation phase
After typically several years of levodopa therapy, wearing-off, on-off fluctuations, and dyskinesia emerge; management shifts to smoothing dopaminergic exposure across the day using adjunct medications, dose-timing changes, and eventually device-aided therapy.
Early / mild disease — starting therapy
Do not start symptomatic medication for symptoms that are present but not yet troublesome or functionally limiting — every antiparkinsonian drug carries adverse-effect potential that is not justified until symptoms actually affect the patient.[4]
1st line, most patients including younger-onset levodopa-carbidopa (or levodopa-benserazide) is the most effective symptomatic therapy for motor symptoms at any age; there is no benefit to delaying levodopa in a patient with troublesome motor symptoms, and the older practice of withholding it in younger patients specifically to avoid future dyskinesia is now considered outdated and is not supported by current evidence.[5]
Alternative, mild disease a monoamine oxidase B (MAO-B) inhibitor (rasagiline) can be used as monotherapy in mild early disease for patients/prescribers who prefer to defer levodopa — expect a smaller symptomatic effect than levodopa.
Alternative, selected patients under 70 a dopamine agonist (pramipexole, ropinirole, or rotigotine) is an option for initial therapy in select younger patients who wish to delay dyskinesia risk and are willing to accept a higher burden of somnolence, orthostatic hypotension, psychiatric adverse effects, and impulse control disorders in exchange — counsel explicitly on impulse control disorders before starting.[2]
Avoid as initial or add-on therapy in patients over 70 dopamine agonists carry a disproportionate confusion, hallucination, and falls burden in older patients and are not recommended as an initial choice in this age group.
Keep the levodopa dose as low as consistently effective throughout treatment — higher cumulative levodopa exposure is one of the main drivers of future motor complications, so this is a proactive dosing principle from the first prescription, not only a later-stage adjustment.[1]
Take levodopa with food early in therapy to reduce nausea; once motor fluctuations emerge, switch to taking it on an empty stomach, separated from protein-rich meals, since dietary protein competes with levodopa for intestinal and blood-brain-barrier transport and can blunt or delay its effect.
Motor fluctuation phase — managing wearing-off and dyskinesia
For predictable end-of-dose wearing-off, first increase levodopa dosing frequency using smaller, more frequent doses before adding a new drug class.
Adjunct for wearing-off add a catechol-O-methyltransferase (COMT) inhibitor (entacapone, given with every levodopa dose) or a MAO-B inhibitor (rasagiline or safinamide) once dose-timing adjustment alone is insufficient.[6]
Adjunct for wearing-off, alternative a dopamine agonist may be added at this phase — unlike in early disease, adjunctive use during motor fluctuations is reasonable even in some older patients, though the same impulse-control-disorder and sedation counselling still applies.
Troublesome dyskinesia add amantadine, the agent with the best evidence for reducing established levodopa-induced dyskinesia; avoid it in patients with significant cognitive impairment, since it can worsen confusion.
Refractory motor fluctuations or dyskinesia despite optimized oral/transdermal therapy refer to a movement disorder specialist for device-aided therapy — deep brain stimulation, intrajejunal levodopa-carbidopa intestinal gel, or continuous subcutaneous levodopa-foscarbidopa infusion (see Non-Pharmacologic Treatment for DBS candidacy).
Never abruptly stop or substantially reduce any dopaminergic medication (levodopa, a dopamine agonist, or an anticholinergic) — abrupt withdrawal risks parkinsonism-hyperpyrexia syndrome, a neuroleptic-malignant-syndrome-like reaction that can be fatal (see Red Flags & Refer). Any dose reduction must be tapered gradually, and planned drug holidays are not a recommended practice.
Non-Pharmacologic Treatment
- Provide patient and caregiver education through Parkinson Canada and other reputable local/national resources from the time of diagnosis onward.
- Recommend a regular, structured exercise program combining aerobic, strength, balance, and gait training (e.g., dance, tai chi, yoga) — this reduces fall risk and improves both motor and nonmotor symptoms, with benefits most pronounced when started early.[1]
- Refer early and proactively to allied health professionals — physiotherapy, occupational therapy, and speech-language pathology — rather than waiting for advanced disease; involve home care services as disability increases.
- Counsel every patient starting a dopamine agonist or levodopa on the possibility of impulse control disorders and sudden sleep attacks before the first dose, not only if problems later emerge.
- Counsel every patient and caregiver, from the first prescription, that dopaminergic medication must never be stopped or reduced abruptly.
