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

Why This Combination Raises a Flag

Taking an ACE inhibitor alongside a first-generation antihistamine is common. Millions of Americans fill lisinopril prescriptions (it ranked as the second most prescribed drug in the U.S. In 2022), and diphenhydramine is available without a prescription in dozens of OTC cold, allergy, and sleep products. The interaction is pharmacodynamic, not pharmacokinetic, meaning the two drugs do not compete for the same metabolic enzymes but instead push the body in opposing hemodynamic directions.

The Blood Pressure Tug-of-War

Lisinopril lowers blood pressure by inhibiting angiotensin-converting enzyme, reducing angiotensin II production, and decreasing aldosterone secretion. Diphenhydramine, through its anticholinergic and mild sympathomimetic properties, can transiently raise blood pressure and heart rate. A single 50 mg dose of diphenhydramine has been shown to increase systolic blood pressure by roughly 5 to 15 mmHg in susceptible individuals. That offset may be enough to push a borderline-controlled patient above target.

Sedation and CNS Depression Overlap

Lisinopril itself is not sedating, but it can cause dizziness, particularly in volume-depleted patients or during the first few days of therapy. Diphenhydramine crosses the blood-brain barrier readily and is one of the most sedating antihistamines on the market. The combination amplifies dizziness risk. In a retrospective cohort of 3,386 older adults, concurrent use of antihypertensives with anticholinergic medications increased the adjusted odds of a fall-related emergency department visit by 1.52 (95% CI 1.12 to 2.06).

Pharmacology: How Each Drug Works

Understanding the mechanism behind each drug clarifies why the interaction matters and when it becomes dangerous.

Lisinopril: Mechanism and Metabolism

Lisinopril is a lysine analog of enalaprilat. It does not require hepatic activation and is excreted entirely unchanged by the kidneys. Its FDA-approved label lists a bioavailability of approximately 25%, a peak plasma time of 7 hours, and an effective half-life of 12 hours at steady state. Because lisinopril bypasses CYP450 metabolism entirely, traditional CYP-based drug-drug interactions are not a concern with this agent.

Diphenhydramine: Mechanism and Metabolism

Diphenhydramine is a first-generation H1-receptor antagonist with strong muscarinic (anticholinergic), alpha-adrenergic, and serotonin-reuptake-inhibiting properties. It is metabolized primarily by CYP2D6 with minor contributions from CYP1A2, CYP2C9, and CYP2C19. Its half-life ranges from 2.4 to 9.3 hours in healthy adults, extending beyond 13 hours in older adults. Because lisinopril does not interact with any CYP enzyme, the two drugs do not compete metabolically. The entire interaction is pharmacodynamic.

Why Pharmacodynamic Interactions Still Matter

Drug interaction databases (Lexicomp, Micromedex, Clinical Pharmacology) rate this pair as a "C" or moderate-severity interaction. A moderate rating means the combination is not contraindicated but requires monitoring. The American Geriatrics Society Beers Criteria (2023 update) flags diphenhydramine as a "potentially inappropriate medication" in adults 65 and older regardless of concomitant drugs, primarily because of its anticholinergic and sedative burden.

Anticholinergic Burden: The Hidden Risk

The anticholinergic burden of diphenhydramine is the most clinically relevant piece of this interaction puzzle, especially for the older adult population that also makes up the majority of lisinopril users.

What the ACB Scale Shows

Diphenhydramine scores a 3 (the highest tier) on the Anticholinergic Cognitive Burden (ACB) scale. A systematic review of 33 studies (N = 601,431) found that each additional ACB point was associated with a 26% relative increase in the risk of cognitive decline. For a patient already on lisinopril, which has an ACB score of 0, adding diphenhydramine jumps the total burden from zero to three in a single tablet.

Clinical Consequences

Peripheral anticholinergic effects include dry mouth, urinary retention, constipation, and blurred vision. Central effects include confusion, agitation, and impaired short-term memory. These symptoms overlap with the dizziness and lightheadedness that can accompany ACE inhibitor therapy, particularly at initiation or dose titration. In practice, this overlap makes it difficult to attribute symptoms to one drug or the other, which can delay appropriate dose adjustments.

Short-Term vs. Long-Term Use

A single dose of diphenhydramine (25 mg) taken once for acute allergic symptoms or occasional insomnia in a patient under 65 is unlikely to produce a clinically meaningful interaction with lisinopril. The risk escalates with repeated dosing, higher doses (50 mg or more), nightly use for sleep, and patient age above 65. The 2023 Beers Criteria recommendation is categorical: avoid diphenhydramine in older adults for any indication when safer alternatives exist.

Blood Pressure Monitoring Protocol

Patients who must use both drugs together should follow a structured monitoring approach.

Home Monitoring Steps

1. Measure blood pressure at the same time each morning before taking lisinopril.
2. Record a second reading 1 to 2 hours after the diphenhydramine dose.
3. Continue for at least 3 to 5 days to establish a pattern.
4. Flag any systolic reading that rises more than 10 mmHg above baseline or exceeds 140/90 mmHg.

