Dehydration in Adults

Dehydration in an adult is a reduction in total body water, most often with an accompanying deficit of sodium and other electrolytes. It is not strictly synonymous with volume depletion — the narrower loss of salt and water specifically from the intravascular space — though the two frequently occur together and are managed the same way at the bedside.^[1] This chapter addresses dehydration in adults specifically: a genuinely different clinical problem from pediatric dehydration, driven in adults by GI losses, heat illness, diuretic and laxative use, poorly controlled diabetes, and — especially in frail and cognitively impaired older adults — simply not drinking enough, rather than by the fontanelle- and skin-turgor-based assessment used in infants and young children.

Dehydration is classified by the ratio of water loss to sodium loss, which determines both the presenting picture and the fluid used to correct it.^[1][2]

^[2]

Dehydration in older adults is genuinely both under- and over-diagnosed. In one series of 339 nursing home residents who became acutely unwell, 91 had biochemical features of dehydration — yet a separate study of 102 consecutive medical admissions coded as dehydration found only 17% had biochemically confirmed dehydration (serum osmolality above 295 mOsm/kg), suggesting many diagnoses rest on non-specific physical signs alone rather than confirmatory testing.^[3] Community and hospital prevalence estimates in older adults range from roughly 17-28%.^[1] The pharmacist's role spans recognizing dehydration risk from the medication list itself, supporting a safe oral rehydration attempt, and knowing when a presentation has crossed into a medical emergency.

Presentation by underlying cause

Regardless of tonicity, the presenting picture reflects both the degree of volume loss and its cause.

Symptoms and signs

Thirst, dizziness or lightheadedness (especially on standing), fatigue, muscle cramps, dark and reduced-volume urine, dry mouth, headache, and — in more severe cases — confusion are the typical complaints. In older adults, atypical presentations are common and just as important: new confusion, constipation, unexplained fever, falls, or a functional decline can be the presenting complaint instead of thirst.^[2]

• Older age — the single largest risk amplifier in adults, through several compounding physiologic changes described in Special Populations.^[3][1]
• Gastrointestinal losses: vomiting, diarrhea, GI bleeding, or ongoing external drainage (e.g., an ostomy or fistula).^[2]
• Heat exposure or exertion, and — separately — an overheated indoor environment in winter.^[2]
• Fever or any acute febrile illness, which increases insensible losses.^[2]
• Diuretic therapy, including self-directed overuse.^[2][4]
• Laxative overuse, prescribed or self-directed.^[2]
• SGLT2 inhibitor therapy, through its osmotic-diuresis mechanism.^[5]
• Poorly controlled diabetes mellitus with osmotic diuresis, and diabetes insipidus.^[2][1]
• Impaired cognition (dementia or delirium), which blunts the behavioural response to thirst and can prevent a person from independently seeking fluids.^[2][3]
• Reduced mobility, dysphagia, or dependence on others for fluids — including social isolation and inadequate caregiver or facility staffing for fluid offering.^[3]
• Four or more chronic conditions, or four or more scheduled medications (polypharmacy) — both independently associated with dehydration risk in older adults who become acutely unwell.^[3]
• Female sex and age over 85, specifically identified as independent risk factors in a nursing-home cohort.^[3]
• Any significant comorbidity — diabetes, congestive heart failure, or chronic kidney disease — or immunocompromised status.^[2]

History

Characterize fluid intake (amount, type, frequency), urine output (frequency, colour, any hematuria), and any vomiting or diarrhea (duration, frequency, presence of blood or mucus). Ask specifically about heat or cold exposure, recent weight loss, underlying illness (diabetes, thyroid disease, renal disease), and — critically for a pharmacist — the full medication list: diuretics, laxatives, ACE inhibitors/ARBs, NSAIDs, metformin, and SGLT2 inhibitors all matter here, both as a cause and because several need to be temporarily held (see Choose Initial Treatment).^[2]

Consider other explanations before attributing findings to dehydration in an older adult

Ask whether dry mouth could instead reflect a medication (many are anticholinergic), and whether muscle weakness could reflect disuse or age-related muscle atrophy rather than volume loss, before attributing either finding to dehydration.^[2]

