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H2 blockers

H2 blockers reduce the amount of acid made by your stomach. They are used in conditions where it is helpful to reduce stomach acid. For example, to help with acid reflux which causes heartburn. Most people who take H2 blockers do not develop any side-effects.

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What are H2 blockers?

H2 blockers are a group of medicines that reduce the amount of acid produced by the cells in the lining of the stomach. They are also called 'histamine H2-receptor antagonists' but are commonly called H2 blockers. They include cimetidine (Tagamet®), famotidine, nizatidine, and have various different brand names.

How do H2 blockers work?

Your stomach normally produces acid to help with the digestion of food and to kill germs (bacteria). This acid is corrosive so your body produces a natural mucous barrier which protects the lining of the stomach from being worn away (eroded).

Upper gastrointestinal tract and acid

Upper GI Tract and acid

In some people this barrier may have broken down allowing the acid to damage the stomach, causing an ulcer. In others there may be a problem with the muscular band at the top of the stomach (the sphincter) that keeps the stomach tightly closed. This may allow the acid to escape and irritate the gullet (oesophagus). This is called 'acid reflux', which can cause heartburn and/or inflammation of the gullet (oesophagitis).

The letter H in their name stands for histamine. Histamine is a chemical naturally produced by certain cells in the body, including cells in the lining of the stomach, called the enterochromaffin-like cells (ECL cells). Histamine stimulates the acid-making cells (parietal cells) in the lining of the stomach to release acid. H2 blockers stop the acid-making cells in the stomach lining from responding to histamine. This reduces the amount of acid produced by your stomach.

By decreasing the amount of acid, H2 blockers can help to reduce acid reflux-related symptoms such as heartburn. This can also help to heal ulcers found in the stomach or in part of the gut (the duodenum).

Note: H2 blockers are a different class of drugs to 'antihistamine drugs' which block H1 receptors in cells that are involved in allergy reactions.

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What conditions are they used to treat?

H2 blockers are commonly used:

At one time they were used as one part of a treatment to get rid of Helicobacter pylori, a germ (bacterium) found in the stomach, which can cause ulcers. However, proton pump inhibitors are now preferred for this use.

What is the best H2 blocker?

No one H2 blocker is thought to work any better than another. However, the newer group of medicines mentioned above - proton pump inhibitors - also reduce the amount of acid produced by the stomach. They include omeprazole, lansoprazole, pantoprazole, rabeprazole, and esomeprazole. In general, proton pump inhibitors are used first because they are better than H2 blockers to reduce the production of stomach acid. However, if you don't get on with a proton pump inhibitor (for example, because of side-effects), your doctor may prescribe an H2 blocker.

Generally, H2 blockers are well absorbed by the body and can provide quick relief of symptoms from some problems. For example, heartburn caused by reflux. However, if you are taking them for other reasons, such as to heal an ulcer, it may take longer for the medication to have an underlying effect.

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Side-effects of H2 blockers

Most people who take H2 blockers do not have any side-effects. However, side-effects occur in a small number of users. The most common side-effects are diarrhoea, headache, dizziness, rash and tiredness. For a full list of side-effects and possible interactions with other drugs, consult the leaflet that comes with your medication.

Ranitidine was used for many years but it has since been withdrawn globally as it contains a chemical that is linked to cancer risk.

Can I buy H2 blockers or do I need a prescription?

You can buy some of these medicines over the counter at pharmacies. They are commonly marketed as medicines for 'relief of heartburn, indigestion, acid indigestion and excess stomach acid' - or similar. However, if you need to use an H2 blocker regularly for more than two weeks, you should consult your doctor.

How long is treatment needed?

This can vary depending on the reason for treating you, so speak with your doctor for advice. In some cases your doctor may prescribe an H2 blocker to use 'as required'. This means you only take it when you need it to relieve your symptoms, rather than every day. In some situations you may be prescribed an H2 blocker to be taken every day.

Who cannot take H2 blockers?

H2 blockers may not be suitable for people with kidney problems or for pregnant or breastfeeding mums. A full list of people who should not take H2 blockers is included with the information leaflet that comes in the medicine packet. If you are prescribed or buy an H2 blocker, read this to be sure you are safe to take it.

Note: taking some H2 blockers can affect how well other medicines work. In particular, tell your doctor if you are taking the blood-thinning medicine warfarin or a medicine for epilepsy, called phenytoin (Epanutin®). You should also tell your doctor if you take theophylline, a medicine commonly used to treat asthma or chronic obstructive pulmonary disease (COPD).

Other considerations

You should consult your doctor if your symptoms worsen, or if you experience any of the following problems which can indicate a serious gut disorder:

  • Bringing up (vomiting) blood. This may be obviously fresh blood but altered blood in vomit can look like ground coffee. Doctors call this 'coffee-ground vomit'.

  • Blood in your stools (faeces). This may be obvious blood, or it may just make your stools black.

  • Unintentional weight loss.

  • Difficulty swallowing, including food getting stuck in the gullet (oesophagus).

  • Persistent abdominal pains or persistent vomiting.

If you are taking antacids you should not take them at the same time as you take your other medication, including H2 blockers. This is because antacids can affect how well other medication is absorbed.

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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