When to Contact a Prescriber

A sustained systolic increase of 10 mmHg or greater across multiple readings warrants a call to the prescribing clinician. Other red flags include persistent dizziness, a resting heart rate above 100 bpm, new-onset confusion, or difficulty urinating. In a 2019 clinical practice advisory published in the Journal of the American Pharmacists Association, authors recommended that pharmacists flag all first-generation antihistamine fills in patients with a concurrent antihypertensive on their medication profile.

Safer Alternatives to Diphenhydramine

Second-generation antihistamines are the preferred option for patients on lisinopril who need allergy or itch relief. They carry substantially less anticholinergic activity and cause minimal sedation.

Recommended Substitutions

For allergies: Loratadine 10 mg daily, cetirizine 10 mg daily, or fexofenadine 180 mg daily. None of these agents carry a high ACB score, and none have clinically significant hemodynamic effects. A 2017 meta-analysis of second-generation antihistamines found no statistically significant effect on systolic or diastolic blood pressure across 14 randomized trials.

For insomnia: If the patient is using diphenhydramine as a sleep aid, consider melatonin 0.5 to 3 mg, cognitive behavioral therapy for insomnia (CBT-I), or a short discussion with a physician about prescription alternatives. The American Academy of Sleep Medicine (AASM) clinical practice guideline recommends CBT-I as first-line therapy for chronic insomnia over any pharmacologic agent, including diphenhydramine.

For cold symptoms: Guaifenesin (expectorant) and saline nasal spray are non-interacting options for congestion and cough. Pseudoephedrine should also be avoided in lisinopril patients because it raises blood pressure through alpha-1 agonism, an even more direct antagonism than diphenhydramine provides.

Special Populations

Adults Over 65

This group faces compounded risk. Age-related reductions in renal clearance extend lisinopril exposure, and parallel reductions in hepatic CYP2D6 activity slow diphenhydramine elimination. The net result is higher plasma concentrations of both drugs for a longer duration. The STOPP/START criteria (version 3, 2023) explicitly recommend stopping first-generation antihistamines in older adults with hypertension because of orthostatic hypotension compounded by sedation.

Patients With Heart Failure

Lisinopril is used in heart failure at doses up to 40 mg daily. Patients with reduced ejection fraction are more sensitive to hemodynamic shifts. Diphenhydramine's anticholinergic effect can increase heart rate, worsening demand-supply mismatch. The ACC/AHA 2022 Heart Failure Guideline advises clinicians to review all OTC medications at every visit and specifically screen for anticholinergic agents.

Patients With Chronic Kidney Disease

Because lisinopril depends entirely on renal elimination, patients with an eGFR below 30 mL/min/1.73 m² already have prolonged drug exposure. Adding diphenhydramine in this population increases the window of overlapping pharmacodynamic effects. Dose reduction of diphenhydramine (12.5 mg rather than 25 mg) and extended dosing intervals (every 8 hours rather than every 4 to 6 hours) are reasonable if the drug cannot be avoided.

What the FDA Labels Say

The lisinopril prescribing information does not list diphenhydramine by name in its drug interactions section. It warns generally about agents that blunt the antihypertensive effect, including NSAIDs and sympathomimetics. The diphenhydramine OTC Drug Facts label warns against concurrent use with sedatives and other CNS depressants and advises users to "ask a doctor before use if you have high blood pressure." That directive is printed on every bottle of Benadryl, ZzzQuil, and store-brand diphenhydramine, though many consumers overlook it.

Counseling Points for Patients

Pharmacists and prescribers should address five topics when a lisinopril patient asks about diphenhydramine.

1. Disclose all OTC medications. Diphenhydramine appears in over 600 OTC products, including PM-labeled pain relievers (Tylenol PM, Advil PM), sleep aids (ZzzQuil, Unisom SleepGels), and combination cold medicines. Patients may not realize they are taking it.
2. Prefer second-generation antihistamines. Loratadine, cetirizine, and fexofenadine provide equivalent allergy relief without the cardiovascular or cognitive side effects.
3. Limit duration. If diphenhydramine is used, keep the course as short as possible. One to three days for acute symptoms is reasonable.
4. Monitor blood pressure. Use a validated home cuff (upper arm, not wrist) and record readings for 3 to 5 days.
5. Avoid alcohol. The sedative interaction between diphenhydramine and alcohol is well documented. Adding lisinopril-related dizziness to that mix significantly increases fall risk.

The 2023 Beers Criteria panel states: "First-generation antihistamines are highly anticholinergic; clearance reduced with advanced age, and tolerance develops when used as hypnotic; risk of confusion, dry mouth, constipation, and other anticholinergic effects or toxicity" [10]. That language applies to every patient on a concurrent antihypertensive, not just those taking lisinopril specifically.

Patients on lisinopril 10 to 40 mg daily who need a one-time dose of diphenhydramine 25 mg for an acute allergic reaction may take it with home blood pressure monitoring and a 3-day time limit, but should switch to cetirizine or loratadine for any ongoing need.