Orthostatic vital signs — the central adult bedside tool

Measure supine blood pressure and pulse after the patient has been lying down for 5-10 minutes, then measure again as soon as the patient stands (or sits, if standing is not possible), and again after 2-3 minutes in that position.^[2]

• A drop in systolic blood pressure of 20 mmHg or more, or in diastolic blood pressure of 10 mmHg or more, from the supine value defines orthostatic hypotension.^[2]
• A pulse increase of more than 20 beats per minute accompanying a blood pressure drop suggests hypovolemia specifically, as opposed to an autonomic- or medication-related cause of orthostatic hypotension without a compensatory tachycardia.^[2]
• Orthostatic vital signs have low sensitivity in older adults — a normal result does not exclude dehydration, and an abnormal result can reflect autonomic dysfunction or antihypertensive therapy rather than volume loss. Interpret alongside the rest of the assessment, not in isolation.^[2][7]

Mucous membranes, skin turgor, and mental status

Check the tongue and oral mucosa for dryness. Test skin turgor over the sternum or the inner aspect of the thigh, rather than the back of the hand — and interpret it cautiously, since decreased skin elasticity with normal aging makes turgor a less reliable sign in older adults regardless of hydration status.^[2]

Assess mental status directly rather than relying on a caregiver's impression; in a study of elderly patients and residents with hypernatremia specifically, four signs were significantly and independently associated with it — abnormal subclavicular/thigh skin turgor, dry oral mucosa, and a recent change in consciousness.^[3] No single sign or bedside test — including thirst itself, which cannot be relied upon in older adults — reliably diagnoses dehydration on its own. One evaluation assessing 49 candidate clinical signs, symptoms, and bedside tests found none reached the accuracy needed to stand alone as a diagnostic test. Combine several findings, and confirm with laboratory testing whenever the presentation is more than mild.^[7][3]

Severity classification

^[2]

Investigations

Laboratory testing is not usually needed for mild dehydration with an obvious cause, but should be obtained for any moderate or severe presentation, or whenever the cause is unclear.^[2]

A single abnormal value in an older adult without clinical signs of dehydration is not itself diagnostic — some older adults have an elevated baseline osmolality, and all three biochemical markers are otherwise fairly stable in a well older adult, so a change from the person's own baseline is more informative than a single reading against a population reference range.^[3]

• Restore circulating volume, matched to the estimated severity of loss.
• Correct the fluid deficit in a way that matches the type of dehydration — replacing isotonic loss with an isotonic fluid, and correcting a hypertonic (free-water) deficit slowly enough to avoid cerebral edema from over-rapid correction.
• Identify and treat the underlying cause, not only the fluid deficit itself — dehydration is always a sign of something else, not a stand-alone diagnosis.
• Identify and temporarily hold medications that worsen volume depletion or blunt the kidney's compensatory response during acute illness, and reliably resume them once the patient has recovered.
• Prevent ongoing losses where possible while rehydration is underway.
• Use oral rehydration wherever it is safe and tolerated — it is effective, safe, and markedly less resource-intensive than intravenous therapy.
• Recognize and escalate promptly when a presentation is beyond what oral rehydration or ward-level care can safely manage.
• Reduce the risk of recurrence through patient, family, and caregiver education specific to the contributing cause identified.

Oral rehydration — composition principles

An effective oral rehydration solution contains both sodium and glucose together, which maximizes co-transported absorption of both across the intestinal wall — a solution containing only one or the other is absorbed far less efficiently.^[2] Avoid replacing GI losses with dilute juice, cola, or weak tea alone — these are low in sodium and can convert an isotonic loss into hypotonic dehydration — and avoid undiluted juice or regular soft drinks, whose high sugar content can itself worsen fluid loss through an osmotic effect in the gut.^[2] Preferred a commercially prepared oral rehydration solution. A home-made solution — 1/2 teaspoon of table salt plus 6 teaspoons of sugar in 1 litre (4 cups) of water — is a recognized alternative, but carries a real risk of a measurement error and should be used with the same caution given to any home-mixed medical preparation.^[2]

Give oral rehydration solution in small, frequent volumes — about 50-100 mL per hour — in addition to replacing any ongoing losses (vomiting, diarrhea, urine output) roughly mL for mL where feasible; total intake over the first 24-48 hours should replace the initial deficit plus continuing losses.^[2] An antiemetic (see Pharmacologic Treatment Options) can be a reasonable adjunct specifically to make an oral rehydration attempt feasible when nausea or vomiting is the limiting factor, before concluding that IV fluids are required.