Surgical / device-aided therapy
Deep brain stimulation (DBS) is the surgical treatment of choice for motor fluctuations and dyskinesia that remain disabling despite optimized medical therapy. The subthalamic nucleus and globus pallidus interna are the most common targets (improving bradykinesia, rigidity, and dyskinesia); a thalamic target is reserved for tremor-predominant disease. DBS is typically offered to patients younger than 70 years of age with more advanced disease whose motor symptoms can no longer be adequately controlled with medication alone.[1]
DBS is not a cure and does not treat symptoms that do not respond to dopaminergic therapy — functioning improves only up to the level of the patient's best on-state, and postural instability typically does not improve. Significant cognitive impairment is a contraindication to DBS, so formal neuropsychological assessment and confirmation of a robust levodopa response are standard parts of the pre-surgical workup.
Special Populations
Breastfeeding: dopamine agonists suppress lactation and are generally avoided during breastfeeding; data on levodopa and other agents in lactation are limited, so specialist input is advised when treatment is genuinely needed during this period.
Pediatrics
PD is a disease of middle-aged and older adults; it does not occur in children. Parkinsonism presenting in a young patient needs a broader differential — genetic and metabolic causes, including Wilson disease — rather than a default assumption of idiopathic PD.
Renal and hepatic impairment
Pramipexole and amantadine are renally cleared and require dose reduction with longer dosing intervals as renal function declines; amantadine in particular can accumulate to toxic levels in significant renal impairment. Rasagiline and safinamide require dose capping or avoidance in hepatic impairment (rasagiline: maximum 0.5 mg/day in mild impairment, avoid in moderate-severe impairment; safinamide: maximum 50 mg/day in moderate impairment, avoid in severe impairment).
Older adults
Avoid starting a dopamine agonist as initial or add-on therapy in patients over 70 years of age, given the disproportionate risk of confusion, hallucinations, and falls in this group. Avoid anticholinergic antiparkinsonian drugs (benztropine, trihexyphenidyl, ethopropazine) in older patients given their anticholinergic burden. Start every antiparkinsonian medication at a low dose and titrate slowly in older patients to minimize acute adverse effects.[1]
Monitoring & Follow-Up
At every visit
- Ask directly about impulse control symptoms (gambling, shopping, hypersexuality, binge eating) in any patient on a dopamine agonist — patients rarely volunteer these unprompted.[2]
- Screen for psychosis (hallucinations, delusions) and assess whether it is tolerated by the patient/caregiver or requires intervention.
- Check orthostatic blood pressure (lying and standing) in any patient on levodopa or a dopamine agonist, especially after a dose increase.
- Ask about falls, freezing episodes, and off-time — a simple symptom/motor-fluctuation diary can help quantify this between visits.
- Review the full medication list for any new dopamine-blocking drug (antipsychotic, antiemetic) that could worsen parkinsonism.
- Screen nonmotor symptoms specifically — mood, sleep quality and REM sleep behaviour disorder symptoms, constipation, and urinary symptoms — since these are easy to miss if not asked about directly.[7]
Periodic
- Cognitive screening at intervals appropriate to disease stage and clinical suspicion, particularly before considering a dopamine agonist, amantadine, or anticholinergic (all can worsen cognition) and before DBS candidacy assessment.
- Fall-risk assessment and home safety review as disease progresses.
- Reassess whether domperidone pretreatment is still needed once GI/orthostatic tolerance to dopaminergic therapy is established.
Refer to a movement disorder specialist
For diagnostic uncertainty, poor or waning levodopa response, disabling motor fluctuations or dyskinesia despite oral/transdermal optimization, or a possible candidate for device-aided therapy.
Common Errors to Avoid
- Stopping or substantially reducing dopaminergic medication abruptly, or allowing an inpatient NPO order or a medication-reconciliation gap to interrupt dosing — risks parkinsonism-hyperpyrexia syndrome.
- Missing drug-induced parkinsonism — continuing an antipsychotic or metoclopramide while a patient is being worked up or treated for presumed idiopathic PD, rather than reviewing the medication list first.
- Using a conventional (typical) antipsychotic, or any atypical antipsychotic other than quetiapine or clozapine, for PD-related psychosis — these worsen motor symptoms.
- Starting a dopamine agonist as initial therapy in a patient over 70 years of age.
- Withholding or delaying levodopa in a functionally impaired patient out of an outdated concern about preserving it for later or triggering earlier dyskinesia.
- Not asking directly about impulse control symptoms in a patient on a dopamine agonist — patients and families often do not recognize or volunteer these as medication-related.
