Nausea and Vomiting of Pregnancy

Clinical Snapshot

Nausea and vomiting of pregnancy (NVP) is one of the most common conditions of early pregnancy. Despite the popular term "morning sickness," symptoms can occur at any time of day and are often present continuously. Virtually everyone who develops NVP has some symptoms by 9 weeks' gestation, more than half by 6 weeks, and most improve by 16 weeks — though a meaningful minority have symptoms that persist beyond 20 weeks.^[1] At the severe end of the same spectrum is hyperemesis gravidarum (HG): intractable vomiting causing dehydration, electrolyte disturbance, nutritional deficiency, and weight loss that can threaten both maternal and fetal wellbeing, and that must prompt investigation for other causes before being accepted as the diagnosis.^[2] The single most important counselling point in this chapter is that treating NVP appropriately — not avoiding treatment out of fear of medication exposure — is the safer choice for mother and fetus, since untreated severe NVP and HG carry real, well-documented risks of their own.

Recognize the Condition

Typical presentation

Nausea, with or without vomiting, beginning in the first trimester — onset after 9 to 10 weeks' gestation is atypical for NVP and should prompt a search for an alternative cause.^[1] Symptoms are frequently accompanied by hypersalivation (ptyalism), a heightened sense of smell, food aversions, and sensitivity to previously tolerated odours.

Severity classification

Mild

Nausea for under 1 hour per day, with no more than 2 episodes of vomiting or retching per day.

Moderate

Nausea for up to 6 hours per day, with no more than 2 episodes of vomiting or retching per day.

Severe

Nausea for more than 6 hours per day, or more than 5 episodes of vomiting or retching per day.

Hyperemesis gravidarum

HG is the severe end of the NVP spectrum: intractable, often continuous vomiting starting in the first trimester that can persist throughout pregnancy, producing dehydration, electrolyte disturbance, nutritional deficiency, and weight loss severe enough to threaten maternal and fetal wellbeing. Because no single confirmatory test exists, HG is a diagnosis of exclusion — reserved for protracted NVP with objective evidence of dehydration and electrolyte imbalance alongside substantial weight loss, once other causes of vomiting in pregnancy have been reasonably excluded.^[2]

Risk Factors

Multifetal (twin or higher-order) pregnancy.
Molar (hydatidiform) pregnancy.
NVP or hyperemesis gravidarum in a previous pregnancy.
A first-degree relative with a history of NVP or hyperemesis gravidarum.
History of motion sickness.
History of migraine.
Nulliparity.
Pre-existing gastrointestinal disorders (e.g., gastroparesis, prior Helicobacter pylori infection) that can compound NVP severity.

Red Flags & Refer

Escalate promptly for any of the following — untreated severe NVP and HG carry real risk, so the threshold to escalate should be low, not high.

Signs of hyperemesis gravidarum: significant dehydration, ketosis, or substantial weight loss.
Inability to tolerate any oral intake, including fluids.
Signs suggesting Wernicke encephalopathy in the setting of prolonged vomiting — visual disturbance, unsteady gait or ataxia, confusion, or new neurological signs — this needs emergency thiamine and urgent assessment, not routine follow-up.
Vomiting accompanied by abdominal pain or fever, which suggests an alternative diagnosis rather than uncomplicated NVP.
Onset of vomiting after 9 to 10 weeks' gestation, or persistence beyond 20 weeks without improvement — both atypical for NVP and warrant evaluation for another cause.
Suspected venous thromboembolism (leg swelling or pain, chest pain, shortness of breath) in a patient with hyperemesis gravidarum — hospitalized HG carries an increased VTE risk and needs thromboprophylaxis assessment.^[2]

Assessment & Diagnosis

History

Frequency, severity, and timing of nausea and vomiting in relation to meals, time of day, and emotional stressors.
The relationship between nausea and vomiting, and, if vomiting occurs, the amount and character of the vomitus.
Whether the presentation is consistent with pregnancy-related nausea or suggests another cause.
A full gastrointestinal review — bowel habit, early satiety, postprandial symptoms, abdominal pain, or unintended weight loss.
Other symptoms such as headache, focal pain, or vertigo.
Diet history, including new foods, food intolerances, and possible contaminated-food exposure.
Exposure to gastroenteritis or other infectious illness.
Past history of migraine, endocrine disorders, or malignancy.
A complete medication history, including prescription, non-prescription, herbal, and recreational drug use (including cannabis), alcohol, and supplements — and any recent changes to the medication regimen that correlate with symptom onset.
Psychosocial stressors and the patient's own concerns, functional impact, and treatment preferences.

