An H2 blocker for acid/heartburn
Cimetidine
An H2 blocker that reduces stomach acid for heartburn and reflux; unlike newer H2 blockers it inhibits liver enzymes and interacts with many medicines.
What is Cimetidine?
Cimetidine is an H2-receptor blocker that reduces the amount of acid the stomach makes, used for heartburn, acid reflux and related conditions. It eases symptoms by lowering stomach acid, although it is generally less powerful than a proton pump inhibitor (PPI). Unlike newer acid-reducing medicines, it inhibits liver enzymes and interacts with many other medicines, so combinations should be checked carefully.
Education and reference only. This is a plain-language guide to Cimetidine — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Cimetidine is an H2-receptor antagonist (an 'H2 blocker') used in the UK to reduce stomach acid in conditions such as heartburn, acid reflux (gastro-oesophageal reflux disease, GORD), indigestion and stomach or duodenal ulcers. It was the first medicine of its type and remains effective, though it is now used less often than newer H2 blockers and proton pump inhibitors (PPIs). It controls acid-related symptoms but does not treat the underlying cause; its main drawback compared with similar medicines is that it interacts with many other drugs.
How it works
The stomach lining contains cells that produce acid when stimulated by a chemical messenger called histamine acting on H2 receptors. Cimetidine blocks these H2 receptors, so the cells produce less acid. With less acid in the stomach, the lining and the gullet are less irritated, which eases heartburn and reflux and helps ulcers heal. Cimetidine also blocks several of the liver's cytochrome P450 enzymes, which are responsible for breaking down many other medicines — this is why it interacts with so many drugs.
Company & origin
Originated / developed by: Developed by Smith, Kline & French (now part of GSK)..
Cimetidine was the first H2 antagonist, developed in the UK in the 1970s by a team led by Sir James Black, and it transformed the treatment of acid-related conditions.
What it treats
Conditions Cimetidine is used for
Practical use
How to take Cimetidine
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it as advised, often around meal times and at bedtime, to cover acid production when symptoms are worst.
- Tell your prescriber or pharmacist about all your other medicines before starting, as cimetidine interacts with many of them.
- Combine it with lifestyle measures such as avoiding large or late meals, losing excess weight if needed, and cutting down on alcohol and smoking.
- Do not take it long term without review, and see a doctor if symptoms persist despite treatment.
- Seek advice promptly for difficulty or pain on swallowing, unintended weight loss, vomiting or signs of bleeding (such as black stools), rather than simply continuing.
- If you take warfarin, phenytoin, theophylline or similar medicines, your prescriber may need to check levels or adjust doses.
Weighing it up
Advantages & disadvantages of Cimetidine
Advantages
- It effectively reduces stomach acid and relieves heartburn, reflux and ulcer symptoms.
- It is well established and was the original medicine of its kind.
- It is taken by mouth and is generally well tolerated.
Disadvantages
- It inhibits liver enzymes and interacts with many medicines, so combinations need checking.
- It is generally weaker than a proton pump inhibitor (PPI) for more troublesome acid problems.
- Newer H2 blockers without the same interaction problems are often preferred.
Practical use
Good to know
The most important practical point with cimetidine is that, unlike newer H2 blockers, it inhibits several liver enzymes and so interacts with many medicines — including warfarin (a blood thinner), phenytoin (for epilepsy) and theophylline (for breathing problems), among others — sometimes raising their levels and risk of side effects. Always check before combining it with other medicines. It is generally a weaker acid-reducer than a PPI, so a PPI may be preferred for more troublesome reflux or ulcers. Simple measures help too — avoiding large or late meals, losing excess weight, and cutting down on alcohol, smoking and trigger foods. If heartburn is frequent, persistent, or comes with difficulty swallowing, unintended weight loss or vomiting, see a doctor rather than just continuing acid treatment.
Who should not take it / use with caution
- People taking medicines that interact significantly with cimetidine, unless this is managed and monitored.
- People with significantly reduced kidney function, who may need a lower dose.
- Anyone who has had a serious allergic reaction to cimetidine.
Monitoring
- Reviewing whether acid symptoms have improved and whether treatment is still needed.
- Checking levels or effects of interacting medicines such as warfarin, phenytoin and theophylline.
- Watching for confusion, particularly in older people and those with kidney problems.
Side effects
- Diarrhoea or other changes in bowel habit.
- Headache and dizziness.
- Tiredness, and occasionally confusion in older people or those with kidney problems.
- Rarely, breast tenderness or enlargement in men with longer use.
Key interactions
- It can raise the effect of warfarin, increasing the risk of bleeding, so clotting may need closer checking.
- It can raise levels of phenytoin (for epilepsy) and theophylline (for breathing problems), increasing the chance of side effects.
- It interacts with many other medicines by affecting liver enzymes, so always check before combining it with anything new.
Available as: Tablets and an oral liquid.
Answers
Cimetidine: frequently asked questions
How is cimetidine different from a PPI like omeprazole?
Both reduce stomach acid, but they work in different ways. Cimetidine blocks histamine-driven acid production, while a proton pump inhibitor (PPI) blocks the final step of acid release and is generally more powerful. A PPI is often preferred for more troublesome reflux or ulcers.
Why does cimetidine interact with so many medicines?
Cimetidine blocks several of the liver enzymes that break down other medicines. This can let those medicines build up to higher levels in the body, raising the risk of side effects. That is why you should always check before combining it with other drugs.
Can I take cimetidine with warfarin?
Cimetidine can increase the effect of warfarin and the risk of bleeding. If both are needed, your prescriber will usually check your clotting (INR) more closely and may adjust your warfarin dose. Always make sure your team knows you take both.
Is cimetidine still used much today?
It is used less often now than newer H2 blockers and proton pump inhibitors, mainly because of its many drug interactions. It remains effective, but a medicine with fewer interactions or a more powerful PPI is frequently chosen instead.
When should I see a doctor rather than keep taking it?
See a doctor if heartburn is frequent or persistent, or if you have difficulty or pain on swallowing, unintended weight loss, vomiting, or signs of bleeding such as black stools. These need proper assessment rather than continued acid treatment alone.
The wider class
About H2 antagonists
Cimetidine belongs to the h2 antagonists class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF
- NICE CKS
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