Diltiazem

High blood pressure and angina are common cardiovascular conditions that benefit from consistent, long-term medication. Diltiazem is a prescription calcium channel blocker used to treat hypertension and angina pectoris in adults. It is available in a range of branded modified-release formulations at different strengths, allowing dosing to be adjusted to individual clinical needs. Diltiazem is also used in the management of certain cardiac arrhythmias in some formulations. Because different diltiazem formulations release the active ingredient at different rates, they are not interchangeable, and prescriptions should always specify the brand name and strength. A prescription can only be issued following a clinical consultation with a registered prescriber.

Diltiazem is a benzothiazepine calcium channel blocker licensed in the United Kingdom for the treatment of angina pectoris and hypertension in adults. Some formulations carry additional licensed indications for the management of supraventricular tachycardias and rate control of atrial fibrillation and flutter, though the indications vary by formulation. Diltiazem is classified as a rate-limiting calcium channel blocker, meaning it slows the heart rate as well as dilating blood vessels. This distinguishes it from the dihydropyridine class of calcium channel blockers, such as amlodipine and nifedipine, which have predominantly vasodilatory effects with less effect on heart rate.

Diltiazem is available in multiple modified-release branded formulations in the UK. Because different formulations use different release technologies, they deliver diltiazem into the body at different rates and over different time periods. The BNF and MHRA guidance state clearly that diltiazem formulations should not be considered interchangeable. Prescriptions must specify the brand name and strength to ensure the patient receives the same formulation consistently. A change in brand without clinical review could alter the therapeutic effect and may carry safety implications.

Non-interchangeability of diltiazem formulations: Different brands of diltiazem modified-release tablets are not interchangeable. Each formulation has its own release profile. Always ensure your prescription specifies the brand name and strength you have been prescribed. Do not switch brands without speaking to your prescriber or pharmacist first.

Diltiazem works by blocking voltage-gated calcium channels in the heart muscle and in the smooth muscle of blood vessel walls. Calcium influx through these channels is necessary for muscle contraction. By blocking these channels, diltiazem produces two complementary cardiovascular effects.

Vasodilation and Blood Pressure Reduction

In blood vessel walls, blocking calcium channels causes the smooth muscle to relax. This leads to vasodilation, reducing peripheral vascular resistance and lowering blood pressure. The antihypertensive effect of diltiazem is well established and it is used as a first-line option in some patient groups, including older adults and those of African or Caribbean family background, as referenced in NICE guideline NG136.

Rate-Limiting Effect on the Heart

Diltiazem also blocks calcium channels in the sinoatrial (SA) node and atrioventricular (AV) node of the heart. This slows the generation and conduction of electrical impulses through the heart, reducing heart rate. In angina, this rate-limiting effect reduces the oxygen demand of the heart muscle, decreasing the frequency and severity of angina episodes. In certain arrhythmias, it helps to control ventricular rate. This is the key distinction from dihydropyridine calcium channel blockers such as amlodipine, which do not significantly slow the heart rate.

Diltiazem and Beta Blockers

Because diltiazem slows the heart rate and reduces atrioventricular conduction, combining it with a beta blocker can produce a significant and potentially dangerous bradycardia and heart block. The combination of diltiazem with a beta blocker is generally contraindicated unless under close specialist supervision. This is a clinically important interaction given the number of patients who take beta blockers for hypertension, angina, or heart failure.

Always take diltiazem exactly as your prescriber instructs. Because different formulations have different dosing schedules, it is essential to follow the specific instructions for the brand you have been prescribed. The information below summarises general guidance applicable to most modified-release diltiazem formulations. Read the Patient Information Leaflet supplied with your specific medicine before starting treatment.

• Take diltiazem at the same time each day, as instructed by your prescriber. Depending on the formulation, this may be once daily or twice daily.
• Most modified-release diltiazem tablets should be swallowed whole and must not be crushed or chewed. Crushing a modified-release tablet destroys the release mechanism and delivers the full dose rapidly, which could cause a dangerous fall in blood pressure or heart rate. Always check the patient information leaflet for your specific brand.
• Diltiazem can generally be taken with or without food, though you should follow the specific instructions in the Patient Information Leaflet for your brand.
• If you miss a dose, take it as soon as you remember on the same day. If you only remember the following day, skip the missed dose and continue with your normal schedule. Do not take two doses on the same day.
• Do not stop taking diltiazem without speaking to your prescriber first. Stopping diltiazem suddenly may cause your blood pressure to rise or angina symptoms to worsen.

Diltiazem is available through this service in multiple branded modified-release formulations. It is essential that prescriptions specify both the brand name and strength, as different formulations are not interchangeable. The formulations available are listed below. Your prescriber will determine which formulation and dose is most appropriate for your clinical circumstances.

Adizem SR (Sustained-Release Capsules)

Adizem SR is a twice-daily sustained-release formulation available in three strengths: 90mg, 120mg, and 180mg. It is used in the management of angina and hypertension. As a twice-daily formulation, dosing is typically morning and evening. Adizem SR capsules should be swallowed whole.