Physical examination

Vital signs, including blood pressure, pulse, respiratory rate, and temperature.
Hydration assessment — jugular venous pressure, mucous membranes, skin turgor, and postural changes in blood pressure and heart rate.
Abdominal examination to look for an alternative cause when the history is not clearly typical of NVP.
A focused examination directed at any suspected underlying cause (e.g., neurologic exam for suspected migraine, assessment for bowel obstruction if postsurgical or malignancy history, nystagmus in vertigo-associated nausea, fundoscopy for papilledema).
Intermittent fetal auscultation or a non-stress test in the second and third trimester; a biophysical profile if fetal compromise is suspected.

Severity scoring — PUQE-24

The modified Pregnancy-Unique Quantification of Emesis (PUQE-24) index is a validated, simple point-sum tool used to grade severity and track response to treatment over time.^[3] Score each of the 3 questions from 1 to 5 and sum the total (range 3 to 15).

Question (over the last 24 hours)	1 point	2 points	3 points	4 points	5 points
For how long have you felt nauseated or sick to your stomach?	Not at all	1 hour or less	2 to 3 hours	4 to 6 hours	More than 6 hours
How many times have you vomited or thrown up?	I did not throw up	1 to 2 times	3 to 4 times	5 to 6 times	7 or more times
How many times have you had retching or dry heaves without bringing anything up?	No time	1 to 2 times	3 to 4 times	5 to 6 times	7 or more times

Total score interpretation: mild NVP is a score of 6 or under; moderate NVP is a score of 7 to 12; severe NVP is a score of 13 or higher.

Other investigations, if indicated by history and exam

Electrolytes, glucose, and a metabolic panel — e.g., calcium in suspected malignancy, glucose in a diabetic patient, potassium/sodium in a patient on diuretics, lipase/amylase if pancreatitis is suspected.
CBC and differential if an infective cause is suspected.
Serum creatinine to assess for a renal cause.
Relevant drug levels (e.g., digoxin, anticonvulsants) if applicable.
Liver function tests.
Thyroid function or fasting cortisol if an endocrine cause is suspected.
Ultrasound of the pregnancy to assess for multiple, molar, or ectopic pregnancy if not already established.
Ultrasound of the liver, gallbladder, or pancreas if indicated.
Abdominal x-ray if bowel obstruction is suspected, or an upper GI series if reflux or gastroparesis is suspected — imaging decisions should weigh maternal benefit against fetal radiation exposure, with the patient involved in that decision.
Upper GI endoscopy or CT of the abdomen, again weighing maternal benefit against fetal risk.
CT of the head if an intracranial cause is suspected.

Treatment Targets

Eliminate nausea and vomiting; where that is not possible, reduce severity, frequency, and duration as much as possible.
Prevent complications of nausea and vomiting, including dehydration, ketosis, and nutritional deficiency.
Prevent recurrence once symptoms are controlled.
Reduce time off work and disruption to daily function.
Correct any consequences of persistent nausea, such as electrolyte abnormalities or nutritional compromise, once they occur.
Prevent Wernicke encephalopathy in any patient with prolonged vomiting through timely thiamine supplementation.

Choose Initial Treatment

Direct treatment rule

1st step, mild NVP begin non-pharmacologic measures (dietary/lifestyle counselling, reassurance) for every patient, and offer a trial of ginger 250 mg PO every 6 hours and/or pyridoxine 25 mg PO every 8 hours.

