The short answer
A specific harmful interaction between grapefruit flesh and diltiazem is uncertain. Juice studies found a small exposure increase in one study and no increase in another; current NHS guidance allows normal food and drink.
What this answer covers
Oral diltiazem with grapefruit flesh. Inspected clinical studies used juice and do not measure exact flesh effects or topical diltiazem interactions.
- Supplement or preparation
- Grapefruit flesh
- Medication ingredient
- diltiazem
A small exposure increase was found in one study
Ten healthy men took immediate-release diltiazem with 250 mL grapefruit juice or water. Average drug exposure increased about 20%, with substantial individual variation. Peak changes were not significant, and BP and heart rate did not differ. This did not demonstrate toxicity or test grapefruit flesh.
Another study found no exposure change
Nine healthy men received diltiazem with repeated juice servings. Total exposure and peak concentration did not change, although half-life increased modestly. The different findings mean an interaction magnitude should not be assumed from another calcium-channel blocker or juice regimen.
Keep dietary advice specific
NHS guidance says people taking diltiazem can eat and drink normally. If you consume large amounts, use concentrated grapefruit products or have symptoms, ask about your exact oral formulation and other medicines. The available studies do not establish a mandatory food ban or protective spacing interval for flesh.
Report symptoms without self-adjusting
Diltiazem can cause dizziness and slow the heart rate independently of grapefruit. Report troublesome dizziness, faintness or unusual pulse changes and avoid driving while dizzy. Keep the prescribed regimen unless your clinician advises otherwise.
Evidence and practical considerations
Side effects & toxicity
Conflicting small juice studies do not establish the current nonspecific grapefruit-flesh safety claim.
- Exact scope
- Oral diltiazem with grapefruit flesh. Inspected clinical studies used juice and do not measure exact flesh effects or topical diltiazem interactions. with Current RxNorm confirms diltiazem RxCUI 3443 IN. Oral hydrochloride once-daily capsules; immediate-release single-dose originals explicitly distinguished. No transfer to topical fissure treatment or IV administration.
- Medication form and route
- Oral
What remains uncertain
- Juice evidence is conflicting and formulation-limited; no direct flesh toxicity study, no quantified event risk or spacing rule.
Factors that may matter: Exact preparation and dose; Oral formulation; Blood pressure and pulse; Other medicines.
Read the supporting source summaries
Source 1: Current prescribing information
dailymed.nlm.nih.gov · Source check Sep 10, 2026
Population: Patients prescribed diltiazem hydrochloride extended-release capsules.
Avoid known CYP3A4 inducers: rifampin lowered concentrations to undetectable. Bradycardia, conduction and hypotension warnings apply.
How this applies: Exact ingredient through the stated labeled salt and oral formulation. Current RxNorm confirms diltiazem RxCUI 3443 IN. Oral hydrochloride once-daily capsules; immediate-release single-dose originals explicitly distinguished. No transfer to topical fissure treatment or IV administration.
Other-drug inducer magnitudes not transferred. Guggul eight-hour sampling is not spacing; small conflicting grapefruit juice studies retained.
Source 2: Original clinical research
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Ten healthy men in a randomized crossover.
Mean individual AUC increased 20%, with considerable variability; peak increase was not significant and BP/heart rate did not differ.
How this applies: Exact preparation and endpoint boundaries retained; this is not a measured diltiazem combination unless explicitly stated.
Abstract-only; immediate-release single dose and juice, not flesh, chronic ER therapy or clinical toxicity.
Source 3: Original clinical research
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Nine healthy men in a randomized crossover.
No AUC or peak change; half-life increased modestly.
How this applies: Exact preparation and endpoint boundaries retained; this is not a measured diltiazem combination unless explicitly stated.
Abstract-only; different regimen, not evidence that every preparation is harmless.
Source 4: National health-service guidance
nhs.uk · Source check Sep 10, 2026
Population: Diltiazem users; oral treatment and topical indications distinguished.
States patients can eat and drink normally, advises discussing cessation and avoiding driving if dizzy.
How this applies: Current original guidance retained as a counterweight to blanket fruit avoidance.
General guidance does not erase the small juice PK signal or establish safety of concentrated botanical products.
Source 5: Medication terminology
rxnav.nlm.nih.gov · Source check Sep 10, 2026
Population: Terminology.
diltiazem is IN.
How this applies: Exact ingredient.
Not clinical evidence.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Article prepared | |
| Sources checked and article drafted. | |
| Research summary published |
Article revision: 42f6d938b46efcc6
This guide brings together the evidence records used by our interaction checker. Each finding retains its own preparation limits, research date, and source summaries.
Full citations require verification. Reading this answer and the source summaries does not.
Taking other supplements or medications?
Use the checker to explore your other combinations. Ask a pharmacist to review the exact products, amounts and medical conditions in your complete list.
Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.