Adizem XL (Once-Daily Extended-Release Capsules)

Adizem XL is a once-daily extended-release formulation available in strengths of 120mg, 180mg, 200mg, 240mg, and 300mg. It is used in the management of angina and hypertension. Once-daily dosing supports consistent blood pressure and heart rate control throughout the day. Adizem XL capsules should be swallowed whole.

Angitil SR (Sustained-Release Capsules)

Angitil SR is a twice-daily sustained-release formulation available in three strengths: 90mg, 120mg, and 180mg. It is used for angina and hypertension. As with other SR formulations, it is taken twice daily and capsules should be swallowed whole.

Angitil XL (Once-Daily Extended-Release Capsules)

Angitil XL is a once-daily extended-release formulation available in two strengths: 240mg and 300mg. It is used in the management of angina and hypertension and should be swallowed whole.

Tildiem Retard (Sustained-Release Tablets)

Tildiem Retard is a twice-daily sustained-release formulation available in two strengths: 90mg and 120mg. It is used for angina and hypertension. Tildiem Retard tablets are designed for twice-daily dosing and should be swallowed whole.

Tildiem LA (Once-Daily Extended-Release Capsules)

Tildiem LA is a once-daily extended-release formulation available in two strengths: 200mg and 300mg. It is used for angina and hypertension, with once-daily dosing. Capsules should be swallowed whole.

Viazem XL (Once-Daily Extended-Release Capsules)

Viazem XL is a once-daily extended-release formulation available in five strengths: 120mg, 180mg, 240mg, 300mg, and 360mg. The range of strengths makes Viazem XL suitable for dose titration across a wide therapeutic range. It is used in the management of angina and hypertension. Capsules should be swallowed whole.

Zemtard XL (Once-Daily Extended-Release Capsules)

Zemtard XL is a once-daily extended-release formulation available in four strengths: 120mg, 180mg, 240mg, and 300mg. It is used for angina and hypertension. Capsules should be swallowed whole.

Like all medicines, diltiazem can cause side effects, although not everyone experiences them. The information below reflects the general side effect profile of diltiazem modified-release formulations as documented in UK SmPCs. The specific side effect profile may vary slightly between formulations. Refer to the Patient Information Leaflet supplied with your specific product for the full list.

Common Side Effects (may affect up to 1 in 10 people)

• Bradycardia (a slow heart rate) - diltiazem slows the heart rate as part of its mechanism. If the heart rate becomes very slow, contact your prescriber.
• Peripheral oedema (swelling of the ankles, feet, or legs), caused by vasodilation.
• Flushing, a feeling of warmth or redness in the face.
• Headache.
• Dizziness or lightheadedness, particularly when standing up quickly.
• Gastrointestinal effects such as nausea, constipation, or abdominal discomfort.

Uncommon Side Effects (may affect up to 1 in 100 people)

• Low blood pressure (hypotension).
• Fatigue and tiredness.
• First degree atrioventricular block (detected on ECG, often without symptoms).
• Skin rash or itching.
• Gingival hyperplasia (overgrowth of gum tissue) with prolonged use.

Serious Side Effects: Seek Urgent Medical Attention

Contact your prescriber or go to your nearest accident and emergency department promptly if you experience any of the following:

• A very slow or irregular heartbeat, or significant worsening of breathlessness.
• Second- or third-degree atrioventricular block - symptoms may include severe dizziness, fainting, or an irregular pulse.
• Signs of heart failure such as increasing breathlessness, rapid weight gain due to fluid retention, or significant ankle swelling.
• Signs of a serious allergic reaction, such as sudden swelling of the face, lips, tongue, or throat; difficulty breathing; or a severe skin reaction.

Report any suspected side effect to your prescriber or pharmacist or via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

Diltiazem Tablets May Be Suitable If You:

• Are an adult with hypertension or stable angina pectoris who requires a rate-limiting calcium channel blocker.
• Have hypertension and have not tolerated or responded adequately to other antihypertensive medicines.
• Are already prescribed a diltiazem formulation and require a repeat prescription through an online prescribing service, provided your prescription specifies the same brand and strength.

Diltiazem Tablets Are Not Suitable If You:

• Have second or third degree atrioventricular block (unless you have a functioning pacemaker).
• Have sick sinus syndrome or sinoatrial block (unless you have a functioning pacemaker).
• Have severe bradycardia (resting heart rate below 40 to 50 beats per minute, depending on formulation).
• Have severe left ventricular dysfunction or decompensated heart failure.
• Are taking a beta-blocker - the combination of diltiazem with a beta-blocker is generally contraindicated due to the risk of significant bradycardia and heart block.
• Are taking intravenous dantrolene (risk of cardiovascular collapse).
• Have a known allergy to diltiazem or any excipient listed in the SmPC.
• Are under 18 years of age (this service is for adults only).

