Diltiazem and Verapamil: Independent Evidence, Risks and Interactions

Table of contents

• Evidence summary
• What diltiazem and verapamil are
• How they work
• What they are prescribed for
• What works and what does not
• Benefits by claim
• Risks and all side effects
• All interactions
• Who should avoid them
• Dosage and how to take it
• Follow the money: who makes them and who funded the evidence
• Related research
• Frequently asked questions
• Sources and funding notes

Evidence summary

Independent evidence and credibility scorecard

Independence tiers: 1 = regulator or government body; 2 = academic or professional-society source with no declared commercial funding for the work; 3 = independent authors with minor or unverified industry ties, or a collaborating manufacturer; 4 = manufacturer-sponsored. Credibility runs from A (highest) to D.

The independent sources cluster heavily in the US: nine of the twelve are American, including every guideline on the cardiology questions and both regulator labels. The verapamil outcome trials are from 2003. The cluster headache sources are European. We found no independent head-to-head trial of diltiazem against a beta-blocker for hypertension or AF that we could open.

What diltiazem and verapamil are

Both are calcium-channel blockers. Doctors split the class in two. The "dihydropyridines" (amlodipine, nifedipine and similar) mainly relax blood vessels. The "non-dihydropyridines" (verapamil and diltiazem) act on the heart as well. They slow the rate, and they slow electrical conduction through the atrioventricular (AV) node. The 2022 US heart failure guideline classes verapamil and diltiazem as harmful in heart failure with reduced ejection fraction (Pharmacy Times, summarising the 2022 AHA/ACC/HFSA guideline).

Forms. Verapamil is sold as immediate-release tablets taken three times a day, extended-release tablets and capsules, and an intravenous solution used in hospitals (DailyMed listing). Diltiazem comes as immediate-release tablets and once-daily extended-release capsules and tablets (Drugs@FDA via openFDA).

Legal status. Both are prescription-only medicines in the United States. We checked the US regulator only; UK and EU licensing was not verified in this review.

How they work

Proven in humans. Diltiazem lengthens the refractory period of the AV node, which is why it can rarely cause very slow heart rates or second- or third-degree AV block (13 of 3,290 patients, 0.40%) (FDA diltiazem label). Verapamil has a negative inotropic effect, meaning it weakens the force of the heartbeat, and the label warns it can worsen heart failure (FDA verapamil label). Both effects explain the uses (slower rate in AF, less workload in angina) and the dangers (heart failure, heart block).

Unknown. Why verapamil prevents cluster headache is not known. A 2019 review says its "mode of action and therapeutic target remain unknown". Possible explanations, such as effects on the body clock or on the pain-signalling molecule CGRP, come from human and animal data and remain unproven (Petersen 2019).

What they are prescribed for

• Verapamil (US label): vasospastic (Prinzmetal) angina, unstable angina at rest, chronic stable angina; rate control with digitalis in chronic atrial flutter or fibrillation; prevention of recurrent paroxysmal SVT; and essential hypertension (FDA verapamil label).
• Diltiazem extended-release capsules (US label): hypertension, chronic stable angina and angina due to coronary spasm (FDA diltiazem label). The oral label we opened does not list AF, so oral diltiazem for rate control is guideline-based use rather than a listed indication in that label.
• Verapamil for cluster headache is not a labelled use in the labels we opened. It is recommended by the European guideline (May et al. 2023).

What works and what does not

Benefits by claim

High blood pressure

INVEST is the best verapamil trial. It randomised 22,576 people aged 50 or older with hypertension and coronary disease at 862 sites in 14 countries to a verapamil sustained-release strategy or an atenolol strategy. Trandolapril or hydrochlorothiazide could be added in each arm. After a mean 2.7 years, the combined outcome of death, non-fatal heart attack or non-fatal stroke occurred in 9.93% vs 10.17% (RR 0.98, 95% CI 0.90 to 1.06). In plain English, the two plans were the same within the margin of chance (Pepine 2003). Death was 7.75% vs 7.90% and stroke 1.16% vs 1.31%. New-onset diabetes was lower with verapamil (7.03% vs 8.23%), but that was exploratory (ACC INVEST summary).

