Ranitidine is a histamine H2 receptor antagonist that reduces gastric acid production in the stomach. It is most commonly used to treat and prevent ulcers of the stomach and intestines as well as gastroesophageal reflux disease (GERD) and Zollinger-Ellison syndrome. Ranitidine was first introduced in 1981 by GlaxoSmithKline under the trade name Zantac and became one of the world's best-selling drugs by the late 1980s.
Mechanism of Action
Ranitidine works by competitively and reversibly binding to histamine H2 receptors located on the gastric parietal cell surface. Histamine is a potent stimulator of acid secretion through its H2 receptor interactions with parietal cells. By blocking these H2 receptors, ranitidine inhibits both basal and nocturnal acid secretion stimulated by histamine, foods, stress, and other factors. It reduces gastric acid levels and volume, allowing ulcers and damaged gastrointestinal tissues to properly heal. Ranitidine has a relatively short half-life of 2.5-3 hours, meaning frequent dosing is required to sustain therapeutic acid suppression throughout the day and night.
Absorption and Metabolism
Orally administered ranitidine is rapidly and almost completely absorbed from the gastrointestinal tract with peak blood levels occurring within 2-3 hours. It has bioavailability of around 50%. Ranitidine is metabolized in the liver by cytochrome P450 enzymes, mainly CYP3A4, to produce a primary active N-oxide metabolite. Both ranitidine and its metabolite are renally excreted in urine, with the majority of the drug being eliminated within 6-8 hours of administration. There is little risk of drug accumulation from regular dosing unless renal function is significantly impaired.
Indications
The primary uses of ranitidine include treatment and prevention of various acid-related gastrointestinal disorders:
- Gastric Ulcer Disease: Ranitidine promotes ulcer healing by reducing gastric acid levels needed for ulcer formation and recurrence. It is effective in healing most gastric ulcers within 4-8 weeks.
- Duodenal Ulcer Disease: Similarly to gastric ulcers, ranitidine treatment consistently achieves duodenal ulcer healing rates over 80% when used for 4-8 weeks.
- Gastroesophageal Reflux Disease (GERD): By inhibiting gastric acid secretion, ranitidine helps control symptoms of heartburn and acid regurgitation in GERD. It provides effective, rapid relief of both erosive/ulcerative esophagitis and non-erosive reflux disease.
- Zollinger-Ellison Syndrome: This rare disorder is characterized by tumors in the pancreas or lungs that secrete excess gastrin and cause severe stomach acid overproduction. Ranitidine is a mainstay long-term treatment due to its powerful acid-blocking effects.
- Prevention of Ulcers Associated with Nonsteroidal Anti-Inflammatory Drug (NSAID) Use: Ranitidine can successfully prevent gastric and duodenal ulcers in patients who require long-term NSAID therapy for conditions like arthritis. This reduces the risk of life-threatening bleeding complications.
Dosage and Administration
- For duodenal or gastric ulcers, the standard oral dose of ranitidine is 150 mg twice daily or 300 mg once at bedtime.
- For maintenance therapy of healed ulcers, the dose is reduced to 150 mg once daily.
- For GERD, the as-needed dose is 150 mg taken 30-60 minutes prior to meals and at bedtime.
- For Zollinger-Ellison syndrome, the usual starting dose is 150 mg four times daily but may need to be increased up to 3600 mg daily based on acid suppression needs.
- For NSAID-associated ulcer prevention, 150 mg twice daily is recommended.
Adverse Effects
Ranitidine is generally well-tolerated with few side effects when taken in recommended doses. Common adverse reactions include headaches, dizziness, constipation or diarrhea. Rare cases of mild reversible confusion, primarily in elderly patients, have also been reported. As with other H2 blockers, ranitidine can cause constipation or diarrhea, abdominal pain or cramps. There have been rare, isolated reports of minor bleeding events, fever or skin reactions.
Drug Interactions
Ranitidine can interact with several other drugs by affecting their absorption or metabolism. Examples include:
- Warfarin: Ranitidine may prolong prothrombin time by enhancing the anticoagulant effects of warfarin. More frequent INR monitoring and warfarin dosage adjustment may be needed.
- Lithium: Increases in lithium levels and toxicity have occurred after ranitidine administration due to potential inhibition of lithium renal clearance. Lithium levels should be monitored.
- Theophylline: Concurrent use with ranitidine may raise theophylline levels.
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