1st line pharmacotherapy — use when mild measures are inadequate, or from the outset for moderate-severe NVP start doxylamine-pyridoxine (Diclectin) 2 tablets PO at bedtime, titrating to a maximum of 4 tablets per day — this is the only Health-Canada-approved medication specifically for NVP and carries one of the largest cumulative pregnancy-safety records of any medication used in pregnancy.^[1]^[4]

Alternative 1st line — use when doxylamine-pyridoxine is not tolerated, not preferred, or not available dimenhydrinate, diphenhydramine, or promethazine are equally reasonable first-line antihistamine options for moderate-severe NVP.^[1]

2nd line — use when 1st-line antihistamine therapy (alone or combined) is inadequate add or switch to a dopamine antagonist: metoclopramide (maximum 5 days of use), prochlorperazine, or chlorpromazine.^[1]

Special situation — refractory NVP or hyperemesis gravidarum consider ondansetron only after standard options have failed, and only after counselling the patient on its evolving safety evidence (see Special Populations); reassess for hyperemesis gravidarum at every step where symptoms remain uncontrolled.^[5]

Special situation — hyperemesis gravidarum with dehydration admit for IV fluid and electrolyte correction and give thiamine before or with any dextrose-containing fluid; reserve corticosteroids for cases refractory to IV fluids and combination antiemetic therapy, preferably after the first trimester.^[2]

Combine drugs from different classes — rather than simply escalating the dose of a single agent — when a patient does not respond adequately to one antiemetic.^[1]

Non-Pharmacologic Treatment

Reassure patients that most mild to moderate NVP is a normal part of pregnancy, is not harmful to the fetus, and usually improves as pregnancy progresses.
Recommend small, bland, frequent meals; avoid fatty, fried, or spicy foods.
Eat at times of day when nausea is less severe, and eat something before getting out of bed in the morning.
Avoid strong food odours; separate solid and liquid intake, drinking small amounts of fluid regularly between meals rather than with meals.
Avoid raw or undercooked meat, fish, or eggs, sprouts, and unpasteurized dairy or juice, given the risk of foodborne illness in pregnancy.
If a standard prenatal vitamin worsens nausea (often due to its iron content or tablet size), switch to folic acid alone or a low-iron multivitamin for the first trimester.
Avoid becoming overheated, increase rest, and reduce work hours or stress where feasible.
Ginger (250 mg PO every 6 hours, up to 1 g per day) has evidence supporting benefit over placebo in randomized trials for mild to moderate NVP, with effectiveness reported as comparable to pyridoxine and to dimenhydrinate in head-to-head trials — a reasonable option for patients who wish to avoid or delay prescription antiemetics.
P6 (Nei Guan) acupressure — located about 5 cm proximal to the wrist crease, between the tendons of palmaris longus and flexor carpi radialis — can be self-applied with fingertip pressure or an acupressure wristband; some practitioners suggest about 5 minutes of pressure every 4 hours. Evidence for acupressure and acupuncture is mixed and has not clearly shown superiority over dietary or lifestyle advice, but the manual technique is safe and low-cost, making it a reasonable adjunct for patients who want a non-drug option.
Discuss cannabis use directly with every patient presenting with NVP — reported use is increasing, cannabis is not an effective or recommended treatment for NVP, and chronic use can itself cause cannabinoid hyperemesis syndrome. Cannabis crosses the placenta, and in-utero exposure has been linked to neonatal withdrawal-like symptoms and longer-term neurodevelopmental effects. Health Canada advises against cannabis use in those who are pregnant, breastfeeding, planning a pregnancy, or of childbearing age without reliable contraception.