Cautions

Particular care is required in patients with first degree atrioventricular block, impaired left ventricular function, or prolonged PR interval on ECG. Patients with liver impairment should be monitored, as diltiazem is extensively metabolised in the liver and blood levels may be higher than expected. Caution is also required in older patients, who may be more sensitive to the heart-rate-lowering and blood-pressure-lowering effects. Diltiazem should be used with caution in patients with Wolff-Parkinson-White syndrome or other accessory pathway conditions.

Important Drug Interactions

Diltiazem has a number of important drug interactions, several of which are potentially serious. Tell your prescriber about all medicines you take, including over-the-counter products and herbal supplements. The following interactions are of particular clinical importance:

• Beta blockers: the combination of diltiazem with any beta-blocker is generally contraindicated. Both reduce heart rate and atrioventricular conduction. Combining them can cause severe bradycardia, heart block, and a significant fall in blood pressure.
• Digoxin: Diltiazem increases digoxin blood levels. Patients taking both medicines require monitoring of digoxin levels and cardiac function.
• Ciclosporin: Diltiazem inhibits the enzyme that metabolises ciclosporin, raising ciclosporin levels significantly. Dose adjustment and close monitoring are required.
• Simvastatin: diltiazem inhibits the metabolism of simvastatin, raising the risk of myopathy and rhabdomyolysis. The dose of simvastatin should not exceed 20mg per day in patients taking diltiazem.
• Lithium: diltiazem may increase the risk of lithium toxicity.
• CYP3A4 substrates: diltiazem inhibits the CYP3A4 enzyme and can raise the blood levels of many other medicines metabolised by this pathway.
• Antiepileptic medicines such as carbamazepine and phenytoin: these reduce diltiazem levels, reducing its effectiveness. Conversely, diltiazem may raise carbamazepine levels.

This is not a complete list of interactions. Your prescriber will carry out a full clinical assessment, including a review of your current medicines, before any prescription is considered.

Diltiazem is a prescription-only medicine. The clinical consultation is the essential first step. A prescription will only be issued if a registered prescriber determines, based on their individual assessment of your circumstances, that diltiazem is clinically appropriate for you.

During your online consultation with Pharmacy Planet, a registered prescriber will review your clinical information. This includes your blood pressure or angina history, heart rate, cardiac rhythm, current medicines, and relevant medical history. For patients seeking a repeat prescription, the prescriber will also confirm the specific brand and strength being prescribed, as diltiazem formulations are not interchangeable and the prescription must specify both.

Not all consultations will result in a prescription being issued. The prescriber may recommend a different formulation, dose, or medicine, or suggest that a face-to-face appointment with your GP or cardiologist is more appropriate. This process is designed to ensure your safety.

If you are already being managed for hypertension, angina, or a heart rhythm condition by your GP or a specialist, please keep them informed of all medicines you are taking, including any prescribed through online services.

• Store below 25°C (or as specified on your individual product packaging, which may vary by brand).
• Store in the original packaging to protect from light and moisture.
• Keep out of the sight and reach of children.
• Do not use after the expiry date printed on the packaging.
• Do not dispose of medicines via household waste or wastewater. Return unused or out-of-date medicine to your pharmacist for safe disposal.

Full storage instructions specific to your brand and formulation are provided in the Patient Information Leaflet supplied with your medicine.

The table below compares calcium channel blocker tablets. Dihydropyridines (such as amlodipine and nifedipine) act mainly on blood vessels. Non-dihydropyridines (verapamil and diltiazem) also slow the heart rate and have antiarrhythmic properties.

Important: This comparison is for general information only and does not replace medical advice. A prescriber will decide which treatment is suitable for you following an online consultation.

Pharmacy Planet is a GPhC-registered online pharmacy providing access to prescription medicines through a clinically led consultation process. Our registered prescribers and pharmacists are committed to patient safety, clinical accuracy, and responsible prescribing.

We follow GPhC standards, MHRA guidance, and ASA advertising regulations in all aspects of our service. Every prescription is reviewed and dispensed by qualified healthcare professionals. We provide clear, factual information to help patients understand their antihypertensive medication and make informed decisions about their health.

If you have questions about diltiazem, your specific formulation, or cardiovascular therapy, our pharmacy team is available to help. We are committed to transparent, patient-centred care.

References

1. Electronic Medicines Compendium (eMC). Diltiazem modified-release formulations — Summaries of Product Characteristics for individual brands. https://www.medicines.org.uk/emc. Accessed August 2026.
2. National Institute for Health and Care Excellence (NICE). Hypertension in adults: diagnosis and management. NICE guideline NG136. https://www.nice.org.uk/guidance/ng136. Accessed August 2026.
3. Joint Formulary Committee. British National Formulary (BNF). Diltiazem hydrochloride. BMJ Group and Pharmaceutical Press. https://bnf.nice.org.uk. Accessed August 2026.
4. NHS. Diltiazem. https://www.nhs.uk/medicines/diltiazem. Accessed August 2026.
5. Medicines and Healthcare products Regulatory Agency (MHRA). Yellow Card — report a side effect. https://yellowcard.mhra.gov.uk. Accessed August 2026.