CONVINCE tested a controlled-onset verapamil capsule against atenolol or hydrochlorothiazide in 16,602 people at risk. Blood pressure fell by almost the same amount (13.6/7.8 vs 13.5/7.1 mm Hg). Events were 364 vs 365 (HR 1.02, 95% CI 0.88 to 1.18). The trial could not prove equivalence, and non-stroke bleeding was more common on verapamil (HR 1.54, 1.16 to 2.04) (Black 2003). Heart failure hospitalisation was higher with verapamil, HR 1.30 (1.00 to 1.69), according to the ACC trial summary (ACC).

A correction on ALLHAT. ALLHAT is often grouped with these trials, but its three arms were chlorthalidone, amlodipine and lisinopril. Diltiazem and verapamil were not tested. It found amlodipine matched chlorthalidone on the main outcome (11.3% vs 11.5% at six years) but caused more heart failure (10.2% vs 7.7%, RR 1.38), and its authors preferred thiazide-type diuretics first (ALLHAT 2002). Those findings should not be assumed to apply to diltiazem or verapamil.

We did not find, among sources we could open, a modern guideline statement on where these two drugs rank among blood-pressure options (the NICE page was blocked to our tools). We can say that the independent trials show a verapamil plan to be a reasonable alternative, not a first choice proven superior.

Atrial fibrillation rate control

The verapamil label lists rate control with digitalis in chronic AF or flutter, with a usual label dose of 240 to 320 mg a day in divided doses for digitalised AF (FDA verapamil label). The 2023 ACC/AHA guideline, according to an emergency-medicine commentary, does not prefer beta-blockers over non-dihydropyridine calcium-channel blockers for most people with AF and a fast heart rate, because few studies compare them. It gives a Class 3: Harm recommendation against intravenous versions in moderate or severe left ventricular systolic dysfunction (ACEP Now). The guideline itself is a published Circulation document (Joglar et al. 2023); we could not open its full text and did not verify exact ejection-fraction thresholds. The commentary says that recommendation rests on low-quality evidence and consensus.

Angina

Both drugs carry angina indications on their US labels. The verapamil label lists vasospastic, unstable-at-rest and chronic stable angina, with a usual label dose of 80 to 120 mg three times daily (FDA verapamil label). The diltiazem label lists chronic stable angina and coronary spasm (FDA diltiazem label). We did not open a randomised angina trial, so we rate independent outcome evidence here as insufficient, not as negative.

Cluster headache prevention

In the best-known trial, 30 people with episodic cluster headache took verapamil 120 mg three times daily or placebo for 14 days; the verapamil group had significantly fewer attacks and used less rescue medicine, with mild side effects (Leone 2000). A 2019 review counted only two randomised and three open-label studies, 183 people in all. In episodic cluster headache the attack burden fell by about one attack a day on average, and about half of chronic patients benefited. The authors still call it the drug of choice (Petersen 2019). The European guideline recommends a daily dose of at least 240 mg (May 2023). Because doses studied in cluster headache are high, heart monitoring matters; we could not verify the exact monitoring advice in the guideline, but the label's conduction warnings apply.

Risks and all side effects

Other verapamil effects listed: gingival hyperplasia (1% or fewer, causal link uncertain); it may reduce neuromuscular transmission in Duchenne muscular dystrophy and worsen myasthenia gravis; in hypertrophic cardiomyopathy, 120 patients on up to 720 mg a day included 3 deaths from pulmonary oedema (FDA verapamil label).

Diltiazem effects listed: dose-related dizziness, sinus bradycardia and first-degree AV block; constipation in under 1%; rash and skin reactions; and, after approval, angioedema (including facial or periorbital swelling), asystole and photosensitivity (FDA diltiazem label). We do not quote diltiazem percentages because the label we opened left out its incidence table.

Overdose: bradycardia, hypotension, heart block and heart failure. Neither drug is removed by dialysis (diltiazem label; verapamil label).

All interactions

Sources for the table: the verapamil and diltiazem labels as served by openFDA, which warns its data are unvalidated. Combining verapamil and diltiazem with each other is not covered in the labels we opened.

