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Diltiazem: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on March 1, 2023.

1. How it works

  • Diltiazem may be used for the treatment of high blood pressure or angina.
  • Experts believe that diltiazem works by inhibiting the movement of calcium ions across cardiac (heart) muscle and the smooth muscle lining blood vessel walls.
  • This effect dilates (widens) blood vessels, reducing how hard the heart has to work to pump blood around the body, which reduces blood pressure. This in turn reduces the demand for oxygen by the heart, which reduces symptoms of angina. Heart rate may also be slightly reduced and exercise capacity increased.
  • Diltiazem belongs to the class of medicines known as calcium channel blockers (may also be called a calcium channel antagonist).

2. Upsides

  • May be used for the treatment of high blood pressure (hypertension), alone or in combination with other agents for high blood pressure.
  • May be used as an ongoing treatment to relieve symptoms such as chest pain in people with chronic stable angina, or angina due to coronary artery spasm. Will not relieve acute episodes of angina (other medications such as sublingual nitroglycerin are used for this purpose).
  • Available as a tablet, an extended-release (long-acting) tablet, and an extended-release capsule.
  • Can improve a person's tolerance to exercise and keep their blood pressure at acceptable levels during exercise. Heart rate at maximal exercise does not change or is slightly reduced.
  • Does not weaken how hard the heart contracts (does not have a negative inotropic effect).
  • Lowers blood pressure in both the standing and the sitting position. Does not tend to cause as much of a drop in blood pressure as some other blood pressure-lowering agents when going from a lying down to a standing position. Does not cause reflex tachycardia (a compensatory fast heart rate that often occurs when blood pressure drops).
  • Generic diltiazem is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Rhinitis, headache, sore throat, constipation, edema (fluid retention), skin rashes (although these are usually transient), and a lack of energy are the most commonly reported side effects.
  • May slightly reduce resting heart rate. However, in some people, it may cause an abnormally slow heart rate; those with preexisting heart conditions are more at risk.
  • May occasionally cause liver injury; however, this is usually reversible on discontinuation.
  • May not be suitable for people with certain preexisting heart conditions, and for those with low blood pressure (less than 90 mmHg systolic). Caution should be used when giving diltiazem to people with kidney or liver disease because not many studies have been done on people with these conditions.
  • May interact with several other drugs including cimetidine, clonidine, digoxin, anesthetics, those that are metabolized by certain hepatic enzymes such as CYP3A4, statins, and some other drugs that also affect the heart (such as beta-blockers).

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

  • Diltiazem may be used for the treatment of high blood pressure or chronic stable angina. Rhinitis, headache, and constipation are commonly reported side effects.

5. Tips

  • Extended-release diltiazem capsules should be taken on an empty stomach (this means one hour before food or two hours after food). Most other diltiazem formulations can be taken either with or without food; however, you should always check the instructions on the label.
  • Extended-release tablets and capsules should be swallowed whole. Do not crush or chew.
  • Take diltiazem around the same time every day. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Take exactly as directed. Do not take more or less of it or take it more often than prescribed by your doctor.
  • Although diltiazem may be used to prevent ongoing angina symptoms such as chest pain, it does not relieve an acute attack of angina. Most people are also prescribed a medication like sublingual nitroglycerin for this purpose. Talk to your doctor if you do not have a medication that you can take for sudden-onset chest pain.
  • Sometimes diltiazem may make you dizzy when you go from a sitting or lying down position to a standing position; the risk is greater if you also take other medications for high blood pressure. Stand up slowly if this happens to you.
  • See your doctor if you have a skin rash that persists or other side effects that are affecting your quality of life.

6. Response and effectiveness

  • The time to peak concentrations varies depending on the formulation used; it can take up to four to six hours for extended-release tablets to reach their peak.
  • Once absorbed, diltiazem is metabolized in the liver to an active metabolite which is approximately 25 to 50% as potent at dilating the coronary arteries as diltiazem.
  • The peak blood pressure-reducing and blood vessel-widening effects occur three to six hours after oral administration of diltiazem extended-release capsules, and at least 50% of the effect is still present after 24 hours.
  • Immediate-release diltiazem tablets are usually taken three or four times a day. Extended-release capsules or tablets are usually taken one or two times a day.

7. Interactions

Medicines that interact with diltiazem may either decrease its effect, affect how long it works, increase side effects, or have less of an effect when taken with diltiazem. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with diltiazem include:

  • anticonvulsants, such as fosphenytoin
  • antifungal agents, such as itraconazole
  • alfentanil
  • amiodarone
  • beta-blockers, such as atenolol, labetalol, or metoprolol
  • buspirone
  • benzodiazepines such as midazolam or triazolam
  • carbamazepine
  • cimetidine
  • cyclosporine
  • grapefruit juice
  • rifampin
  • lovastatin or simvastatin
  • quinidine
  • NSAIDs, such as diclofenac, ibuprofen, and indomethacin, may decrease the blood pressure-lowering capabilities of diltiazem
  • other medications that inhibit or induce CYP 3A4 or CYP2D6.

Note that this list is not all-inclusive and includes only common medications that may interact with diltiazem. You should refer to the prescribing information for diltiazem for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use diltiazem only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: March 1, 2023.