Diltiazem

Why this drug is interesting

Diltiazem occupies an odd position in UK emergency medicine. It appears in the RCUK 2025 adult tachyarrhythmia algorithm as a rate-control option for atrial fibrillation.³ It is the default first-line agent for acute rate control across most of North America, and the emergency medicine literature you will read about AF with rapid ventricular response is full of it.

And yet there is no intravenous diltiazem preparation licensed in the UK. Searching the electronic Medicines Compendium returns oral tablets, modified-release capsules and a 2% topical preparation for anal fissure — no solution for injection. The drug that dominates the American literature on acute rate control is, in a British resus room, an oral tablet.

The UK formulation gap

This is worth stating carefully because it changes what a UK clinician can actually do.

• Available in the UK: immediate-release tablets (typically 60 mg), a wide range of modified-release capsules and tablets, and topical 2% for anal fissure
• Not available as a licensed UK product: any solution for injection or infusion
• Consequence: the intravenous bolus-then-infusion regimens described in North American emergency medicine are not directly transferable to UK practice

What this means for the algorithm

The RCUK 2025 algorithm lists beta-blocker, verapamil, diltiazem or digoxin for rate control in probable AF with an EF above 40%.³ The algorithm is agnostic about route. In practice, for a patient who needs rate control now, the UK choice narrows to an intravenous beta-blocker such as metoprolol or intravenous verapamil — with diltiazem entering as an oral agent for ongoing control.

Pharmacology

Mechanism

Diltiazem is a benzothiazepine calcium channel blocker. Like verapamil it blocks L-type calcium channels, slowing conduction and prolonging refractoriness in the sino-atrial and atrioventricular nodes, and producing coronary and peripheral vasodilatation.

Its distinguishing feature is where it sits on the spectrum between the two non-dihydropyridines: diltiazem is less negatively inotropic than verapamil and more vasoselective. That is the entire clinical basis for preferring it in a patient whose contractility you are worried about — though "less negatively inotropic" is not "safe in heart failure", which is why it still disappears from the algorithm below an EF of 40%.

Kinetics

Oral bioavailability

Around 40% — substantial first-pass metabolism

Half-life

Roughly 3–5 hours for immediate-release; modified-release preparations extend the dosing interval, not the drug's half-life

Metabolism

Hepatic, largely CYP3A4; diltiazem is itself a CYP3A4 inhibitor

Active metabolite

Desacetyldiltiazem, with about half the coronary vasodilator activity

Indications and dosing

UK licensed indications

Diltiazem preparations are licensed for angina — chronic stable and vasospastic — and, in the case of many modified-release preparations, for hypertension. Rate control in atrial fibrillation is a common and well-supported use but is generally off-label for UK oral preparations; check the SmPC for the product you are prescribing.

Rate control in the RCUK algorithm

• EF > 40% — rate control with a beta-blocker, verapamil, diltiazem or digoxin
• EF < 40% — beta-blocker or digoxin only
• Anticoagulate if the arrhythmia has lasted more than 24 hours

Practical notes

• Immediate-release diltiazem is short-acting and requires three-times-daily dosing — rarely a sensible long-term choice, but useful for initial titration
• The elderly and those with hepatic impairment need lower doses
• Bradycardia and heart block are dose-related; check the pulse and the ECG before increasing

Contraindications and adverse effects

Contraindications and cautions

• Severe bradycardia, sick sinus syndrome, second- or third-degree AV block without a pacemaker
• Left ventricular failure with pulmonary congestion; significantly impaired LV function
• Cardiogenic shock and severe hypotension
• Atrial fibrillation or flutter with an accessory pathway — the same hazard as every AV nodal blocker; see Verapamil and Adenosine
• Concurrent intravenous beta-blockade — additive AV nodal blockade and negative inotropy
• Pregnancy and breastfeeding — generally avoided
• Hepatic and renal impairment — reduce dose

Adverse effects

• Bradycardia and AV block
• Ankle oedema — common, dose-related, and frequently misattributed to heart failure
• Hypotension, flushing, headache, dizziness
• Constipation — less prominent than with verapamil
• Rash; rarely severe cutaneous reactions including erythema multiforme
• Gum hyperplasia and hepatic dysfunction with chronic use

Interactions

• Beta-blockers — additive bradycardia and negative inotropy; particular caution with intravenous combinations
• Statins metabolised by CYP3A4 — simvastatin and atorvastatin exposure rises; dose limitation applies
• Digoxin — concentrations may rise
• Ciclosporin, carbamazepine, theophylline — concentrations raised by CYP3A4 inhibition
• Ivabradine, dantrolene — avoid

Critical appraisal

1. The comparative evidence between intravenous diltiazem and intravenous beta-blockade for acute AF rate control genuinely favours diltiazem on speed and success of rate control in several systematic reviews — which is exactly the literature a UK clinician cannot act on, because the preparation does not exist here. That is a formulation problem, not a clinical disagreement.
2. Rate control versus rhythm control is the larger question. For a patient with recent-onset AF, whether rate control is even the right strategy depends on symptoms, duration, stroke risk and local pathways — see Flecainide for the cardioversion side of that decision.
3. The EF > 40% threshold is a pragmatic line, not a precise one. In practice the EF is frequently unknown in the ED, and the honest fallback is clinical assessment: a patient who looks like decompensated heart failure should not receive a non-dihydropyridine calcium channel blocker whatever the last echocardiogram said.
4. Ankle oedema is under-recognised as a drug effect and is a common reason patients on diltiazem are investigated, or diuresed, for heart failure they do not have.
5. Brand-specific MR prescribing is poorly observed in practice, particularly at admission and discharge, and is a realistic source of avoidable harm that has nothing to do with emergency prescribing decisions.

References

1. 1
   Diltiazem hydrochloride — UK products listed on the electronic Medicines Compendium. Checked 20 Aug 2026: immediate-release tablets, modified-release tablets and capsules, and topical 2% preparations are listed; no solution for injection or infusion is listed.
2. 2
   Diltiazem hydrochloride — dosing, brand-specific modified-release prescribing and safety monograph. BNF, NICE. bnf.nice.org.uk
3. 3
   Resuscitation Council UK. Adult tachyarrhythmia algorithm, 2025. resus.org.uk — mirrored at lifesupport.resusdoc.uk.
4. 4
   NICE NG196. Atrial fibrillation: diagnosis and management. National Institute for Health and Care Excellence.
5. 5
   Hindricks G, Potpara T, Dagres N, et al. 2020 ESC Guidelines for the diagnosis and management of atrial fibrillation. Eur Heart J 2021;42(5):373–498.
6. 6
   Comparative evidence for intravenous diltiazem versus metoprolol in AF with rapid ventricular response — umbrella review of systematic reviews and meta-analyses, 2024. PMC Relevant to practice outside the UK, where an intravenous preparation is available.
7. 7
   Adizem-SR — Summary of Product Characteristics. electronic Medicines Compendium. Source of the statement that patients should not be changed between different presentations. Verified 21 Aug 2026.