Who should avoid them

• Heart failure with reduced ejection fraction or severe left ventricular dysfunction. Verapamil label contraindication; guidelines class both as harmful in this group (Pharmacy Times). The diltiazem label contraindicates acute heart attack with pulmonary congestion.
• Sick sinus syndrome or second- or third-degree AV block without a working pacemaker (both labels).
• Systolic blood pressure under 90 mm Hg or cardiogenic shock (both labels).
• AF or flutter with an accessory bypass tract (verapamil label).
• Pregnancy. Verapamil is Pregnancy Category C: no teratogenicity in animals but no adequate human trials, and it crosses the placenta. The diltiazem label says to use it only if the benefit justifies the risk, citing fetal lethality in animals at 5 to 10 times the human dose.
• Breastfeeding. Both drugs pass into breast milk; the verapamil label advises stopping nursing while taking it, and the diltiazem label advises an alternative feeding method if the drug is essential.
• Children. The verapamil label says safety and effectiveness are not established.
• Liver disease. In severe liver dysfunction verapamil's half-life extends to about 14 to 16 hours, and the label suggests roughly 30% of the normal dose. Diltiazem bioavailability rose about 69% in cirrhosis.
• Hypertrophic cardiomyopathy, Duchenne muscular dystrophy, myasthenia gravis (verapamil precautions).
• Older adults. The verapamil label notes 40 mg three times daily may suit the elderly or those with reduced liver function. Diltiazem trials did not include enough people aged 65 and over to say whether they respond differently.

Sources: FDA verapamil label; FDA diltiazem label.

Dosage and how to take it

These are label and trial doses, not advice. A prescriber individualises the dose.

• Verapamil immediate-release (label): dose is titrated; do not exceed 480 mg a day. Angina: usual 80 to 120 mg three times daily. Digitalised AF: 240 to 320 mg a day divided. SVT prevention: 240 to 480 mg a day divided. Hypertension: usual initial 240 mg a day (80 mg three times daily), with no evidence that doses above 360 mg a day add effect (FDA label).
• Diltiazem extended-release capsule (label): hypertension starts at 180 to 240 mg once daily, usual range 240 to 360 mg, up to 480 mg in some. Angina starts at 120 or 180 mg once daily, up to 480 mg. Maximum blood-pressure effect is usually seen by 14 days. Avoid alcohol, which speeds release (FDA label).
• Cluster headache doses studied: 120 mg three times daily for 14 days (Leone); 360 mg a day in both randomised studies reviewed (Petersen); the guideline gives at least 240 mg a day (May).
• Product forms are not interchangeable. Extended-release tablets and capsules differ by brand; the diltiazem label advises switching to the nearest equivalent total daily dose when changing products.

Follow the money: who makes them and who funded the evidence

Who makes and sells them

• Diltiazem brands: Cardizem tablets (NDA 018602, approved 5 November 1982 as a new molecular entity), Cardizem CD (1991) and Cardizem LA (2003) are listed under sponsor Bausch (Drugs@FDA via openFDA). Generic extended-release diltiazem is sold by firms including Ingenus, named as the adverse-reaction contact on the label we opened, and repackaged by firms such as Bryant Ranch Prepack (FDA diltiazem label).
• Verapamil brands: Verelan (NDA 019614, 1990) and Verelan PM (1998) are listed under sponsor Azurity, and intravenous verapamil under Exela (Drugs@FDA via openFDA). DailyMed lists 136 verapamil label records, including generic tablets from Zydus and extended-release products from Cadila and Nivagen (DailyMed). We could not confirm the current holder of the Calan brand, so we do not name one.
• Incentives (inference): both drugs are decades old and heavily generic, so few firms gain from funding new trials. That may explain why large verapamil outcome trials date from 2003, and why we found no head-to-head trial of diltiazem against newer options that we could open.

Who funded the evidence

• INVEST: the University of Florida is the lead sponsor and Abbott is a registered industry collaborator; the registry names the verapamil products as Isoptin SR or verapamil SR (ClinicalTrials.gov). PubMed lists NHLBI grants N01-HV-68163 and U01HL64924-02 (PubMed). The extent of Abbott's support is not given in anything we opened. We did not see the JAMA funding statement, because the JAMA page was not accessible to our tools.
• CONVINCE: the sponsor ended it "due to commercial considerations", so investigators saw under one third of the planned events (ACC; AAFP). The sponsor is not named in those pages.
• ALLHAT: NHLBI, NIH (grant N01-HC-35130) (PubMed).
• Cluster headache: no funding statement in the Leone record; a TrygFonden grant appears on the Petersen review (PubMed).
• Guidelines: the AF guideline record lists NIH and VA grants (PubMed); the heart failure and cluster headache guideline records list no funding information.