IV fluid selection and rate

First-line normal saline (0.9% sodium chloride) is the preferred initial isotonic crystalloid for most adult dehydration and hypovolemia requiring IV therapy.^[1] Alternative a balanced crystalloid such as lactated Ringer's is a reasonable alternative, particularly for larger-volume replacement, since large volumes of normal saline can produce a hyperchloremic metabolic acidosis; use cautiously if significant hyperkalemia or renal failure is present, given its potassium content.^[1] Individualize the rate to the estimated severity of loss and to the patient's cardiac and renal status — a rate appropriate for a young, healthy adult with severe isotonic loss can readily precipitate fluid overload in an older adult with heart failure or significant renal impairment; see Special Populations.

Hypertonic (hypernatremic) dehydration — correct slowly

Medication review — hold the right medications, for the right reason, and resume them

This is one of the most genuinely pharmacist-specific interventions in this chapter. During acute illness with reduced oral intake, vomiting, or diarrhea, several medication classes should be temporarily held — not stopped permanently — because they either worsen volume contraction directly or blunt the kidney's ability to compensate for it.

Diabetes Canada's expert committee frames the diuretic/RAAS-blocker/NSAID hold as a formal recommendation (Grade D, Consensus): adults with diabetes and chronic kidney disease should be given a written sick-day medication list.^[4] The same principle reasonably extends to any adult, with or without diabetes or CKD, who is acutely unwell with reduced intake or significant GI losses. Resume all held medications once the patient is eating and drinking normally and clinically recovered — an open-ended hold is itself a medication error.^[4][5]

Separately, review any patient with recurrent or unexplained dehydration for laxative overuse, prescribed or self-directed — an under-recognized, genuinely modifiable contributor, particularly stacked on top of reduced intake in a frail older adult.^[2]

• Estimate baseline fluid requirements and use them to set a concrete daily target rather than a vague "drink more" instruction — a practical adult formula is 100 mL/kg for the first 10 kg of body weight, 50 mL/kg for the next 10 kg, and 15-20 mL/kg for each kg after that (roughly 1.5-2 L/day for an average-weight adult, though requirements are genuinely individual).^[2]
• For older adults, especially in a care setting: offer fluids at fixed routine points in the day (with each medication round, before/after bathing, before/after any therapy session) rather than relying on the person to ask.^[3]
• Offer the person's preferred drinks and a genuine choice of beverage, and include "wet" foods (soup, yoghurt, gelatin, pureed fruit) as a meaningful secondary fluid source when plain drinking is difficult.^[3]
• Keep fluids within reach for anyone with reduced mobility, and prompt rather than assume — family and visitors can meaningfully help simply by offering a drink each visit.^[3]
• Reduce environmental heat exposure where possible (hydration before/during/after heat exposure or exertion), and remember that an overheated indoor environment in winter carries the same insensible-loss risk as summer heat.^[2]
• For diarrheal illness specifically: start oral rehydration solution early — from the beginning of the illness — and continue it frequently rather than waiting for signs of dehydration to appear.^[2]
• Anti-diarrheal medication is not a substitute for rehydration and is not generally recommended as primary therapy for acute dehydrating diarrheal illness.^[2]
• Escalate to IV (or subcutaneous, where available and appropriate) fluid replacement rather than persisting with oral rehydration alone whenever oral intake is not tolerated (e.g., persistent vomiting) or is not correcting the deficit.^[2]

Older adults — the central population for this chapter

Several physiologic changes compound in older age to raise both the likelihood and the severity of dehydration, and each is genuinely relevant to how a pharmacist assesses and counsels this population.

