Lisinopril and Diphenhydramine Interaction: Safety, Risks, and What to Monitor

Lisinopril is a prescription angiotensin-converting enzyme (ACE) inhibitor used for hypertension, heart failure, and after certain heart attacks. Diphenhydramine (brand name Benadryl, and the active ingredient in many "PM" combination products and sleep aids such as ZzzQuil) is a first-generation, over-the-counter H1-antihistamine with anticholinergic and sedating properties. This article does not cover second-generation antihistamines (loratadine, cetirizine, fexofenadine), which behave differently and are discussed below as alternatives.

Direct answer

Lisinopril and diphenhydramine do not have a known pharmacokinetic interaction: lisinopril is cleared unchanged by the kidneys and is not metabolized by liver enzymes, while diphenhydramine is metabolized mainly through hepatic pathways, so the two drugs are not competing for the same clearance route. The concern is pharmacodynamic instead. Diphenhydramine's anticholinergic and mild pressor properties run counter to lisinopril's blood-pressure-lowering mechanism, and its sedating effect can compound the dizziness some patients feel on an ACE inhibitor, particularly older adults. Most drug interaction references classify this pairing as moderate, meaning it is not a reason to avoid diphenhydramine outright but does warrant caution with dose, duration, and monitoring, especially in people 65 and older or those with heart failure or reduced kidney function.

Why this pairing gets flagged

Lisinopril is one of the most widely prescribed antihypertensives in the United States, and diphenhydramine is present in dozens of OTC cold, allergy, and sleep products, so co-exposure is common even when a patient never mentions taking "an antihistamine." The interaction rating is pharmacodynamic rather than metabolic: lisinopril lowers blood pressure by inhibiting the renin-angiotensin-aldosterone pathway, while diphenhydramine has anticholinergic and mild sympathomimetic activity that can nudge blood pressure and heart rate in the opposite direction and add central nervous system depression on top of any dizziness the ACE inhibitor already causes.

The blood pressure question

Some published reports describe modest, transient increases in blood pressure after diphenhydramine dosing in certain individuals, consistent with its anticholinergic and adrenergic activity. The magnitude of that effect at typical over-the-counter doses (25 to 50 mg) is not consistently established across the literature available for this review, and a specific milligram-of-mercury figure should not be treated as a fixed, reproducible number. What is consistent across sources is the direction of the effect and the practical implication: diphenhydramine is not blood-pressure-neutral, and it should not be assumed to be harmless in someone whose hypertension is only marginally controlled.

Sedation and dizziness overlap

Lisinopril can cause dizziness, especially early in treatment or in a volume-depleted patient. Diphenhydramine crosses the blood-brain barrier readily and is one of the more sedating antihistamines still sold without a prescription. Combining the two makes it harder to tell which drug is causing a given episode of lightheadedness, and it plausibly raises the risk of falls in older adults, a population where both anticholinergic medications and antihypertensives are already independently linked to fall risk in the broader geriatric pharmacology literature. The exact size of that added risk for this specific drug pair has not been established in a dedicated trial and should be treated as a plausible, not proven, extension of that broader literature.

Pharmacology in brief

Lisinopril: a lysine analog of enalaprilat that does not require hepatic activation and is excreted unchanged by the kidneys. According to its FDA-approved prescribing information, oral bioavailability is roughly 25%, peak plasma concentration occurs around 7 hours after dosing, and the effective half-life is approximately 12 hours at steady state. Because it bypasses CYP450 metabolism, classic CYP-based interactions are not a concern with lisinopril.

Diphenhydramine: a first-generation H1-receptor antagonist with anticholinergic (muscarinic-blocking) and mild alpha-adrenergic activity. It is metabolized primarily in the liver, with a half-life generally reported between roughly 2.5 and 9 hours in healthy adults and longer in older adults due to reduced clearance. Because lisinopril is not hepatically metabolized, there is no meaningful metabolic competition between the two drugs; whatever interaction exists is pharmacodynamic.

Drug interaction references (such as Lexicomp and Micromedex) commonly rate this combination as moderate, meaning monitoring is advised but the combination is not contraindicated.

Anticholinergic burden: the more durable concern

Diphenhydramine carries a high anticholinergic burden by common clinical scoring systems (it is typically rated at the top tier on the Anticholinergic Cognitive Burden scale), while lisinopril has essentially none. Cumulative anticholinergic exposure across all of a patient's medications, not just this one pair, has been associated in observational literature with a higher risk of confusion, dry mouth, constipation, urinary retention, and, with chronic use, cognitive decline in older adults. That literature is largely about total anticholinergic load rather than this specific two-drug combination, so it supports caution about diphenhydramine in general rather than a precise, drug-pair-specific risk number.

Current geriatric prescribing guidance, including the American Geriatrics Society's Beers Criteria, lists first-generation antihistamines such as diphenhydramine among medications to generally avoid in adults 65 and older because of anticholinergic and sedative burden, independent of what antihypertensive the patient is also taking. The exact current wording and edition of that guidance should be confirmed against the primary publication before being quoted directly in a clinical context.