Documented integrity events

The one documented event we can source is the early closure of CONVINCE by its sponsor for commercial reasons, which left the question of equivalence unresolved. We found no regulatory ban, warning letter or recall for these two drugs in the sources we opened, but we did not search recall databases.

Related research

For wider, evidence-graded context, see these Pure City Research guides:

• Hypertension
• Blood pressure medicines
• Amlodipine (the dihydropyridine comparator)
• Atrial fibrillation
• Stable angina
• Vasospastic angina
• Heart failure with reduced ejection fraction
• Heart block
• Bradycardia
• Bisoprolol (a beta-blocker)
• Simvastatin
• Statins and other cholesterol-lowering treatments
• Chronic constipation
• Buspirone (interacts with diltiazem)

Frequently asked questions

Sources and funding notes

What we could not verify. NICE hypertension, AF and angina guidance returned access blocks, so we did not use them. We did not open the JAMA full text for INVEST or CONVINCE, so author conflicts of interest are unchecked. The CONVINCE sponsor is not named in the pages we opened. We read the 2023 AF guideline and 2022 heart failure guideline through a PubMed record and secondary summaries, not the full text. The openFDA labels we used are repackager or generic labels, and openFDA warns its data are unvalidated. The diltiazem label omitted its adverse-reaction table. We did not find or open a randomised angina trial or a modern diltiazem outcome trial, and we did not search recall databases. We could not confirm the current Calan brand holder or UK and EU licensing. Drug-company donations of study drugs in ALLHAT were not confirmed.

1. FDA verapamil hydrochloride tablet label (openFDA; repackaged generic, 2024 version) — regulator text; US government.
2. FDA diltiazem extended-release capsule label (openFDA, effective August 2025) — regulator text; US government.
3. FDA diltiazem label, nursing, hepatic, geriatric and overdose sections (openFDA) — regulator text; US government.
4. FDA simvastatin label (openFDA) — regulator text; US government.
5. openFDA Drugs@FDA, Cardizem applications — US government database; sponsor Bausch.
6. openFDA Drugs@FDA, verapamil NDAs — US government database; sponsors Azurity and Exela.
7. DailyMed verapamil label listing — US National Library of Medicine; 136 records.
8. Pepine et al., INVEST, JAMA 2003 — randomised trial; NHLBI grants listed; Abbott collaborator.
9. ClinicalTrials.gov record NCT00133692 — registry; lead sponsor University of Florida, collaborator Abbott.
10. ACC INVEST trial summary — American College of Cardiology; secondary summary.
11. Black et al., CONVINCE, JAMA 2003 — randomised trial; industry sponsor (unnamed in record) closed it early.
12. ACC CONVINCE trial summary — secondary summary.
13. American Family Physician summary of CONVINCE — secondary summary; American Academy of Family Physicians.
14. ALLHAT Collaborative Research Group, JAMA 2002 — randomised trial; NHLBI grant N01-HC-35130.
15. Goldstein et al., Circulation 1991 — post-hoc analysis of a randomised trial; no funding in record.
16. ACC MDPIT trial summary — secondary summary; sponsor not stated.
17. Joglar et al., 2023 ACC/AHA/ACCP/HRS AF guideline — society guideline; NIH and VA grants listed.
18. ACEP Now commentary on the 2023 AF guideline — American College of Emergency Physicians; secondary summary.
19. Heidenreich et al., 2022 AHA/ACC/HFSA heart failure guideline — society guideline; no funding in record.
20. Pharmacy Times clinical overview — trade publication; secondary source for the harm classification.
21. Leone et al., Neurology 2000 — randomised trial; no funding in record.
22. Petersen et al., Headache 2019 — narrative review; TrygFonden grant listed.
23. May et al., EAN cluster headache guideline, Eur J Neurol 2023 — society guideline; no funding in record.

Educational research reviewed 6 October 2026. This article is not medical advice; talk to a qualified clinician or pharmacist before starting, stopping or combining treatments.