Pharmacologic Treatment Options

Drug / intervention	Role	Dose / use	Important adverse effects	Teaching points
Ginger Preferred	1st step, mild NVP	250 mg PO every 6 hours, up to 1 g/day	Heartburn, belching, mouth/throat irritation	Reasonable first option for patients who want to avoid prescription antiemetics; caution with anticoagulant or antiplatelet therapy.
Pyridoxine Preferred	1st step, mild NVP	25 mg PO every 8 hours	Generally well tolerated	A reasonable evidence-based initial step, alone or before adding doxylamine; escalate promptly if not effective within about a week.
Doxylamine-pyridoxine Preferred	1st-line pharmacotherapy for NVP	2 tablets PO at bedtime; may add 1 tablet AM + 1 tablet mid-afternoon (maximum 4 tablets/day)	Sedation, dry mouth, epigastric pain, headache	Only Health-Canada-approved product for NVP; delayed-release — swallow whole, do not crush or split.
Dimenhydrinate Preferred	Alternative 1st line, moderate-severe NVP	50 mg PO/PR/IV every 4 to 6 hours as needed	Sedation, anticholinergic effects	Do not combine with diphenhydramine — dimenhydrinate contains diphenhydramine as its active moiety.
Diphenhydramine Preferred	Alternative 1st line, moderate-severe NVP	25 mg PO every 4 to 6 hours, or 50 mg PO every 6 to 8 hours, as needed	Sedation, anticholinergic effects	Do not combine with dimenhydrinate.
Promethazine Preferred	Alternative 1st line, moderate-severe NVP	12.5 to 25 mg PO/IM every 4 to 6 hours as needed	Sedation, anticholinergic effects	Give IM deep into a large muscle only; avoid IV administration given extravasation-related tissue injury risk.
Metoclopramide Alternative	2nd line, dopamine antagonist	5 to 10 mg PO every 8 hours as needed, maximum 5 days	Diarrhea, headache, hyperprolactinemia, extrapyramidal symptoms including oculogyric crisis	Discontinue immediately if a dystonic reaction occurs; do not exceed 5 days of use.
Prochlorperazine Alternative	2nd line, dopamine antagonist	5 to 10 mg PO every 6 to 8 hours as needed	Sedation, anticholinergic effects, extrapyramidal symptoms	Parenteral/rectal forms are used in some inpatient settings for patients unable to tolerate oral therapy.
Chlorpromazine Alternative	2nd line, dopamine antagonist	10 to 25 mg PO every 4 to 6 hours as needed	Sedation, anticholinergic effects, extrapyramidal symptoms	Generally reserved for when other 2nd-line agents are ineffective or unavailable.
Ondansetron Special situation	Refractory NVP or hyperemesis gravidarum	4 mg PO every 8 hours or 8 mg PO every 12 hours; 8 mg IV over 15 minutes every 12 hours	Headache, constipation, QTc prolongation	Counsel on the small increased oral-cleft signal and conflicting cardiac-malformation data before starting; prefer after the first trimester where possible.
Methylprednisolone Special situation	Refractory hyperemesis gravidarum	15 to 20 mg IV every 8 hours for 24 hours, then transition to an oral taper	Hyperglycemia, infection risk, mood changes	Reserve for cases refractory to IV fluids and combination antiemetics; use after the first trimester where possible.
Prednisone Special situation	Oral corticosteroid taper after IV therapy	40 to 50 mg PO daily, then taper to the lowest dose controlling symptoms	Hyperglycemia, mood changes, infection risk	Step-down agent once clinical improvement occurs; may need to continue until the gestational age at which HG would typically resolve.

Special Populations

Pregnancy — trimester-specific considerations

Every agent recommended above has a reassuring pregnancy-safety record when used as directed. Ondansetron is the notable exception: a large cohort study found no increased risk of cardiac malformations or congenital malformations overall after adjusting for confounders, but did find a small increased risk of oral clefts with first-trimester exposure, and other cohorts have reported conflicting signals for cardiac septal defects specifically.^[5] Health Canada updated the Zofran and Zofran ODT product monograph in November 2020 to reflect this signal and states that ondansetron use in pregnancy is not recommended; ondansetron is not authorized in Canada specifically for NVP.^[6] In practice, ondansetron continues to be used for NVP and hyperemesis gravidarum refractory to other agents given the genuine difficulty of managing severe symptoms otherwise — this should only follow explicit counselling on the oral-cleft signal and the conflicting cardiac data, with a shared decision to proceed, and use after the first trimester where the option to delay exposure exists. Systemic corticosteroids carry a weak signal for oral clefts with first-trimester exposure, so reserve them for after the first trimester wherever the clinical situation allows.

Breastfeeding

The agents used for NVP and HG in pregnancy are also generally considered compatible with breastfeeding.^[1]

Pediatric

Not applicable — NVP and hyperemesis gravidarum are pregnancy-specific conditions.

Renal/hepatic impairment

Reduce the ondansetron dose in moderate-severe hepatic impairment (no more than 8 mg per day). Reduce the metoclopramide dose in significant renal or hepatic impairment. No specific adjustment is required for doxylamine-pyridoxine, dimenhydrinate, diphenhydramine, promethazine, ginger, pyridoxine, or thiamine at standard NVP doses, though standard caution applies in severe organ impairment.

Multiple or molar gestation

Both carry an increased risk of NVP and hyperemesis gravidarum; confirm the diagnosis with ultrasound if not already established, since management and counselling differ from a singleton, non-molar pregnancy.

