Diltiazem XR Drug Interactions: What to Avoid and What to Tell Your Doctor

Diltiazem XR has a long list of clinically significant drug interactions. Because it affects both the heart's electrical system and a major liver enzyme system (CYP3A4), it can raise or lower the blood levels of many other common medications — and vice versa. Before starting Diltiazem XR, your doctor and pharmacist need to know about every medication you take, including over-the-counter drugs and supplements.

Contraindicated Combinations: Never Take Together

These combinations are considered absolutely contraindicated — meaning the risks are severe enough that they should not be used together under any circumstances:

Dantrolene (muscle relaxant): Combining IV dantrolene with calcium channel blockers has caused cardiovascular collapse and fatal hyperkalemia. The combination is contraindicated even for the oral form as a precaution.

Ivabradine (Corlanor — used for heart failure/angina): Diltiazem significantly inhibits the metabolism of ivabradine, dramatically increasing ivabradine blood levels. The combination causes severe bradycardia (dangerously slow heart rate).

Flibanserin (Addyi — used for hypoactive sexual desire disorder): Diltiazem inhibits CYP3A4, which metabolizes flibanserin. The combination causes severe hypotension and syncope. Avoid diltiazem within 2 days of flibanserin use.

Major Interactions: Use With Extreme Caution

These combinations require very close monitoring or dose adjustments:

Beta-blockers (metoprolol, atenolol, carvedilol, etc.): Both diltiazem and beta-blockers slow the heart rate and AV conduction. The combination can cause severe bradycardia, heart block, or heart failure. If prescribed together, patients require close monitoring of heart rate and ECG.

Digoxin: Diltiazem increases digoxin blood levels by 20–30% through inhibition of P-glycoprotein. This raises the risk of digoxin toxicity. Digoxin levels should be checked after starting diltiazem, and the digoxin dose may need to be reduced.

Amiodarone: Both drugs slow AV conduction. Combining them significantly increases the risk of bradycardia, sinus arrest, or complete heart block.

Cyclosporine and tacrolimus (immunosuppressants): Diltiazem inhibits CYP3A4, which metabolizes cyclosporine and tacrolimus. This significantly raises their blood levels, increasing the risk of nephrotoxicity and other toxicities. Dose reductions of cyclosporine or tacrolimus are usually required.

Statins (especially simvastatin, lovastatin): Diltiazem inhibits CYP3A4, increasing statin blood levels. This raises the risk of statin-related myopathy and rhabdomyolysis. Simvastatin doses should be limited to 10 mg/day with diltiazem. Pravastatin or rosuvastatin (less CYP3A4-dependent) are preferred alternatives.

Ranolazine (Ranexa — for angina): Diltiazem inhibits CYP3A4 metabolism of ranolazine, raising ranolazine levels. Ranolazine doses should be limited to 500 mg twice daily when combined with diltiazem.

Moderate Interactions: Tell Your Doctor

These interactions may require dose adjustments or added monitoring:

Carbamazepine, phenytoin, phenobarbital, rifampin: These are CYP3A4 inducers that speed up diltiazem metabolism, reducing its blood levels and potentially making it less effective.

Azole antifungals (fluconazole, itraconazole, ketoconazole): Strong CYP3A4 inhibitors that increase diltiazem blood levels, potentially causing excessive blood pressure lowering, bradycardia, or edema.

Macrolide antibiotics (erythromycin, clarithromycin): CYP3A4 inhibitors that can increase diltiazem exposure; monitor for excessive blood pressure lowering.

General anesthetics: Additive cardiac depression. Tell your anesthesiologist and surgeon if you take Diltiazem XR before any procedure.

Foods to Avoid: Grapefruit and Grapefruit Juice

Grapefruit and grapefruit juice contain compounds called furanocoumarins that inhibit CYP3A4 enzymes in the gut wall and liver. This can significantly increase diltiazem absorption and blood levels, potentially causing excessive blood pressure lowering, bradycardia, or other side effects.

Avoid grapefruit, grapefruit juice, and Seville oranges (used in marmalades) while taking Diltiazem XR.

Supplements That May Interact With Diltiazem XR

St. John's Wort: A powerful CYP3A4 inducer that can significantly reduce diltiazem blood levels and effectiveness. Do not take together.

Magnesium supplements: High doses may enhance blood pressure lowering effects; discuss with your doctor.

What to Tell Your Doctor and Pharmacist

Before starting Diltiazem XR, give your prescriber and pharmacist a complete list of:

All prescription medications (including those from other specialists)

All over-the-counter medications (including NSAIDs like ibuprofen and naproxen, which can reduce blood pressure lowering efficacy)

All vitamins and supplements, including herbal products

Any planned surgeries or dental procedures (anesthesiologists need to know)

For more on Diltiazem XR side effects, see Diltiazem XR Side Effects: What to Expect and When to Call Your Doctor.

Need help finding your Diltiazem XR prescription? Use medfinder to locate pharmacies near you with your specific formulation in stock.