Evidence-status assessment for this interaction pair

A practical decision framework

Single, occasional OTC dose (25 mg), under 65, no heart failure or advanced kidney disease: Generally low added risk for a one-off use, such as an acute allergic reaction. Home blood pressure monitoring is reasonable but not strictly necessary for a single dose.

Repeated or nightly dosing, any age, or a single dose in someone 65 or older: Higher-caution scenario. Anticholinergic and sedative effects accumulate with repeated dosing, and older adults clear diphenhydramine more slowly. A second-generation antihistamine or a non-antihistamine sleep strategy is preferable, and this is a reasonable point to involve a pharmacist or prescriber before continuing.

Heart failure, eGFR below 30 mL/min/1.73 m², or blood pressure already poorly controlled: Avoid routine diphenhydramine use if possible. If it cannot be avoided, use the lowest effective dose, the shortest duration, and involve the prescribing clinician rather than self-managing.

Blood pressure monitoring if the combination cannot be avoided

1. Measure blood pressure at roughly the same time each morning before the lisinopril dose, using a validated upper-arm cuff.
2. If diphenhydramine is taken, record a second reading one to two hours afterward.
3. Continue for several days to see whether a pattern emerges rather than reacting to a single reading.
4. Contact the prescriber for a sustained rise well above the patient's usual baseline, a resting heart rate persistently above 100 bpm, new confusion, or difficulty urinating.

Safer alternatives to diphenhydramine

For allergy symptoms, second-generation antihistamines such as loratadine, cetirizine, and fexofenadine are generally preferred for patients on lisinopril because they cross into the central nervous system far less and carry a much lower anticholinergic burden. For sleep, non-antihistamine approaches such as cognitive behavioral therapy for insomnia, or a conversation with a prescriber about other options, are worth considering before reaching for diphenhydramine-based sleep aids. For congestion, saline nasal spray or guaifenesin avoids the interaction entirely; pseudoephedrine should also be used cautiously in lisinopril patients since it raises blood pressure through a more direct adrenergic mechanism.

Special populations

Adults over 65: Reduced renal clearance can extend lisinopril exposure, and reduced hepatic clearance can extend diphenhydramine exposure, so both drugs may linger longer than expected. This is the population most guideline bodies specifically caution against using diphenhydramine in at all, independent of the antihypertensive interaction.

Heart failure: Patients on higher lisinopril doses for reduced ejection fraction are generally more sensitive to hemodynamic shifts. Anticholinergic-related increases in heart rate are a theoretical concern in this group; clinicians managing heart failure typically review all OTC medications at each visit for this reason.

Chronic kidney disease: Lisinopril depends entirely on renal elimination, so reduced kidney function already prolongs its effect. Adding a drug like diphenhydramine, which also accumulates with impaired clearance, widens the window during which both drugs are active. If diphenhydramine cannot be avoided, a lower dose and longer interval between doses is a reasonable, clinician-guided approach rather than a self-directed one.

What the FDA labels actually say

The FDA-approved lisinopril prescribing information does not name diphenhydramine specifically in its drug interactions section; it warns generally about agents that can blunt the antihypertensive effect, such as NSAIDs and sympathomimetics. Standard OTC drug facts labeling for diphenhydramine warns against combining it with other sedating drugs and instructs users to ask a doctor before use if they have high blood pressure. That instruction appears on Benadryl, ZzzQuil, and store-brand diphenhydramine packaging, though it is easy to overlook.

Evidence boundary

Established: Lisinopril and diphenhydramine have no known pharmacokinetic interaction. Diphenhydramine carries meaningful anticholinergic and sedative effects, and its OTC label directs people with high blood pressure to check with a doctor first. Geriatric prescribing guidance generally advises against diphenhydramine in older adults.

Plausible but not firmly quantified for this pair: A modest blood-pressure-raising effect from diphenhydramine, and an increased fall or dizziness risk when combined with an antihypertensive like lisinopril. These follow reasonably from known pharmacology and broader anticholinergic-burden research, but a dedicated study isolating lisinopril plus diphenhydramine specifically was not identified for this review.

Not established: A precise numeric estimate of how much diphenhydramine raises blood pressure at typical OTC doses, or a precise increase in fall or hospitalization risk attributable specifically to this two-drug combination. Readers and clinicians should treat any specific percentage or millimeter-of-mercury figure for this exact pairing with caution until confirmed against a primary study.

Counseling points

1. Ask about all OTC medications. Diphenhydramine is an ingredient in many combination products, including PM-labeled pain relievers, sleep aids, and cold medicines, and patients may not recognize it by name.
2. Favor second-generation antihistamines for ongoing allergy needs.
3. If diphenhydramine is used, keep it short-term, generally a few days at most, not a nightly habit.
4. Monitor blood pressure at home with a validated upper-arm cuff if both drugs are used together.
5. Avoid alcohol, which compounds the sedating effects of diphenhydramine and any dizziness from lisinopril.