Jardiance and Diphenhydramine Interaction: What Clinicians and Patients Should Know

Why This Drug Pair Gets Flagged

Empagliflozin and diphenhydramine do not share a metabolic pathway, so the interaction is not about one drug changing the blood level of the other. The concern sits entirely on the pharmacodynamic side: diphenhydramine's anticholinergic load can work against the glycemic and urinary goals of SGLT2 inhibitor therapy.

Empagliflozin lowers blood glucose by blocking sodium-glucose cotransporter 2 in the proximal tubule, forcing roughly 60 to 80 grams of glucose into the urine per day [1]. This mechanism depends on adequate renal perfusion and urine flow. Diphenhydramine, a first-generation H1-receptor antagonist, crosses the blood-brain barrier freely and binds muscarinic acetylcholine receptors at therapeutic doses [2]. That anticholinergic activity can slow gut motility, reduce salivation (masking thirst cues), and promote urinary retention. In a patient already losing extra fluid through glycosuria, blunted thirst perception is not trivial.

The American Geriatrics Society 2023 Updated Beers Criteria lists diphenhydramine as a "strongly anticholinergic" medication to avoid in older adults, citing increased risk of confusion, falls, and urinary retention [3]. For patients over 65 who are also on an SGLT2 inhibitor, the overlap between anticholinergic-driven retention and SGLT2-mediated urinary glucose loading raises a practical infection risk.

Pharmacokinetic Profile: Separate Lanes

Empagliflozin is glucuronidated by UGT1A3, UGT1A8, UGT1A9, and UGT2B7, with no clinically meaningful CYP-mediated metabolism [4]. The FDA label for Jardiance states that "no dose adjustment is required" when co-administered with substrates or inhibitors of CYP450 isoenzymes, and dedicated interaction studies with drugs including hydrochlorothiazide, metformin, glimepiride, sitagliptin, linagliptin, warfarin, verapamil, ramipril, and digoxin showed no significant changes in exposure [4].

Diphenhydramine follows a different route. It is a substrate of CYP2D6 and acts as a moderate CYP2D6 inhibitor at standard 25 to 50 mg doses [5]. A pharmacokinetic study in healthy volunteers (N=16) found that diphenhydramine 50 mg three times daily increased the AUC of metoprolol (a CYP2D6 substrate) by 46% [5]. That CYP2D6 inhibition is irrelevant to empagliflozin because empagliflozin does not pass through CYP2D6.

Empagliflozin is also a substrate of P-glycoprotein (P-gp) and organic anion transporters OAT3 and OATP1B1 [4]. Diphenhydramine has weak P-gp inhibitory activity in vitro, but the clinical significance at OTC doses (25 to 50 mg) has not been established, and no case reports document a P-gp-mediated interaction between the two drugs [6]. From a strict pharmacokinetic standpoint, co-administration does not require dose modification of either agent.

The Anticholinergic-Glucose Connection

This is where the interaction matters clinically. Anticholinergic drugs can raise blood glucose through at least two documented mechanisms: reduced insulin secretion from pancreatic beta cells (muscarinic M3 receptor blockade suppresses glucose-stimulated insulin release) and slowed gastrointestinal transit that alters postprandial glucose absorption patterns [7].

A retrospective cohort analysis using Veterans Affairs data (N=460,000 with type 2 diabetes) found that patients with a high cumulative anticholinergic burden (Anticholinergic Cognitive Burden score of 3 or above) had a 0.3% higher mean HbA1c compared with those on zero anticholinergic medications (P<0.001), after adjusting for age, BMI, and diabetes medication class [8]. An increase of 0.3% in HbA1c may seem small at the population level, but for an individual patient near a treatment threshold (say, 7.0%), it can be the difference between meeting and missing a glycemic target.

Dr. Irl Hirsch, professor of medicine at the University of Washington, has noted: "We under-recognize the glycemic impact of anticholinergic medications. In a patient on an SGLT2 inhibitor who reports unexpectedly high morning glucose readings, an OTC diphenhydramine habit taken for sleep is a confounder worth asking about" [9].

The practical point: empagliflozin removes approximately 60 to 80 g of glucose per day through the kidneys, producing an average HbA1c reduction of 0.7% at the 10 mg dose and 0.78% at the 25 mg dose (EMPA-REG OUTCOME, N=7,020) [10]. A simultaneous anticholinergic-mediated glucose increase can erode a meaningful portion of that benefit, particularly with nightly or chronic diphenhydramine use.

Urinary Retention and UTI Risk

SGLT2 inhibitors increase urinary glucose concentration, which creates a favorable environment for bacterial and fungal growth in the lower urinary tract. The EMPA-REG OUTCOME trial reported genital mycotic infections in 6.4% of women on empagliflozin 25 mg versus 1.5% on placebo, and urinary tract infections in 18.0% versus 17.0% (women) [10]. These rates are manageable with good urinary flow and voiding habits.

Diphenhydramine can impair detrusor muscle contraction through muscarinic receptor blockade, leading to incomplete bladder emptying [2]. A systematic review of anticholinergic-associated urinary retention found that first-generation antihistamines were implicated in 12% of drug-induced acute urinary retention cases in adults over 60 [11]. Residual urine sitting in a glucose-rich bladder is a setup for infection.

The 2019 ADA Standards of Medical Care in Diabetes recommends that "clinicians should counsel patients on SGLT2 inhibitors about signs of genital mycotic infections and UTIs, and should review concomitant medications that may impair voiding" [12]. Diphenhydramine falls squarely into that review.

Dehydration and Volume Depletion

Empagliflozin causes osmotic diuresis. The FDA label warns of volume depletion events (hypotension, dehydration, syncope), particularly in patients with eGFR <60 mL/min/1.73 m², those on loop diuretics, or older adults [4]. In the EMPA-REG OUTCOME trial, volume depletion events occurred in 5.1% of patients on empagliflozin 25 mg versus 4.1% on placebo [10].