Monitoring & Follow-Up

Reassess symptom severity (informally, or with a repeat PUQE-24 score) and track weight at each follow-up visit. Review the effect of a new or escalated antiemetic within a few days of starting it rather than waiting for a routine visit, since ineffective therapy should be escalated promptly rather than continued unchanged. For hospitalized HG, check electrolytes and weight daily while on IV fluids, and step down to oral therapy as soon as it is tolerated. Watch for medication-specific issues at every review: extrapyramidal symptoms or oculogyric crisis with dopamine antagonists (stop immediately if these occur), QTc-prolongation risk factors before and during ondansetron use, and hyperglycemia or infection signs during corticosteroid therapy. Reinforce that thiamine must be given before or with any dextrose-containing fluid in a patient with prolonged vomiting. Assess hospitalized HG patients for thromboprophylaxis with low-molecular-weight heparin unless contraindicated, and reassess Wernicke encephalopathy risk (visual changes, ataxia, confusion) at every review of a patient with prolonged vomiting.

Treatment Failure / Next Step

Confirm the current regimen is actually being taken as prescribed before escalating — including whether the patient can keep oral doses down at all.
Add a second antiemetic from a different drug class rather than simply increasing the dose of the first.
If first-line antihistamine-based therapy (doxylamine-pyridoxine, dimenhydrinate, diphenhydramine, or promethazine) is inadequate, add or switch to a dopamine antagonist (metoclopramide, prochlorperazine, or chlorpromazine).
If symptoms remain refractory, consider ondansetron after explicit safety counselling, and reassess for hyperemesis gravidarum.
If the patient cannot tolerate oral intake, has ketosis, significant weight loss, or electrolyte disturbance, admit for IV fluid and electrolyte correction, giving thiamine before or with any dextrose-containing fluid.
If HG persists despite IV fluids and combination antiemetic therapy from at least two drug classes, consider a corticosteroid (methylprednisolone or hydrocortisone IV, then an oral prednisone taper), preferably after the first trimester.
At any step, reassess for an alternative diagnosis if the pattern deviates from typical NVP — persistent abdominal pain, fever, onset after 9 to 10 weeks, or persistence beyond 20 weeks without improvement.

Common Errors to Avoid

Withholding effective antiemetic therapy out of unfounded fear of medication risk to the fetus — untreated severe NVP and HG carry real maternal and fetal risk, and appropriate treatment is the safer choice.
Starting a dextrose-containing IV fluid in a patient with prolonged vomiting before giving thiamine, risking precipitation of Wernicke encephalopathy.
Continuing metoclopramide beyond 5 days or exceeding its maximum daily dose, given the dose- and duration-dependent risk of extrapyramidal effects and tardive dyskinesia.
Reaching for ondansetron as a first-line agent without first trying better-established options, or without counselling the patient on its evolving safety evidence.
Combining dimenhydrinate and diphenhydramine, unintentionally duplicating the same active antihistamine.
Attributing every case to routine "morning sickness" without asking about abdominal pain, fever, or a symptom pattern atypical for NVP.
Missing hyperemesis gravidarum until dehydration or weight loss is already advanced, rather than escalating care at the first signs of inability to tolerate oral intake.
Not asking about cannabis use, or assuming it is a safe or effective treatment for NVP.
Forgetting to reassess and step down therapy as symptoms improve toward the expected resolution window.

Clinical Decision Algorithm

Stepwise management of nausea and vomiting of pregnancy

Nausea, with or without vomiting, in the first trimester; onset after 9-10 weeks or symptoms with abdominal pain/fever should prompt evaluation for an alternative cause

Classify severity (mild/moderate/severe) or calculate PUQE-24; begin dietary counselling, reassurance, and prenatal vitamin adjustment for every patient

Mild symptoms

Trial ginger 250 mg PO every 6 hours and/or pyridoxine 25 mg PO every 8 hours

Improved

Continue; reassess routinely as pregnancy progresses

Inadequate response

Escalate to doxylamine-pyridoxine (or dimenhydrinate/diphenhydramine/promethazine)

Moderate-severe symptoms from the outset

Start doxylamine-pyridoxine 2 tablets PO at bedtime (titrate to max 4 tablets/day); dimenhydrinate, diphenhydramine, or promethazine are reasonable alternatives