- Giving levodopa with a high-protein meal in a patient who already has motor fluctuations, without recognizing why the response has become erratic.
- Missing the sudden-sleep-attack driving-safety conversation when starting or increasing any dopaminergic medication.
Clinical Decision Algorithm
Drug Tables
How to use this table: Doses are adult doses and require patient-specific confirmation, including renal/hepatic adjustment. Parkinson disease does not occur in children, so no pediatric dosing line is given for any drug below. Select any drug name to open its complete Pharmasist Drug Database page.
Levodopa preparations
The most effective symptomatic therapy at every disease stage; keep the total daily dose as low as consistently effective.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Levodopa/carbidopa, immediate-release | Initial: 50/12.5 mg PO BID Usual maintenance: 100/25 mg to 150/37.5 mg PO TID, titrated to effect | Nausea, vomiting, orthostatic hypotension, dyskinesia (dose-dependent with long-term use), hallucinations, confusion | Antihypertensives/vasodilators may increase hypotension; contraindicated with nonselective MAOIs (phenelzine, tranylcypromine) | Take with food early in therapy to reduce nausea; once fluctuations develop, separate from protein-rich meals. Never stop abruptly. |
| Levodopa/carbidopa, controlled-release | Initial: 100/25 mg PO BID Usual maintenance: 200/50 mg PO TID | Same as immediate-release | Same as immediate-release | Reduced peak clinical effect and more erratic absorption than the immediate-release formulation; in practice reserved for bedtime dosing rather than routine daytime use, since starting therapy with controlled-release confers no advantage over immediate-release. |
| Levodopa/benserazide | Initial: 50/12.5 mg PO BID Usual maintenance: 100/25 mg to 150/37.5 mg PO TID | Same as levodopa/carbidopa | Same as levodopa/carbidopa | Equivalent alternative combination; the two decarboxylase-inhibitor combinations are not interchangeable tablet-for-tablet without confirming the levodopa content. |
| Levodopa/carbidopa/entacapone | Dosing: One tablet, strength matched to the patient's previous levodopa dose, PO TID | Dyskinesia, orthostatic hypotension, nausea, sleep disorder, orange/brown urine discolouration, hallucinations | Contraindicated with nonselective MAOIs; avoid with dobutamine, dopamine, epinephrine, isoproterenol (entacapone reduces their metabolism) | For patients already established on levodopa plus entacapone who are candidates for combining them into one tablet; not for entacapone-naive initiation. |
| Levodopa/carbidopa, intestinal gel | Dosing: Individualized morning bolus plus continuous daytime infusion, based on the patient's previous total levodopa requirement | Nausea, vomiting, orthostatic hypotension, dyskinesia, hallucinations, confusion; device-related complications (tube-site infection, intestinal obstruction) | Same as levodopa/carbidopa | Continuous intrajejunal infusion via a percutaneous gastrostomy tube and external pump, for motor fluctuations refractory to oral/transdermal therapy; initiated and managed only by specialized PD clinics. No formulation-specific Drug Database monograph exists for this delivery system, so this row is not linked. |
| Foslevodopa/foscarbidopa, subcutaneous infusion | Dosing: Individualized continuous infusion based on the previous total levodopa requirement, up to a maximum equivalent to 4260 mg levodopa/day | Infusion-site reactions (edema, erythema, nodule, cellulitis/abscess) are the most common; hallucinations, dyskinesia, falls | Same as levodopa/carbidopa | Continuous 24-hour subcutaneous infusion via a wearable pump; rotate the infusion site and replace the infusion set at least every 3 days. Also specialist-clinic-initiated. |
Dopamine agonists
Non-ergot agents (pramipexole, ropinirole, rotigotine) are preferred over ergot-derived bromocriptine given its pulmonary/cardiac fibrosis risk. Avoid initiating any agent in this class in patients over 70.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Pramipexole | Initial: 0.125 mg PO TID Usual maintenance: 0.5-1 mg PO TID | Somnolence/sudden sleep attacks, impulse control disorders, orthostatic hypotension, hallucinations, nausea | Antihypertensives/vasodilators/tricyclic antidepressants may increase hypotension | Counsel on impulse control disorders before starting. Taper gradually; never stop abruptly. |
| Ropinirole | Initial: 0.25 mg PO TID Usual maintenance: 3-6 mg PO TID | Same as pramipexole | Same as pramipexole, plus ciprofloxacin raises ropinirole levels | Same counselling as pramipexole. |
