Nitrates

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What are nitrate drugs?

Nitrate drugs include: glyceryl trinitrate (GTN), isosorbide dinitrate and isosorbide mononitrate. Each has various brand names. Nitrate drugs do not alter the underlying cause of angina. (Angina is usually caused by narrowing of the coronary arteries due to a build up of a fatty substance called atheroma. See separate leaflet called 'Angina'.) However, nitrate drugs are good at easing and preventing angina pains.

How do nitrates work?

An angina pain develops if part of the heart muscle does not get as much blood and oxygen as it needs. (Blood flow to heart muscle is restricted because the coronary arteries are narrowed.) Nitrates mainly work by relaxing the blood vessels in the body. This causes them to dilate (widen). This then makes it easier for the heart to pump blood and reduces the 'strain' on the heart. This means the heart muscle does not need as much blood and oxygen supply.

Nitrates also relax and widen the coronary arteries which increases the flow of blood to the heart muscle.

Short acting nitrate preparations

GTN tablets or sprays
GTN is commonly used to ease angina pains. Many people who have angina always carry their GTN spray or tablets with them. You take a dose under your tongue 'as required' when a pain develops. GTN is absorbed quickly into the bloodstream from under the tongue. A dose works to ease the pain within a minute or so. If the first dose does not work, take a second dose after five minutes. (If the pain persists for 15 minutes despite taking GTN, then call an ambulance.)

GTN tablets 'go off' after a few weeks. Therefore, you need a fresh supply of tablets every eight weeks, and return any unused tablets to the pharmacist. You may prefer to use a GTN spray which has a much longer shelf life than tablets.

Some people with angina take a GTN tablet or a spray before certain types of exercise. For example, before climbing stairs. They know a pain is likely to develop with the exercise, but a dose of GTN is likely to prevent it.

Isosorbide dinitrate
This is sometimes used as an alternative to GTN to ease angina pains when they develop. Again, it comes in tablet and spray form.

Long-acting nitrate preparations

If you have frequent angina pains, you are likely to be prescribed one or more drugs which aim to prevent the pains from developing. You need to take these each day as prescribed. Several groups of drugs can prevent angina pains and include beta-blockers, calcium antagonists, and long-acting nitrates.

All of the nitrates (glyceryl trinitrate (GTN), isosorbide dinitrate, and isosorbide mononitrate) come in long-acting preparations. A long acting preparation takes longer to start working, so is not much use for immediate pain relief. But, it works for much longer after each dose than a short acting preparation (which loses its effect after 20 minutes or so). Some preparations are 'slow release' or 'modified release' tablets. When you swallow these they gradually release a steady amount of nitrate which is absorbed into the body. Some preparations come as skin patches or ointments which release a steady amount of nitrate into the bloodstream through the skin.

Tolerance to nitrates
If nitrate stays in your bloodstream all the time, your body becomes used to it and the nitrate then has much less of an effect. To overcome this 'tolerance' the dose schedule aims to leave your blood free of nitrate, or with very low levels, for a few hours each day. This is why the slow release tablets may not be prescribed at equal intervals throughout the day. This can result in your body being free of nitrate in the early hours when you are asleep.

What are the possible side-effects or problems with nitrates?

The most common side-effects are a throbbing headache, flushing, and dizziness. (These may occur as the blood vessels 'widen' when you take nitrate.) They are unpleasant, but not serious. In particular, many people get a headache within a few minutes of taking a dose of GTN. If you develop these side-effects, they often become less severe with continued use of the medication.

You should not take nitrates if you have various other disorders. For example: hypertrophic obstructive cardiomyopathy, aortic stenosis, constrictive pericarditis, mitral stenosis or closed-angle glaucoma. (This is the less common form of glaucoma. Nitrates are fine if you have the more common type of glaucoma called open angle glaucoma.)

Nitrates interfere with some other drugs which may cause problems. In particular, you should not take sildenafil (viagra) or similar drugs used for erectile dysfunction (impotence) if you are taking a nitrate.

Other side-effects or problems are uncommon. Read the information leaflet which comes with your particular brand for a full list of possible side-effects.

© EMIS and PIP 2006   Updated: June 2006   PRODIGY Validated

The authors and editors of this article are employed to create accurate and up to date content reflecting reliable research evidence, guidance and best clinical practice. They are free from any commercial conflicts of interest. Find out more about updating.

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