• Blunted thirst response — older adults have a measurably reduced thirst sensation in response to fluid deprivation, and an impaired hormonal (antidiuretic hormone) response to dehydration; both are more pronounced in people with dementia. Thirst cannot be relied upon as a warning sign in this population — encourage scheduled fluid offering rather than waiting to feel thirsty.^[3][1]
• Reduced total body water reserve — total body water declines with age, largely reflecting reduced muscle mass, leaving a smaller physiologic buffer before a given fluid loss becomes clinically significant.^[7][1]
• Reduced renal concentrating ability — aging kidneys are less able to concentrate urine to conserve free water, raising hypernatremic-dehydration risk specifically, compounded further by any limitation on increasing fluid intake (immobility, dysphagia).^[2][1]
• Polypharmacy — four or more medications, and four or more chronic conditions, are each independently associated with dehydration risk when an older adult becomes acutely unwell; diuretic use specifically is a recognized risk factor for both electrolyte disturbance and dehydration in nursing home residents.^[3]

Classic signs — skin turgor, thirst, and orthostatic hypotension — all have low sensitivity in older adults; dehydration instead often presents atypically, as new confusion, constipation, unexplained fever, or falls.^[2] Combine several findings and confirm with laboratory testing for anything beyond mild dehydration, rather than relying on any single bedside sign (see Assessment & Diagnosis).^[7]

Diabetes, and SGLT2 inhibitor therapy specifically

Poorly controlled diabetes produces an osmotic diuresis that itself causes hypertonic dehydration; SGLT2 inhibitor therapy adds a pharmacologic osmotic-diuresis mechanism on top of this, and carries a specific risk of euglycemic diabetic ketoacidosis, volume depletion, and acute kidney injury during acute illness — hold it temporarily during acute illness with reduced intake or dehydration risk, and before major surgery, resuming only once the patient has recovered.^[5]

Chronic kidney disease

Reduced ability to both concentrate and dilute urine narrows the safe margin between under- and over-hydration. This is specifically the population for whom Diabetes Canada's sick-day medication list recommendation is written — every adult with diabetes and CKD should be given a written list identifying which medications to hold during acute illness.^[4]

Heart failure

Dehydration and early decompensated heart failure can look similar in an older adult on chronic diuretic therapy — both can present with fatigue, confusion, and abnormal orthostatic findings. Do not adjust a chronic diuretic regimen unilaterally; involve the prescriber, and resist the urge to over-resuscitate with IV fluid in this population once volume is restored.

Pregnancy and breastfeeding

Hyperemesis gravidarum is a genuine cause of significant dehydration in pregnancy; oral rehydration solution is safe, and isotonic IV fluids are used as needed for more significant loss. Involve the patient's obstetric provider for persistent vomiting rather than managing it as routine dehydration alone.

Renal and hepatic impairment

Adjust IV fluid choice, rate, and any potassium replacement to renal function, and monitor renal function closely during and after correction — both under- and over-correction carry real risk. Use acetaminophen cautiously (lower total daily dose) in significant hepatic impairment.

• For mild dehydration managed at home or as an outpatient: re-evaluate every 24 hours for the first 2 days, including a recheck of body weight, and confirm the patient (or caregiver) knows to return promptly if symptoms worsen or oral intake cannot keep pace with losses.^[2]
• For moderate dehydration: reassess blood pressure and mental status within 1 hour of starting oral rehydration therapy — this is the specific decision point for escalating to physician consultation and IV fluids.^[2]
• Recheck electrolytes and renal function after IV fluid therapy or after correcting a significant electrolyte derangement, and before resuming any temporarily held sick-day medication.
• In hypernatremic dehydration, monitor serum sodium closely during correction — more frequently than for routine rehydration — to confirm the actual rate of correction is tracking the intended rate, not just at the end of a calculated infusion.^[6]
• In long-term care or with a recurrent-dehydration history, daily weight is a practical, low-burden ongoing monitoring tool.
• Confirm every temporarily held sick-day medication (diuretic, ACE inhibitor/ARB, NSAID, metformin, SGLT2 inhibitor) is explicitly restarted once the patient is eating and drinking normally and clinically recovered — this is easy to lose track of once the acute illness has resolved, and an indefinite, unintended hold is itself a medication error.^[4][5]
• For anyone with a modifiable contributing cause identified (diuretic or laxative overuse, inadequate fluid offering in a care setting, an unaddressed swallowing difficulty), close the loop with a concrete follow-up plan rather than treating the acute episode as the end of the encounter.