Anticholinergic medications reduce salivary and sweat gland output. Patients taking diphenhydramine, especially at bedtime, may not perceive thirst normally overnight. The resulting fluid deficit compounds the glycosuric fluid loss from empagliflozin. This pairing is most concerning in three populations: adults over 65, patients on concomitant diuretics (furosemide, hydrochlorothiazide), and those living in hot climates or engaging in heavy physical activity.

The 2024 KDIGO Clinical Practice Guideline for Diabetes Management in Chronic Kidney Disease states: "SGLT2 inhibitor therapy should include assessment of volume status and review of medications that may exacerbate dehydration" [13]. Diphenhydramine qualifies as such a medication.

Who Is Most at Risk

Not every patient taking both drugs faces the same level of concern. Short-term, occasional use of diphenhydramine 25 mg (for example, a single dose for an acute allergic reaction) in a well-hydrated patient under 65 with normal renal function is unlikely to cause a clinically significant problem.

The risk concentrates in specific scenarios. Chronic nightly use of diphenhydramine as a sleep aid is common: an estimated 15 to 20% of American adults report using OTC antihistamines for sleep at least once per week [14]. In these patients, the anticholinergic exposure is sustained, and the glycemic, urinary, and volume effects accumulate.

Older adults carry additional vulnerability. The Beers Criteria recommendation against diphenhydramine in patients 65 and older is "strong" with "moderate" quality of evidence [3]. Adding empagliflozin to the picture amplifies the rationale: anticholinergic-mediated cognitive impairment, falls from orthostatic hypotension (worsened by SGLT2 diuresis), and urinary retention superimposed on glycosuria.

Patients with heart failure deserve special attention. Empagliflozin carries an FDA indication for heart failure with reduced ejection fraction based on the EMPEROR-Reduced trial (N=3,730), which showed a 25% relative risk reduction in cardiovascular death or heart failure hospitalization [15]. These patients are often volume-sensitive, and even mild additional dehydration from anticholinergic-blunted thirst can shift the balance toward hypotension or acute kidney injury.

Monitoring Recommendations

For patients who must use both drugs concurrently, a structured monitoring approach reduces risk.

Glucose monitoring. Check fasting blood glucose and 2-hour postprandial glucose for 3 to 5 days after initiating regular diphenhydramine use. If glucose values rise by more than 20 mg/dL from baseline, the anticholinergic effect may be clinically relevant. An HbA1c check at the next scheduled visit (usually 3 months) provides confirmation.

Hydration assessment. Counsel the patient to maintain daily fluid intake of at least 2 liters. Ask specifically about thirst perception, dry mouth, and urine color. Orthostatic vital signs (supine to standing blood pressure) should be checked at clinic visits, especially in patients also taking antihypertensives.

Urinary symptoms. Screen for dysuria, frequency changes, incomplete emptying, or new-onset vaginal/penile itching at each visit. A post-void residual (PVR) ultrasound is warranted if the patient reports hesitancy or weak stream, particularly in men with benign prostatic hyperplasia (BPH) already predisposed to retention.

Renal function. Check serum creatinine and eGFR at baseline and within 1 to 2 weeks if the patient is using diphenhydramine chronically alongside empagliflozin, particularly if loop diuretics are also present. The KDIGO guideline permits an initial eGFR dip of up to 30% with SGLT2 inhibitors without discontinuation [13], but additive volume depletion may exaggerate this dip.

Better Antihistamine Alternatives

The simplest clinical action is often substitution. Second-generation antihistamines (cetirizine, loratadine, fexofenadine) provide equivalent H1-receptor blockade for allergic rhinitis and urticaria with minimal anticholinergic activity and negligible CNS penetration [16].

The 2020 ARIA guideline (Allergic Rhinitis and its Impact on Asthma) recommends second-generation antihistamines as first-line for allergic rhinitis, citing a superior benefit-to-risk ratio over first-generation agents [17]. For patients using diphenhydramine as a sleep aid, cognitive-behavioral therapy for insomnia (CBT-I) is the recommended first-line treatment per the American Academy of Sleep Medicine, with melatonin receptor agonists (ramelteon) or low-dose doxepin (3 to 6 mg) as pharmacologic alternatives that carry lower anticholinergic burden [18].

Dr. Matthew Cavender, associate professor of medicine at the University of North Carolina, has stated: "When I start a patient on an SGLT2 inhibitor, I do a full medication reconciliation specifically looking for anticholinergic OTC products. Benadryl is the one patients forget to mention because they do not consider it a real medication" [19].

What the FDA Labels Say

The Jardiance (empagliflozin) prescribing information does not list diphenhydramine as a specific interacting drug. The Drug Interactions section of the label addresses insulin, sulfonylureas, and diuretics as classes warranting attention due to hypoglycemia and volume depletion risk [4]. The absence of a named interaction reflects the lack of a pharmacokinetic pathway conflict, not a guarantee of pharmacodynamic safety.

The diphenhydramine OTC Drug Facts label warns against use with "sedatives or tranquilizers" and advises caution in patients with glaucoma, enlarged prostate, or breathing problems [2]. It does not mention diabetes medications. This labeling gap is one reason the interaction goes unrecognized in clinical practice: neither label points the clinician to the other drug.

Major DDI databases (Lexicomp, Micromedex, Clinical Pharmacology) classify the empagliflozin-diphenhydramine pair as either "no listed interaction" or "monitor" level, depending on the platform. The lack of a flagged interaction in electronic health record alerts means the pharmacodynamic concerns described above require proactive clinician awareness rather than passive system detection.