Improved

Continue; step down and reassess as symptoms resolve toward 16-20 weeks

Inadequate response

Add or switch to a 2nd-line dopamine antagonist: metoclopramide (max 5 days), prochlorperazine, or chlorpromazine

Still refractory: consider ondansetron after safety counselling; reassess for hyperemesis gravidarum at every step^[5]

Hyperemesis gravidarum management

Hyperemesis gravidarum: protracted NVP with dehydration, ketosis/electrolyte disturbance, and significant weight loss, other causes reasonably excluded

Admit for IV normal saline with added potassium chloride, guided by daily electrolytes; give thiamine 100 mg before or with any dextrose-containing fluid or parenteral nutrition^[2]

Continue or add antiemetics from at least 2 different drug classes; use the parenteral or rectal route if oral is not tolerated

Improving

Transition to oral intake and oral antiemetics as tolerated; step down and reassess

Refractory despite IV fluids and combination antiemetic therapy

Consider a corticosteroid (methylprednisolone or hydrocortisone IV, then an oral prednisone taper), preferably after the first trimester

Assess need for thromboprophylaxis (LMWH) while hospitalized; reassess Wernicke encephalopathy signs at every review

Drug Tables

How to use this table: doses are adult doses for NVP/HG specifically and require patient-specific confirmation. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

Antihistamines (H1-receptor antagonists)

First-line alternatives to doxylamine-pyridoxine for moderate-severe NVP.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Dimenhydrinate	Oral or rectal: 50 mg every 4 to 6 hours as needed Intravenous: 50 mg every 4 to 6 hours as needed	Sedation, anticholinergic effects (dry mouth, blurred vision, urinary retention), rare paradoxical stimulation	Additive sedation with other CNS depressants; may mask ototoxicity of concurrently administered ototoxic drugs	Dimenhydrinate is a salt of diphenhydramine combined with 8-chlorotheophylline — do not combine with diphenhydramine to avoid duplicate antihistamine dosing.
Diphenhydramine (oral)	Standard: 25 mg PO every 4 to 6 hours as needed Alternative: 50 mg PO every 6 to 8 hours as needed	Sedation, anticholinergic effects, paradoxical excitation in some patients	Additive sedation with other CNS depressants; additive anticholinergic burden with other anticholinergic drugs	First-line alternative to doxylamine-pyridoxine; do not combine with dimenhydrinate.
Diphenhydramine (injectable)	Intravenous or deep intramuscular: 10 to 50 mg every 6 hours as needed	Sedation, anticholinergic effects	Additive sedation with other CNS depressants	Reserve for inpatient use when oral therapy is not tolerated; a distinct injectable-only formulation from the oral tablet product above.
Promethazine	Oral: 12.5 to 25 mg every 4 to 6 hours as needed Intramuscular: 12.5 to 25 mg deep IM every 4 to 6 hours as needed	Sedation, anticholinergic effects, extrapyramidal symptoms (less common than with phenothiazine antiemetics at antipsychotic doses)	Additive sedation with other CNS depressants	Give IM injections deep into a large muscle only; avoid intravenous administration given the risk of severe tissue injury with extravasation.

Combination Product — Doxylamine/Pyridoxine

Doxylamine succinate/pyridoxine hydrochloride (delayed-release, marketed in Canada as Diclectin) is the only medication specifically approved by Health Canada for NVP, with one of the largest cumulative pregnancy-safety experiences of any medication used in pregnancy. Its predecessor combination (marketed in the United States as Bendectin, which also contained the antispasmodic dicyclomine) was voluntarily withdrawn from the US market in 1983 amid litigation alleging a teratogenic effect, despite an absence of supporting epidemiologic evidence; the doxylamine-pyridoxine combination without dicyclomine was later re-approved in the US as Diclegis and, as a once-nightly extended-release formulation, Bonjesta — only the original delayed-release formulation (Diclectin) is marketed in Canada. A 2018 reanalysis of previously unpublished trial data found no significant benefit of the combination over placebo,^[7] which is why a trial of pyridoxine alone (see Natural Health Products below) is also a reasonable evidence-based first step — but the combination remains recommended as first-line pharmacotherapy by SOGC and ACOG^[1]^[4] given its exceptional safety record; the debate concerns the size of its benefit over pyridoxine alone, not its safety.^[8]

Drug	Dosage	Adverse effects	Drug interactions	Comments
Doxylamine-pyridoxine	Initial: 2 tablets PO at bedtime Titration, if daytime symptoms persist: add 1 tablet in the morning and 1 tablet in the mid-afternoon Maximum: 4 tablets per day	Sedation, dry mouth, epigastric pain, headache	Additive sedation with other CNS depressants and other antihistamines; MAO inhibitors may prolong and intensify the antihistamine effect	Delayed-release — swallow whole, do not crush or split; take on a regular schedule rather than only when symptoms occur, since levels need to be established before symptoms peak; taper rather than stop abruptly.