| Rotigotine, transdermal patch | Initial: 2 mg/24h patch once daily Usual maintenance: 6-8 mg/24h (maximum 16 mg/24h) | Same dopamine agonist class effects, plus application-site reactions | Same as pramipexole | Continuous transdermal delivery avoids pulsatile receptor stimulation. Rotate the application site; do not reuse a site within 14 days. |
| Bromocriptine | Initial: 1.25 mg PO BID Usual maintenance: 5-10 mg PO TID | Same dopamine agonist class effects, plus headache, psychosis, erythromelalgia, pleural/pulmonary fibrosis | Same as pramipexole | Ergot-derived; not recommended as a first-choice dopamine agonist given its fibrosis risk. Obtain a baseline chest x-ray and monitor ECG, ESR, and renal function periodically. |
| Apomorphine | Test dose: 0.2 mL (2 mg) SC under medical supervision with pre/post blood pressure and pulse monitoring Usual maintenance: 2-6 mg SC PRN per off-episode, at least 2 hours apart Maximum: 6 mg/dose, 5 doses/day, 20 mg total daily dose | Orthostatic hypotension, somnolence/sudden sleep attacks, confusion, hallucinations, nausea/vomiting, injection-site reactions | Contraindicated with 5-HT3 antagonists (e.g., ondansetron) — severe hypotension/loss of consciousness; caution with other hypotensive drugs (e.g., tricyclic antidepressants) | Rescue therapy for off-episodes not responding to other adjustments. Requires domperidone pretreatment (up to 3 days beforehand) and a supervised, monitored first dose. Subcutaneous only — never intravenous. |
Monoamine oxidase B (MAO-B) inhibitors
Modest symptomatic effect; rasagiline and safinamide are also useful adjuncts for wearing-off. No convincing evidence that any agent in this class is disease-modifying.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Rasagiline | Monotherapy: 1 mg PO once daily Adjunctive therapy: 0.5-1 mg PO once daily | Insomnia, confusion, hallucinations, increased dyskinesia (as an adjunct), nausea, orthostatic hypotension | Avoid concurrent serotonergic drugs and meperidine (serotonin syndrome/hypertensive crisis risk); use 0.5 mg/day with ciprofloxacin or other CYP1A2 inhibitors | Maximum 0.5 mg/day in mild hepatic impairment; avoid in moderate-severe impairment. |
| Safinamide | Initial: 50 mg PO once daily Usual maintenance: may increase to 100 mg PO once daily after 2 weeks, based on response | Same as rasagiline; hypertension is more common than orthostatic hypotension with this agent | Same as rasagiline | Approved only as an add-on for wearing-off, not as monotherapy. Maximum 50 mg/day in moderate hepatic impairment; avoid in severe impairment. |
| Selegiline | Dosing: 2.5 mg PO once daily to 5 mg PO BID | Same as rasagiline | Same as rasagiline, plus oral contraceptives may inhibit its metabolism | Give before early afternoon to reduce insomnia risk. No convincing evidence of disease modification despite early hopes. |
Catechol-O-methyltransferase (COMT) inhibitors
Adjunct for wearing-off only — has no effect if not given together with levodopa; not indicated in non-fluctuating early disease.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Entacapone | Dosing: 200 mg PO with each levodopa dose, up to 8 times daily (maximum 1600 mg/day) | Increased dyskinesia, nausea, sleep disorder, anorexia, diarrhea, hallucinations, orange/brown urine discolouration | Potentiates levodopa; avoid with dobutamine, dopamine, epinephrine, isoproterenol; contraindicated with nonselective MAOIs (hypertensive crisis risk) | Give with every levodopa dose. Also available as a fixed combination — see levodopa/carbidopa/entacapone above. |
Opicapone, a longer-acting, once-daily COMT inhibitor, is approved in some other jurisdictions but is not available in Canada.
N-methyl-D-aspartate (NMDA) receptor antagonist
Used specifically to reduce established levodopa-induced dyskinesia, not for early motor symptoms.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Amantadine | Usual dosing: 100 mg PO BID | Livedo reticularis, leg/ankle edema, dry mouth, insomnia, anxiety, impaired concentration, orthostatic hypotension | Anticholinergic agents may increase anticholinergic-type effects | Avoid or use with extreme caution in patients with cognitive impairment — can worsen confusion. Renally cleared; reduce dose in renal impairment. Avoid in pregnancy. |
Anticholinergic agents
Should not be used as first-line PD therapy given limited efficacy (mainly tremor, little effect on bradykinesia) and a substantial side-effect burden; avoid in older patients and in anyone with dyskinesia or motor fluctuations.
