Usage
Cimetidine is prescribed for treating and preventing ulcers in the stomach and duodenum. It also addresses conditions where the stomach produces too much acid, such as Zollinger-Ellison syndrome. Additionally, it’s used for gastroesophageal reflux disease (GERD), heartburn, and preventing upper gastrointestinal bleeding. Its pharmacological classification is H2-receptor antagonist. Cimetidine works by blocking histamine H2 receptors in the stomach, reducing acid secretion.
Alternate Names
Cimetidine is often marketed under the brand name Tagamet HB. There are no widely used alternate names.
How It Works
Pharmacodynamics: Cimetidine competitively inhibits histamine at H2 receptors on parietal cells in the stomach, decreasing basal and stimulated gastric acid secretion. This reduces gastric volume and hydrogen ion concentration.
Pharmacokinetics:
- Absorption: Cimetidine is well absorbed orally.
- Metabolism: It undergoes hepatic metabolism, primarily via CYP450 enzymes (CYP1A2, CYP2C9, CYP2D6, and CYP3A4).
- Elimination: Primarily renal excretion (70%), with some fecal elimination.
Mode of Action: Cimetidine binds to H2 receptors on parietal cells, preventing histamine from binding and stimulating acid secretion.
Elimination Pathways: Predominantly renal excretion; some hepatic metabolism.
Dosage
Standard Dosage
Adults:
- Duodenal Ulcer: 800 mg orally at bedtime or 300 mg four times daily with meals and at bedtime.
- Gastric Ulcer: 800 mg orally at bedtime or 300 mg four times daily.
- GERD: 400 mg orally four times daily or 800 mg twice daily.
- Zollinger-Ellison Syndrome: 300 mg four times daily with meals and at bedtime; may increase up to 2400 mg daily.
Children:
- 1 month - 1 year: 10-20 mg/kg/day orally divided every 6-12 hours.
- 1-18 years: 20-40 mg/kg/day orally divided every 6 hours (maximum 1600 mg/day).
- Neonates: 5-10 mg/kg/day orally every 8-12 hours.
Special Cases:
- Elderly Patients: Reduce dosage due to potential for confusion and drug interactions. Start with lower doses (e.g., 300 mg twice daily) and adjust based on renal function.
- Patients with Renal Impairment: Dosage adjustment is necessary based on creatinine clearance.
- Patients with Hepatic Dysfunction: Use with caution. Dosage adjustment may be required.
- Patients with Comorbid Conditions: Individualized dosing based on specific conditions.
Clinical Use Cases
Dosage adjustments may be required in these settings; consult specialized guidelines:
- Intubation: May be used to prevent stress ulcers.
- Surgical Procedures: Prophylactic use to reduce gastric acid secretion.
- Mechanical Ventilation: Use for stress ulcer prophylaxis.
- Intensive Care Unit (ICU) Use: Stress ulcer prophylaxis.
- Emergency Situations: Not typically used in emergency situations like status epilepticus or cardiac arrest.
Dosage Adjustments
Adjustments based on creatinine clearance:
- CrCl > 30 mL/min: Standard dosage.
- CrCl 15-30 mL/min: Reduce dosage (e.g., 600 mg/day).
- CrCl < 15 mL/min: Further reduce dosage (e.g., 400 mg/day or 200 mg every 12 hours).
- Hemodialysis: Cimetidine is removed by hemodialysis.
Side Effects
Common Side Effects
Headache, dizziness, diarrhea, nausea, vomiting, confusion (especially in the elderly), gynecomastia (breast enlargement in men), impotence.
Rare but Serious Side Effects
Neutropenia, thrombocytopenia, aplastic anemia, pancytopenia, agranulocytosis, cardiac arrhythmias, hypotension, hepatitis, acute interstitial nephritis, seizures, hallucinations, delirium, encephalopathy (particularly in patients with renal or hepatic impairment).
Long-Term Effects
Potential for gynecomastia and impotence with prolonged use.
Adverse Drug Reactions (ADR)
Severe allergic reactions (anaphylaxis), severe blood dyscrasias, hepatotoxicity, nephrotoxicity, cardiac events.
Contraindications
Hypersensitivity to cimetidine or other H2 receptor antagonists.
Drug Interactions
Cimetidine interacts with numerous drugs due to its effects on CYP450 enzymes. It can increase the levels of warfarin, phenytoin, theophylline, metformin, and some antidepressants. It may decrease the effectiveness of ketoconazole, itraconazole, and some antacids. Alcohol may increase the risk of CNS side effects.
Pregnancy and Breastfeeding
Cimetidine is classified as Pregnancy Category B. While generally considered safe, use only if clearly needed and the benefit outweighs the risk. Cimetidine enters breast milk. Breastfeeding is generally not recommended during cimetidine use. Famotidine or nizatidine are preferred alternatives if needed during breastfeeding.
Drug Profile Summary
- Mechanism of Action: H2-receptor antagonist; reduces gastric acid secretion.
- Side Effects: Headache, dizziness, diarrhea, gynecomastia, confusion. Rarely: blood dyscrasias, hepatotoxicity.
- Contraindications: Hypersensitivity to cimetidine.
- Drug Interactions: Numerous; affects CYP450 metabolism of other drugs.
- Pregnancy & Breastfeeding: Use cautiously in pregnancy if benefits outweigh risks; not recommended while breastfeeding.
- Dosage: Varies depending on indication and patient characteristics.
- Monitoring Parameters: Renal function, liver function, complete blood count (especially with long-term use).
Popular Combinations
Cimetidine is sometimes used in combination with antacids for symptom relief.
Precautions
- General: Assess renal and hepatic function before and during treatment.
- Specific Populations: Reduce dosage in elderly and patients with renal/hepatic impairment.
- Lifestyle Considerations: Avoid alcohol as it may increase CNS side effects.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for Cimetidine?
A: Dosage depends on the indication and patient factors. Refer to the Dosage section above for detailed guidelines.
Q2: What is the mechanism of action of Cimetidine?
A: Cimetidine competitively inhibits histamine at H2 receptors, reducing gastric acid secretion.
Q3: What are the common side effects of Cimetidine?
A: Common side effects include headache, dizziness, diarrhea, and confusion.
Q4: What are the serious side effects of Cimetidine?
A: Rare but serious side effects include blood dyscrasias, hepatitis, and cardiac arrhythmias.
Q5: Can Cimetidine be used in pregnancy?
A: Cimetidine is Pregnancy Category B. Use cautiously only if clearly needed and benefits outweigh risks.
Q6: Can Cimetidine be used during breastfeeding?
A: Not generally recommended. Alternative H2 blockers like famotidine or nizatidine are preferred.
Q7: What are the major drug interactions with Cimetidine?
A: Cimetidine interacts with many drugs metabolized by CYP450 enzymes. Consult drug interaction resources for a comprehensive list.
Q8: How should Cimetidine dosage be adjusted in renal impairment?
A: Dosage reduction is needed based on creatinine clearance. See the Dosage section for specific recommendations.
Q9: What are the contraindications to using Cimetidine?
A: Hypersensitivity to cimetidine or other H2 receptor antagonists.
Q10: What are some important monitoring parameters for patients on Cimetidine?
A: Monitor renal function, liver function, and complete blood count, particularly during long-term therapy.
