Cimetidine is a substituted imidazole compound that acts as a histamine H2-receptor antagonist (H2 blocker). H2 blockers are commonly used to treat gastric and duodenal ulcers, gastroesophageal reflux disease, and to prevent conditions associated with excessive gastric acid. Drugs in this class, including cimetidine, ranitidine, and famotidine, may also exert immunomodulatory effects by interacting with H2 receptors on immune cells.
Some studies suggest that these immunomodulatory properties contribute to cimetidine’s antitumor activity. Initially thought to enhance immune function, more recent research indicates that cimetidine inhibits tumor growth and metastasis through multiple mechanisms, including anti-adhesion and anti-angiogenesis effects.
BRAND NAMES
Tagamet – the most widely known brand.
Cimetidine Generic – available in tablet, oral solution, and injectable forms.
MECHANISM OF ACTION
Cimetidine is a histamine H2-receptor antagonist that reduces gastric acid secretion. By blocking H2 receptors on the parietal cells of the stomach, it inhibits both basal and stimulated acid production. This action helps heal ulcers, manage gastroesophageal reflux, and prevent complications from excess acid. Additionally, it may have immunomodulatory and antitumor effects via H2 receptor interactions on immune cells.
PHARMACOKINETICS
Absorption: Cimetidine is rapidly absorbed from the gastrointestinal tract with peak plasma levels reached in 1–2 hours after oral administration.
Distribution: It is moderately distributed throughout body tissues and crosses the placenta and into breast milk.
Metabolism: Partially metabolized in the liver, with some of the drug excreted unchanged.
Excretion: Primarily eliminated via the kidneys, with both unchanged drug and metabolites excreted in urine.
PHARMACODYNAMICS
Cimetidine reduces gastric acid secretion by blocking histamine H2 receptors on parietal cells. This decreases gastric acidity and promotes healing of gastric and duodenal ulcers. It also reduces acid reflux in the esophagus, alleviating symptoms of heartburn. Its immunomodulatory effects may enhance immune responses and contribute to antitumor activity. The drug’s onset is rapid, but maximal effect is usually observed after repeated dosing.
ADMINISTRATION
Cimetidine can be administered orally as tablets, capsules, or liquid, and intravenously in hospital settings. The oral route is most common for chronic therapy. It should be taken with or after meals to maximize efficacy and reduce stomach irritation. Dosing frequency is usually 1–4 times daily, depending on the condition being treated. Consistent timing is important to maintain therapeutic blood levels.
DOSAGE AND STRENGTH
Common oral doses range from 200 mg to 800 mg, depending on the condition.
For ulcers, typical adult dosing is 400 mg twice daily or 800 mg at bedtime.
Pediatric dosing is weight-based and lower.
Adjustments are needed in patients with renal impairment.
Extended therapy may be required for chronic conditions.
DRUG INTERACTIONS
Cimetidine inhibits several cytochrome P450 enzymes (especially CYP1A2, CYP2C19, CYP3A4), leading to increased levels of drugs metabolized by these enzymes. Examples include:
Warfarin
Phenytoin
Theophylline
Diazepam
Monitoring is required to prevent toxicity when used concurrently with these drugs.
FOOD INTERACTIONS
Cimetidine absorption is not significantly affected by food, but taking it with meals may reduce stomach upset. Alcohol does not have a major interaction but should be limited in patients with ulcers. High-fat meals do not alter efficacy. Consistent administration with regard to meals is recommended for optimal results.
CONTRAINDICATIONS
Cimetidine should not be used in individuals who have a known hypersensitivity to the medication. Use of clarithromycin should be approached with caution in patients with significant kidney or liver impairment. It should be used cautiously in individuals with a history of gastrointestinal bleeding. Pregnancy and breastfeeding require medical supervision. Severe drug interactions may necessitate avoidance or dose adjustment.
SIDE EFFECTS
Headache and dizziness.
Gastrointestinal disturbances such as diarrhea, constipation, or nausea.
Gynecomastia or reduced libido in males with long-term use.
Rash, itching, or allergic reactions.
Rare: confusion, hallucinations, or liver enzyme alterations.
OVER DOSE
Symptoms of cimetidine overdose may include drowsiness, confusion, rapid heart rate, and dizziness. Severe cases may lead to kidney or liver complications. Treatment is mainly supportive, including monitoring vital signs and renal function. Hemodialysis may be considered in severe renal impairment. Prompt medical attention is necessary in cases of suspected overdose.
TOXICITY
Cimetidine toxicity is uncommon but may occur with high doses or prolonged use. It can cause central nervous system effects (confusion, hallucinations), liver enzyme elevation, and rarely cardiac arrhythmias. Long-term therapy may lead to hormonal effects such as gynecomastia in men. Monitoring renal and liver function reduces the risk of serious toxicity.
