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Diltiazem

Prescription ·Strong evidence ·Reviewed May 2026

Diltiazem is a benzothiazepine-type non-dihydropyridine calcium channel blocker that affects both vascular smooth muscle and cardiac conduction tissue. It is used for hypertension, angina, and rate control in atrial fibrillation/flutter. Its dual action on vessels and the AV node makes it versatile.

Evidence tier
Strong
Indexed sources
10 2018–2024
Interactions
8
Category
Prescription
Read the sources → Compare profiles
What it's good for
  • Effective blood pressure reduction2
  • Rate control in atrial fibrillation/flutter1,2
  • Angina relief including vasospastic (Prinzmetal) angina
  • Coronary vasodilation5
What to watch for
  • Bradycardia
  • Peripheral edema
  • Dizziness
  • Severe sinus bradycardia or sick sinus syndrome without pacemaker
  • Second- or third-degree AV block without pacemaker6

The bottom line

Evidence rating strong. Most-documented uses: effective blood pressure reduction, rate control in atrial fibrillation/flutter, angina relief including vasospastic (prinzmetal) angina. 10 sources indexed (2018–2024), with 8 interaction records on file.

The science

How it works, mechanistically.

Core mechanism

Inhibits L-type calcium channels in both vascular smooth muscle and cardiac tissue. Causes vasodilation (reducing blood pressure), slows conduction through the AV node (rate control), and reduces myocardial contractility and oxygen demand. Also has coronary vasodilatory properties, making it effective for vasospastic angina.6,7

Class
Non-Dihydropyridine Calcium Channel Blocker
Absorption
Fat-soluble; take with food
Dosing

Dosing & protocol.

Common range
120–360 mg daily (extended-release once daily; immediate-release 3–4 times daily) (as prescribed by your physician)
Recommended form
Extended-release capsule or tablet preferred for hypertension; immediate-release or IV for acute rate control

Extended-release formulations can be taken with or without food; immediate-release absorption is not significantly affected by food

Safety

Full safety detail.

Side effects

  • Bradycardia
  • Peripheral edema
  • Dizziness
  • Headache
  • Constipation
  • Flushing
  • AV block (dose-dependent)

Contraindications

  • Severe sinus bradycardia or sick sinus syndrome without pacemaker
  • Second- or third-degree AV block without pacemaker6
  • Severe hypotension
  • Concurrent IV beta-blocker administration6
  • Acute myocardial infarction with pulmonary congestion1
  • Accessory bypass tract (WPW syndrome) with atrial fibrillation5
Interactions

Interaction records.

SeriousConflict

St. John's Wort

St. John's Wort induces CYP3A4, reducing diltiazem plasma levels significantly. This can lead to uncontrolled hypertension or angina.

Recommendation: Avoid combining St. John's Wort with diltiazem.

ModerateCaution

Berberine

Diltiazem is itself a moderate CYP3A4 inhibitor that depends on the same enzyme for clearance. Adding berberine, which inhibits CYP3A4, CYP2D6, and CYP2C9 in humans, can raise diltiazem levels and amplify its negative chronotropic and vasodilatory effects. The result can be excessive bradycardia, hypotension, or AV block, particularly in older adults or those on other rate-slowing drugs.

Recommendation: Avoid combining berberine with diltiazem unless your prescriber agrees. If used together, check your pulse and blood pressure twice daily for 2 weeks after starting berberine and report a resting heart rate below 50 bpm, lightheadedness, or readings below your usual range.

ModerateCaution

Resveratrol

Resveratrol is a mechanism-based inhibitor of CYP3A4 and a P-glycoprotein blocker. In rats, oral resveratrol increased diltiazem AUC and Cmax 1.5-fold by inhibiting both intestinal CYP3A and efflux transport. The extrapolated human effect would amplify diltiazem's bradycardia and hypotension, particularly when patients use high-dose resveratrol supplements (250-1000 mg/day).

Recommendation: Avoid high-dose resveratrol supplements while on diltiazem. If you do use resveratrol, separate doses by at least 4 hours, check pulse and blood pressure for 2 weeks, and reduce or stop the supplement if your HR drops below 50 bpm or you become lightheaded.

ModerateCaution

Quercetin

Quercetin pretreatment in rabbits roughly doubled the oral bioavailability of diltiazem by inhibiting CYP3A4 and intestinal P-glycoprotein. While human data are limited, large-dose quercetin supplements (500-1000 mg/day) are plausibly able to raise diltiazem levels and accentuate bradycardia, hypotension, or AV-nodal blockade.

Recommendation: Avoid high-dose quercetin supplements while on diltiazem. If you use quercetin, keep doses modest (under 500 mg/day), separate from diltiazem by at least 4 hours, and monitor pulse and blood pressure for 2 weeks.

ModerateCaution

L-Theanine

L-Theanine modestly lowers stress-induced blood pressure increases and has mild relaxing effects on vascular tone. Layered on top of diltiazem's bradycardia, vasodilation, and negative inotropy, the combination can produce additive drops in heart rate or blood pressure, particularly in patients who are already well-controlled or volume-depleted.

Recommendation: If you take diltiazem, L-theanine doses up to 200-350 mg/day supplemental elemental magnesium are usually well-tolerated, but check your blood pressure and pulse when you start. Reduce or stop the supplement if you develop dizziness, fatigue, or a resting heart rate below 50 bpm.

ModerateCaution

Taurine

Taurine produces small but consistent reductions in blood pressure (about 3-4 mmHg systolic in meta-analysis) and slows heart rate through autonomic modulation. Added to diltiazem's negative chronotropic and vasodilatory effects, the combination can produce additive bradycardia or hypotension, particularly in older patients or those on additional rate-slowing medications.

Recommendation: If you take diltiazem, taurine doses up to 1.5-3 g/day are usually tolerable, but check blood pressure and pulse when you start. Hold or reduce taurine if your HR drops below 50 bpm or you develop new dizziness or fatigue.

ModerateCaution

Magnesium Citrate

Magnesium is a physiologic calcium antagonist that lowers blood pressure by about 2/1.8 mmHg at supplemental doses and also blunts sinoatrial and AV-nodal conduction. Combined with diltiazem's rate-slowing and vasodilatory effects, the result can be additive bradycardia, hypotension, or AV-nodal slowing, particularly with high magnesium doses or IV use.

Recommendation: If you take diltiazem, keep oral magnesium citrate to 200-350 mg/day supplemental elemental magnesium and monitor blood pressure and pulse after starting. Reduce or stop magnesium if your HR drops below 50 bpm, you develop dizziness, or your BP falls below your usual range. Avoid IV magnesium without specialist supervision.

ModerateCaution

Garlic Extract

Aged garlic extract reduces systolic and diastolic blood pressure by about 8 and 5 mmHg in hypertensive patients. Combined with diltiazem's vasodilatory and rate-slowing effects, the additive antihypertensive response can be welcome but also risks symptomatic hypotension or dizziness, especially in older patients or those at goal blood pressure.

Recommendation: If you take diltiazem, monitor blood pressure and pulse for 2-4 weeks after starting garlic extract. Reduce or stop garlic if readings drop below your usual range, you become dizzy, or your heart rate falls below 50 bpm.

Search all 8 interaction records for Diltiazem →

Sources

Sources, by evidence tier.

Numbered references. Citations throughout the page link here.

Meta-analyses & systematic reviews

5

Reference material

5
Keep exploring

Deep dives & adjacent profiles.

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This page is educational. Do not start, stop, or change a supplement or medication based on it without checking with a qualified healthcare professional.

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