Natural Health Products

Reasonable first steps for mild NVP, or as an adjunct to prescription therapy.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Ginger (Zingiber officinale)	Standard: 250 mg PO every 6 hours, up to 1 g per day	Heartburn, belching, mouth or throat irritation	Theoretical additive bleeding risk with anticoagulant or antiplatelet therapy	Give a trial of several days before judging effect; equally effective to pyridoxine and to dimenhydrinate in head-to-head trials.
Pyridoxine (vitamin B6)	Standard: 25 mg PO every 8 hours	Generally well tolerated at this dose; paresthesia and photosensitivity reported with prolonged high-dose use	None significant at this dose	A reasonable evidence-based first step, alone or before adding doxylamine, for mild NVP; more recent systematic-review evidence has questioned the added benefit of the combination product over pyridoxine alone, so escalate promptly if symptoms do not improve.

Dopamine Antagonists (Phenothiazines and Prokinetics)

Second-line — use when first-line antihistamine-based therapy is inadequate. Domperidone is used for this indication in some jurisdictions but carries a dose-related QT-prolongation warning and is not covered in this table.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Metoclopramide (oral)	Standard: 5 to 10 mg PO every 8 hours as needed Maximum duration: 5 days	Diarrhea, abdominal cramping, headache, drowsiness, hyperprolactinemia, extrapyramidal symptoms including oculogyric crisis, rare tardive dyskinesia with prolonged use	Additive sedation with other CNS depressants; additive extrapyramidal risk with other dopamine antagonists	Limit to 5 days of use given the dose- and duration-dependent risk of extrapyramidal effects; discontinue immediately if a dystonic reaction occurs.^[2]
Metoclopramide (injectable)	Intravenous or intramuscular: 5 to 10 mg every 8 hours as needed, maximum 5 days Administration: give IV doses by slow injection over at least 3 minutes	Same as oral formulation	Same as oral formulation	Reserve for inpatient or hyperemesis gravidarum management when oral therapy is not tolerated; slow IV administration reduces the risk of acute dystonic reactions compared with rapid bolus.
Prochlorperazine	Oral: 5 to 10 mg PO every 6 to 8 hours as needed	Sedation, anticholinergic effects, extrapyramidal symptoms	Additive sedation with other CNS depressants; additive extrapyramidal risk with other dopamine antagonists	Intramuscular, intravenous, and rectal formulations are used in some inpatient settings for patients unable to tolerate oral therapy.
Chlorpromazine	Oral: 10 to 25 mg PO every 4 to 6 hours as needed	Sedation, anticholinergic effects, extrapyramidal symptoms, orthostatic hypotension	Additive sedation with other CNS depressants; additive extrapyramidal risk with other dopamine antagonists	Generally reserved for when other 2nd-line agents are ineffective or unavailable.

Serotonin 5-HT3 Receptor Antagonist

Reserve for refractory NVP or hyperemesis gravidarum after explicit safety counselling — see Special Populations.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Ondansetron	Oral: 4 mg every 8 hours, or 8 mg every 12 hours, as needed Intravenous: 8 mg every 12 hours, given over 15 minutes	Headache, constipation, dizziness, transient ECG changes, QTc prolongation	Increased risk of serotonin syndrome or neuroleptic-malignant-syndrome-like reactions with other serotonergic drugs; may reduce the analgesic effect of tramadol; CYP3A4 inducers reduce ondansetron concentrations	Counsel on the small increased oral-cleft signal and conflicting cardiac-malformation data with first-trimester exposure before starting; use caution with other QT-prolonging drugs or QT risk factors.