1. Moderate dehydration whose blood pressure and mental status have not stabilized within 1 hour of oral rehydration therapy: escalate to physician/prescriber consultation and IV fluids rather than continuing oral rehydration alone.^[2]
2. IV fluid therapy not correcting volume status or electrolytes as expected: reassess the fluid choice and rate against the patient's cardiac and renal status, and confirm the estimated tonicity (isotonic/hypotonic/hypertonic) was correct — treatment that is not working is often treating the wrong classification.
3. Serum sodium correcting faster or slower than the intended rate in hypernatremic dehydration: adjust the fluid composition and rate promptly — both under-correction (ongoing symptoms) and over-correction (cerebral edema risk) are genuine hazards, not just theoretical ones.^[6]
4. Recurrent dehydration episodes in the same patient: this is a signal to review the medication list for diuretic or laxative overuse, reassess an underlying cause (poorly controlled diabetes, swallowing difficulty), and build an explicit hydration plan with the patient, family, or care facility rather than treating each episode in isolation.^[3]
5. Suspected acute kidney injury not improving with fluid resuscitation: this needs prompt nephrology or hospital-level reassessment rather than continued fluid administration alone.
6. Any diagnostic uncertainty, or a patient who does not fit the expected pattern (e.g., persistent hypotension despite adequate volume replacement): reconsider the differential — dehydration is a sign of an underlying process, not a stand-alone diagnosis, and sepsis, hemorrhage, or a cardiac cause may need to be actively ruled out.^[2]

• Diagnosing dehydration in an older adult from a single non-specific sign (dry mouth, mild confusion, reported reduced intake) without combining several findings or confirming with laboratory testing when the presentation is more than mild.^[7][3]
• Relying on thirst as a warning sign in an older adult — it is genuinely blunted with age and cannot be trusted the way it can in a younger adult.^[3]
• Treating skin turgor as equally reliable across all ages — decreased skin elasticity with normal aging makes it a weaker sign in older adults regardless of hydration status.^[2]
• Replacing GI losses with dilute juice, cola, or weak tea rather than a sodium-and-glucose-containing oral rehydration solution.^[2]
• Correcting hypernatremic (hypertonic) dehydration too quickly — this is a genuine cerebral edema risk, not a theoretical one; respect the 0.5-1 mEq/L-per-hour ceiling.^[6]
• Continuing diuretics, ACE inhibitors/ARBs, NSAIDs, metformin, or SGLT2 inhibitors through an acute illness with reduced intake or significant GI losses, rather than temporarily holding them.^[4][5]
• Leaving a sick-day medication hold open-ended rather than explicitly restarting it once the patient has recovered.^[4]
• Reaching for an antidiarrheal agent as primary therapy for acute dehydrating diarrheal illness instead of rehydration.^[2]
• Missing laxative overuse as a contributing, modifiable cause in a patient with recurrent or unexplained dehydration.^[2]
• Assuming a patient with heart failure and abnormal orthostatic findings on chronic diuretic therapy is dehydrated (or, conversely, assuming they are fluid-overloaded) without a careful individual assessment — both errors are plausible in this population, and unilaterally adjusting the diuretic dose without the prescriber compounds either one.

How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

Oral Rehydration and Electrolyte Replacement

First-line for mild-to-moderate dehydration when oral intake is tolerated. Give in small, frequent volumes and continue replacing ongoing losses.

Intravenous Fluids

For moderate-to-severe dehydration, or when oral rehydration is not tolerated or not correcting the deficit. Individualize the rate to severity and to cardiac/renal status.

Antiemetics to Support an Oral Rehydration Attempt

A reasonable adjunct specifically when nausea or vomiting is the limiting factor for oral rehydration, before concluding IV fluids are required.

Sick-Day Medications to Temporarily Hold

Hold during acute illness with reduced oral intake, vomiting, or diarrhea; resume once the patient is eating and drinking normally and clinically recovered. An open-ended hold is itself a medication error.

Safe Default Analgesic/Antipyretic

A reasonable default while other sick-day medications are held.