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Benztropine | Dosing: 1-2 mg PO BID | Dry mouth, blurred vision, constipation, urinary retention, precipitation of angle-closure glaucoma, confusion, memory impairment | Amantadine may increase anticholinergic effects | Taper gradually; abrupt discontinuation risks a parkinsonism-hyperpyrexia-like withdrawal reaction. |
| Trihexyphenidyl | Initial: 1 mg PO once daily Usual maintenance: 2 mg PO TID | Same as benztropine | Same as benztropine | Same tapering caution as benztropine. |
| Ethopropazine | Dosing: 25 mg PO BID to 50 mg PO TID | Same as benztropine | Same as benztropine | Same tapering caution as benztropine. |
Nonmotor symptom management
PD does not have a separate Clinical Guidelines chapter of its own for these problems, so real dosing for the most pharmacist-relevant nonmotor issues is included directly below. Domperidone carries a specific dose-related cardiac risk in this population.[7][8]
| Drug | Dosage | Adverse effects | Drug interactions | Comments |
|---|---|---|---|---|
| Domperidone | Dosing: 10-20 mg PO 30 minutes before each dopaminergic dose | QT prolongation and ventricular arrhythmia risk, particularly with existing cardiovascular disease | Additive QT effect with other QT-prolonging drugs | For nausea or orthostatic hypotension at the start of levodopa/dopamine agonist therapy, or before an apomorphine test dose. Crosses the blood-brain barrier poorly, so it does not blunt levodopa's central effect. Reassess ongoing need after the first months of therapy. |
| Midodrine | Dosing: 2.5-10 mg PO TID, during daytime hours | Supine hypertension, scalp tingling, piloerection, urinary retention | Additive hypertension with other vasopressors | For orthostatic hypotension not controlled by conservative measures (increased salt/fluid intake, avoiding alcohol, deprescribing contributing drugs). Avoid the last dose within 4 hours of bedtime to reduce supine hypertension. |
| Fludrocortisone | Dosing: 0.1-0.2 mg PO once daily | Supine hypertension, fluid retention, hypokalemia | Additive hypokalemia with other potassium-wasting drugs | Alternative or add-on for orthostatic hypotension refractory to conservative measures. |
| Clonazepam | Dosing: 0.25-1 mg PO before sleep | Sedation, cognitive impairment, falls (particularly in older adults) | Additive CNS depression with other sedatives, opioids, or alcohol | For REM sleep behaviour disorder; more effective than melatonin but with a higher rate of side effects. |
| Melatonin | Dosing: 3-12 mg PO before sleep | Daytime drowsiness, headache | Minimal | Alternative for REM sleep behaviour disorder with a better tolerability profile than clonazepam, though somewhat less effective. |
| Quetiapine | Dosing: 12.5-25 mg PO at bedtime, titrated slowly to response | Sedation, orthostatic hypotension, QT prolongation | Additive CNS depression and hypotension with other sedating/hypotensive drugs | Often tried first for PD psychosis to avoid the mandatory blood monitoring required with clozapine, though trial evidence for benefit is less consistent than for clozapine. |
| Clozapine | Dosing: 6.25-50 mg PO at bedtime, titrated slowly to response | Agranulocytosis (requires mandatory neutrophil monitoring), sedation, orthostatic hypotension, sialorrhea | Additive CNS depression and hypotension with other sedating/hypotensive drugs | Lowest risk of worsening motor symptoms among all antipsychotics and the strongest trial evidence for PD psychosis, but mandatory absolute neutrophil count monitoring limits its routine first-line use. |
| Trospium | Dosing: 20 mg PO BID, or 60 mg extended-release PO once daily | Dry mouth, constipation; lower CNS penetration than older anticholinergics | Additive anticholinergic effects with other anticholinergic drugs | Preferred over oxybutynin-type anticholinergics for urinary urgency/incontinence in PD, given lower blood-brain-barrier penetration and lower confusion risk. |
| Mirabegron | Dosing: 25-50 mg PO once daily | Hypertension, headache | None significant | Nonanticholinergic option for urinary urgency, avoiding any additional confusion risk. |
| Sildenafil | Dosing: 25-100 mg PO 1 hour before activity, maximum once per 24 hours | Headache, flushing, hypotension | Contraindicated with nitrates; caution with alpha-blockers and antihypertensives | For erectile dysfunction; interacts with the same nitrate/hypotension precautions as in the general population. |