Corticosteroids (Refractory Hyperemesis Gravidarum)

Reserve for HG refractory to IV fluids and combination antiemetic therapy, and use after the first trimester where possible given a weak signal for oral clefts with first-trimester systemic corticosteroid exposure.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Methylprednisolone	Acute: 15 to 20 mg IV every 8 hours for 24 hours	Hyperglycemia, increased infection risk, mood changes, fluid retention	None specific to this indication	Canadian-context dosing for refractory HG; transition to an oral prednisone taper once clinical improvement occurs.
Hydrocortisone (alternative regimen)	Alternative regimen: 100 mg IV every 12 hours, then convert to an oral taper once improving	Hyperglycemia, increased infection risk, mood changes, fluid retention	None specific to this indication	An internationally recognized alternative IV regimen to methylprednisolone for refractory HG.^[2]
Prednisone	Oral taper: 40 to 50 mg PO daily, then taper gradually to the lowest dose that controls symptoms	Hyperglycemia, mood changes, increased infection risk, insomnia	None specific to this indication	Oral step-down agent after initial IV corticosteroid therapy; in some cases needs to continue until the gestational age at which HG would typically have resolved.

Thiamine — Wernicke Encephalopathy Prevention

Give thiamine before or with any dextrose-containing fluid or parenteral nutrition in a patient with prolonged vomiting — administering dextrose to a thiamine-deficient patient can precipitate Wernicke encephalopathy.^[2]

Drug	Dosage	Adverse effects	Drug interactions	Comments
Thiamine (vitamin B1)	Prevention, prolonged vomiting: 100 mg PO or IV once daily Before/with any dextrose-containing IV fluid: give on each day dextrose is administered, before or together with the dextrose Suspected or confirmed Wernicke encephalopathy: urgent high-dose parenteral thiamine per local protocol, with emergent neurology input	Generally well tolerated; rare hypersensitivity reactions with rapid IV administration	None significant	A missed or delayed diagnosis of Wernicke encephalopathy in HG is associated with poor rates of full recovery, making this an essential, easily-preventable safety step rather than an optional add-on.

References

The Management of Nausea and Vomiting of Pregnancy — Journal of Obstetrics and Gynaecology Canada, Society of Obstetricians and Gynaecologists of Canada (Campbell K, Rowe H, Azzam H, Lane CA) — SOGC Clinical Practice Guideline No. 339. J Obstet Gynaecol Can 2016;38(12):1127-1137
The Management of Nausea and Vomiting in Pregnancy and Hyperemesis Gravidarum (Green-top Guideline No. 69) — BJOG, Royal College of Obstetricians and Gynaecologists (Nelson-Piercy C, et al.). BJOG 2024;131(7):e1-e30
Validity of a modified Pregnancy-Unique Quantification of Emesis and Nausea (PUQE) scoring index to assess severity of nausea and vomiting of pregnancy — American Journal of Obstetrics and Gynecology (Lacasse A, Rey E, Ferreira E, et al.). Am J Obstet Gynecol 2008;198(1):71.e1-e7
ACOG Practice Bulletin No. 189: Nausea and Vomiting of Pregnancy — Obstetrics & Gynecology, American College of Obstetricians and Gynecologists (Committee on Practice Bulletins-Obstetrics). Obstet Gynecol 2018;131(1):e15-e30
Association of maternal first-trimester ondansetron use with cardiac malformations and oral clefts in offspring — JAMA (Huybrechts KF, Hernandez-Diaz S, Straub L, et al.). JAMA 2018;320(23):2429-2437
Health Product InfoWatch, November 2020 — Zofran and Zofran ODT (ondansetron) safety labelling update — Health Canada. Health Canada Health Product InfoWatch, November 2020
Doxylamine-pyridoxine for nausea and vomiting of pregnancy randomized placebo controlled trial: prespecified analyses and reanalysis — PLoS One (Persaud N, Meaney C, El-Emam K, et al.). PLoS One 2018;13(1):e0189978
Effectiveness of delayed-release doxylamine and pyridoxine for nausea and vomiting of pregnancy: a randomized placebo controlled trial — American Journal of Obstetrics and Gynecology (Koren G, Clark S, Hankins GD, et al.). Am J Obstet Gynecol 2010;203(6):571.e1-e7

Professional reference only. Apply clinical judgment and verify patient-specific decisions against current authoritative guidance and product information.