cimetidine at a glance
Medicine class
Histamine H2-receptor antagonist
Main role
Reduces gastric acid secretion in ulcers, reflux and other acid-related conditions
Administration
Taken by mouth as tablets, syrup or oral solution
Key practical issue
Has numerous clinically important medicine interactions
What is cimetidine used for?
Selected products are licensed for conditions in which suppressing gastric acid is beneficial.
Duodenal, benign gastric, recurrent or stomal ulcers, including ulcers associated with non-steroidal anti-inflammatory medicines
Oesophageal reflux disease and persistent dyspeptic symptoms
Prevention of gastrointestinal bleeding from stress ulceration in seriously or critically ill patients
Zollinger–Ellison syndrome
Reducing malabsorption and fluid loss in short bowel syndrome
Reducing degradation of pancreatic enzyme supplements in pancreatic insufficiency
Reducing acid-aspiration risk around general anaesthesia for specified products; suitability is formulation-specific
How does cimetidine work?
Cimetidine competitively blocks histamine H2 receptors on stomach parietal cells. This reduces resting and stimulated acid production, gastric-juice volume and pepsin output.
cimetidine preparations and strengths
Oral solution
Route: Oral
Strengths: 200 mg/5 mL
Tablet
Route: Oral
Strengths: 200 mg, 800 mg
How to take cimetidine
Follow the prescription label and use only the formulation supplied. Food has little effect on absorption.
Swallow tablets with water.
Measure syrup or oral solution with an appropriate medicine-measuring device, not a household spoon.
Check liquid ingredients if there is hereditary fructose intolerance, sensitivity to colourings or preservatives, or concern about alcohol or propylene glycol.
cimetidine side effects
Common or expected effects
- Headache
- Dizziness
- Diarrhoea
- Tiredness
- Skin rash
- Muscle pain
Get urgent medical advice
- Severe allergic reaction or anaphylaxis
- Blood-cell disorders causing unusual bleeding, persistent fever, mouth ulcers or recurrent infection
- Confusion or hallucinations, particularly in older or seriously ill people
- Hepatitis or jaundice
- Pancreatitis
- Markedly slow, fast or irregular heartbeat, heart block or fainting
- Kidney inflammation
- Inflamed blood vessels or a severe skin reaction
cimetidine in pregnancy
Available information on H2-receptor antagonists has not identified clear pregnancy risks. However, cimetidine crosses the placenta, and its product information advises avoiding it unless the prescriber considers it essential. Formulation ingredients must also be considered.
cimetidine while breastfeeding
Cimetidine passes into breast milk. H2-receptor antagonists can be used while breastfeeding a full-term, healthy infant, but cimetidine is least preferred because milk levels are higher and interactions are more likely. Premature or unwell infants need individual assessment.
cimetidine interactions
Cimetidine can inhibit liver enzymes, compete for renal secretion and change gastric acidity. A pharmacist or prescriber should review all prescribed, non-prescription and herbal products.
Warfarin and related coumarin anticoagulants
May increase anticoagulant exposure; closer clotting-time monitoring may be required when cimetidine is started or stopped.
Phenytoin or theophylline
Reduced clearance may increase exposure and toxicity, requiring clinical or concentration monitoring.
Intravenous lidocaine
Reduced metabolism may increase exposure and adverse-effect risk.
Common questions about cimetidine
Answers are fully visible for fast scanning and source review.
Is cimetidine an antacid?
No. It reduces acid production by blocking H2 receptors; antacids mainly neutralise acid already in the stomach.
Why does cimetidine interact with many medicines?
It inhibits several liver enzymes, competes with some medicines for kidney secretion and changes stomach acidity.
Can cimetidine cause confusion?
Sources
These sources were used to prepare and review this medicine page.
Prepared and reviewed by the iatroX editorial team.