| Tadalafil | On-demand: 10-20 mg PO before activity Daily dosing option: 2.5-5 mg PO once daily | Headache, back pain, flushing | Contraindicated with nitrates; caution with alpha-blockers and antihypertensives | Longer duration of action than sildenafil or vardenafil. |
| Vardenafil | Dosing: 10 mg PO before activity, maximum once per 24 hours | Headache, flushing, hypotension | Contraindicated with nitrates; caution with alpha-blockers and antihypertensives | Similar profile to sildenafil. |
| Polyethylene glycol 3350 | Dosing: 17 g (1 capful) PO once daily, dissolved in liquid | Bloating, abdominal cramping | None significant | First-line for the near-universal problem of PD-related constipation, alongside adequate hydration, fibre, and exercise. |
| Lactulose | Dosing: 15-30 mL PO once to twice daily | Bloating, flatulence, cramping | None significant | Alternative osmotic laxative for constipation. |
| Psyllium | Dosing: 1 tsp to 1 tbsp PO once to three times daily with a full glass of water | Bloating; requires adequate fluid intake to avoid worsening obstruction | May reduce absorption of other oral drugs if taken simultaneously — separate dosing by 2 hours | Bulk-forming fibre option for constipation; probiotics may also help, though no specific product or dose is established as preferred. |
| OnabotulinumtoxinA | Dosing: Specialist-administered dose per institutional protocol | Excessive dry mouth, local injection discomfort, rare dysphagia from diffusion | None significant | For sialorrhea not controlled by simpler measures; injected into the salivary glands by a specialist, not a routine outpatient prescription. |
| Donepezil | Dosing: 5 mg PO once daily, may increase to 10 mg after 4-6 weeks | Nausea, diarrhea, insomnia, vivid dreams; can worsen tremor | Additive bradycardia with beta-blockers | For PD-related dementia; modest benefit. Watch for deterioration in motor function with initiation. |
| Rivastigmine | Oral: 1.5 mg PO BID initial, titrated as tolerated Transdermal patch: 4.6 mg/24h initial, titrated to 9.5-13.3 mg/24h | Nausea, vomiting, diarrhea; can worsen tremor | Additive bradycardia with beta-blockers | For PD-related dementia; the transdermal patch is often better tolerated than oral dosing. |
Evidence note: Drug inventory, dosing, adverse effects, and comments reflect standard Canadian practice, cross-checked against the current Canadian Parkinson disease guideline.[1]
References
- Canadian guideline for Parkinson disease, 2nd edition — Canadian Medical Association Journal (Grimes D, Fitzpatrick M, Gordon J, et al.). CMAJ 2019;191(36):E989-E1004
- Longitudinal analysis of impulse control disorders in Parkinson disease — Neurology (Corvol JC, Artaud F, Cormier-Dequaire F, et al.). Neurology 2018;91(3):e189-e201
- MDS clinical diagnostic criteria for Parkinson's disease — Movement Disorders (Postuma RB, Berg D, Stern M, et al.). Mov Disord 2015;30(12):1591-1601
- Initiation of pharmacological therapy in Parkinson's disease: when, why, and how — Lancet Neurology (de Bie RMA, Clarke CE, Espay AJ, et al.). Lancet Neurol 2020;19(5):452-461
- Dopaminergic therapy for motor symptoms in early Parkinson disease practice guideline summary: a report of the AAN Guideline Subcommittee — Neurology, American Academy of Neurology (Pringsheim T, Day GS, Smith DB, et al.). Neurology 2021;97(20):942-957
- Update on Treatments for Parkinson's Disease Motor Fluctuations: An International Parkinson and Movement Disorder Society Evidence-Based Medicine Review — Movement Disorders (de Bie RMA, Katzenschlager R, Swinnen BEKS, et al.). Mov Disord 2025;40(4):776-794
- Update on treatments for nonmotor symptoms of Parkinson's disease: an evidence-based medicine review — Movement Disorders (Seppi K, Ray Chaudhuri K, Coelho M, et al.). Mov Disord 2019;34(2):180-198
- Ventricular tachyarrhythmia and sudden cardiac death with domperidone use in Parkinson's disease — British Journal of Clinical Pharmacology (Renoux C, Dell'Aniello S, Khairy P, et al.). Br J Clin Pharmacol 2016;82(2):461-472
Professional reference only. Apply clinical judgment and verify patient-specific decisions against current authoritative guidance and product information.