Cimetidine is an H2 blocker that reduces stomach acid. It treats heartburn, reflux, gastritis and stomach ulcers. It was the first medicine of its kind, and it has largely been replaced by newer options because it interacts with a great many other medicines.
What is Cimetidine?
Cimetidine is a histamine H2 receptor antagonist — an H2 blocker. It is available as tablets and as an injection, and it is a Schedule 2 or 3 medicine depending on the form.
Histamine is best known for its role in allergy, but it does something quite different in the stomach: it binds H2 receptors on acid-producing cells and tells them to secrete acid. Cimetidine blocks those receptors, so less acid is produced.
That is a different mechanism from an antihistamine for hay fever, which blocks H1 receptors — the two are not interchangeable.
Cimetidine has a notable place in medical history. Introduced in the 1970s, it was the first drug designed rationally from receptor theory rather than found by screening, and it transformed ulcer treatment — before it, severe ulcers were often treated surgically.
It has since been largely superseded, for two reasons.
Proton pump inhibitors are stronger. A PPI such as the esomeprazole in Trustan blocks the final step in acid production and suppresses acid far more completely and for longer.
Cimetidine interacts with an unusually large number of medicines. It inhibits liver enzymes that break down other drugs, raising their levels. Newer H2 blockers such as famotidine do not have this problem, which is why they are generally preferred now.
What is Cimetidine commonly used for?
- Heartburn and acid reflux.
- Gastro-oesophageal reflux disease (GORD).
- Stomach (gastric) and duodenal ulcers, for healing and prevention.
- Gastritis.
- Indigestion with an acid component.
- Zollinger-Ellison syndrome, a rare acid-overproducing condition.
- Preventing stress ulcers, in hospital settings.
Where an H2 blocker still fits usefully:
Night-time acid breakthrough. Some people on a PPI still get symptoms overnight, and an H2 blocker at bedtime can cover that gap.
Stepping down from a PPI. Because stopping a PPI abruptly causes rebound acid, switching to an H2 blocker for a period is a recognised way to come off it more comfortably.
As-needed use. H2 blockers work within about an hour — slower than an antacid, faster than a PPI — and last several hours, which suits predictable symptoms such as a heavy evening meal.
For persistent daily reflux or a confirmed ulcer, a PPI is usually the better choice.
How Cimetidine fits into this medicine category
| Group | Examples | Speed and strength |
|---|---|---|
| Antacids | Mist Alba, Gaviscon | Minutes; short-lived |
| H2 blockers | Cimetidine, famotidine | About an hour; moderate, several hours |
| Proton pump inhibitors | Esomeprazole (Trustan), pantoprazole (Peploc) | Days to full effect; strongest, longest |
| Antispasmodics | Hyoscine butylbromide (Buscopan) | For cramping, not acid |
| Antiemetics | Metoclopramide (Adco-Contromet) | For nausea, not acid |
Famotidine is the H2 blocker generally preferred today, because it lacks cimetidine’s interactions and its anti-androgen effects. See the digestive, stomach and nausea medicines hub.
What to check before using Cimetidine
Tell every prescriber and pharmacist that you take it. The interaction list is the main reason for care with this particular medicine, and it is easily overlooked because cimetidine seems like an ordinary indigestion tablet.
Cimetidine raises the levels of, among others:
- Warfarin — increasing bleeding risk.
- Phenytoin and carbamazepine (Partid).
- Theophylline.
- Some benzodiazepines, increasing sedation.
- Some antidepressants and antiarrhythmics.
It also reduces the absorption of medicines that need stomach acid, including some antifungals and HIV medicines.
Tell your prescriber or pharmacist if you:
- Take any regular medicine — this is the key question.
- Have kidney or liver disease.
- Are pregnant or breastfeeding.
- Are older, where confusion is more likely.
- Have had stomach cancer or are being investigated for it — acid suppression can mask symptoms.
Report alarm symptoms rather than self-treating: difficulty or pain on swallowing, unintended weight loss, persistent vomiting, vomiting blood, black tarry stools, or anaemia.
Follow the label. This page does not set doses.
Possible cautions and when to ask a pharmacist or doctor
Interactions are the main issue. If you are on several medicines, ask whether famotidine would be a better choice — it does the same job without the interaction burden.
Anti-androgen effects. Cimetidine can cause breast enlargement and tenderness in men (gynaecomastia), reduced libido and, rarely, erectile difficulty. These are dose-related, reversible on stopping, and specific to cimetidine among the H2 blockers — another reason famotidine is generally preferred. Raise it rather than tolerating it.
Confusion in older adults, particularly with kidney impairment, is recognised.
Common effects — headache, dizziness, diarrhoea, tiredness — are usually mild.
Do not use it long term without review. If you need acid suppression daily and indefinitely, the reason should be clear and the choice of medicine revisited.
Lifestyle measures genuinely reduce reflux: smaller meals, not eating within three hours of lying down, raising the head of the bed, reducing alcohol, losing excess weight, and stopping smoking.
Seek medical attention for vomiting blood or material like coffee grounds, black tarry stools, difficulty swallowing, unintended weight loss, severe abdominal pain, confusion, unusual bruising or bleeding, or yellowing of the skin or eyes. Chest pain spreading to the arm or jaw, or with breathlessness or sweating, may be cardiac rather than digestive and needs emergency assessment.
Related medicine guides
- Browse Digestive, Stomach & Nausea Medicines guides
- Read: Hyospasmol used for
- Read: Adco Contromet used for
Frequently asked questions
What is cimetidine used for? Heartburn, acid reflux, gastritis and stomach ulcers. It is an H2 blocker that reduces acid production.
How is it different from omeprazole? Cimetidine blocks histamine H2 receptors; a PPI blocks the final acid pump. PPIs suppress acid far more completely and for longer.
Why is it used less than it once was? It interacts with a great many other medicines, and newer H2 blockers such as famotidine do the same job without that problem.
Is it an antihistamine? It blocks histamine H2 receptors in the stomach. Allergy antihistamines block H1 receptors — the two are not interchangeable.
Why has it caused breast tenderness? Cimetidine has a mild anti-androgen effect, which can cause breast enlargement or tenderness in men. It is reversible, and an alternative avoids it.
How quickly does it work? Within about an hour — slower than an antacid, faster than a PPI — lasting several hours.