Usage
Famotidine is prescribed for conditions involving excessive stomach acid production. These include:
- Gastroesophageal Reflux Disease (GERD)
- Erosive esophagitis
- Benign gastric ulcers
- Duodenal ulcers
- Pathological hypersecretory conditions (e.g., Zollinger-Ellison syndrome)
- Stress ulcer prophylaxis
- Prevention of aspiration pneumonitis (Mendelson’s syndrome)
- Treatment of heartburn, dyspepsia
- Nausea and vomiting in pregnancy (adjunct therapy)
Pharmacological Classification: H2-receptor antagonist.
Mechanism of Action: Famotidine competitively inhibits histamine at H2-receptors in gastric parietal cells, reducing basal and nocturnal gastric acid secretion.
Alternate Names
International/Regional Variations: No significant variations.
Brand Names: Pepcid, Pepcid AC, Pepcid Complete (combined with antacids), Zantac 360.
How It Works
Pharmacodynamics: Famotidine reversibly binds to H2 receptors of parietal cells, reducing both basal and stimulated gastric acid secretion (stimulated by food, histamine, gastrin, caffeine, insulin). It affects gastric volume minimally but profoundly influences hydrogen ion concentration (pH).
Pharmacokinetics:
- Absorption: Oral absorption is incomplete (around 40-45%). Food slightly alters absorption.
- Metabolism: Minimal hepatic metabolism.
- Elimination: Primarily renal excretion (65-70% unchanged), with some biliary excretion. The elimination half-life is approximately 2.5-3.5 hours.
Mode of Action: Competitive inhibition of histamine at the H2 receptors of gastric parietal cells.
Receptor Binding: Selectively binds to H2-receptors.
Enzyme Inhibition: Not applicable to its primary mechanism, although minor effects on hepatic enzyme systems exist.
Neurotransmitter Modulation: Minimal direct effects, primarily acts peripherally on gastric parietal cells.
Elimination Pathways: Primarily via renal excretion as unchanged drug, with some biliary excretion.
Dosage
Standard Dosage
Adults:
- GERD: 20 mg twice daily or 40 mg once daily at bedtime for up to 6-12 weeks.
- Duodenal Ulcer: 40 mg at bedtime or 20 mg twice daily for up to 8 weeks. Maintenance: 20 mg at bedtime.
- Gastric Ulcer: 40 mg at bedtime for up to 8 weeks.
- Hypersecretory Conditions: Initial: 20 mg every 6 hours; titrate up to a maximum of 160 mg every 6 hours as needed.
Children:
- GERD (3 months - 1 year): Initial: 0.5 mg/kg twice daily. May increase to 1 mg/kg twice daily (maximum 40 mg/day).
- GERD (1-16 years): 0.5 mg/kg twice daily (maximum 80 mg/day).
- Duodenal Ulcer (1-16 years): 0.5 mg/kg daily at bedtime or divided twice daily (maximum 40 mg/day).
Special Cases:
- Elderly Patients: Start at lower doses.
- Patients with Renal Impairment: Reduce dose or increase dosing interval (e.g., 10 mg every other day for CrCl < 30 ml/min, or adjust based on CrCl).
- Patients with Hepatic Dysfunction: No specific dosage adjustments are typically necessary.
- Patients with Comorbid Conditions: Dosage adjustments may be required.
Clinical Use Cases
- Intubation/Surgical Procedures: 20 mg IV or 40 mg oral 1-3 hours pre-operatively.
- Mechanical Ventilation/ICU Use: 20 mg IV or PO every 12 hours (24 hours if CrCl < 50 ml/min).
- Emergency Situations (e.g., anaphylaxis): 20 mg IV.
Dosage Adjustments
Dosage adjustments based on CrCl for renal impairment.
Side Effects
Common Side Effects: Headache, dizziness, constipation, diarrhea.
Rare but Serious Side Effects: Arrhythmias, thrombocytopenia, agranulocytosis, aplastic anemia, liver dysfunction, confusion, hallucinations.
Long-Term Effects: Generally well-tolerated, but potential for vitamin B12 deficiency with long-term use.
Adverse Drug Reactions (ADR): Hypersensitivity reactions (rare), Stevens-Johnson syndrome (very rare), QT prolongation (rare, especially in renal impairment).
Contraindications
Hypersensitivity to famotidine or other H2-receptor antagonists.
Drug Interactions
- Antacids: may reduce famotidine absorption.
- Ketoconazole, itraconazole: decreased absorption of these antifungals.
- Drugs metabolized by CYP450 enzymes: potential for minor interactions.
- Procainamide: reduced renal clearance of procainamide.
Pregnancy and Breastfeeding
Pregnancy Safety Category: B. Considered safe for use during pregnancy.
Breastfeeding: Small amounts excreted in breastmilk; generally considered safe but monitor the infant.
Drug Profile Summary
- Mechanism of Action: H2-receptor antagonist, reduces gastric acid secretion.
- Side Effects: Headache, dizziness, constipation, diarrhea (common); arrhythmias, blood dyscrasias, liver dysfunction, CNS effects (rare).
- Contraindications: Hypersensitivity.
- Drug Interactions: Antacids, ketoconazole, itraconazole, procainamide.
- Pregnancy & Breastfeeding: Generally safe.
- Dosage: See dosage section above.
- Monitoring Parameters: Renal function (especially in impaired patients), liver function tests (if indicated), ECG (in cases of suspected QT prolongation).
Popular Combinations
Famotidine is often combined with antacids for symptomatic relief of heartburn (e.g., Pepcid Complete).
Precautions
- Renal impairment: dose adjustment necessary.
- Liver disease: monitor patients.
- Elderly: start with lower doses.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for Famotidine?
A: See dosage section above.
Q2: How should famotidine be administered?
A: Orally, intravenously, or via nasogastric tube/feeding tube.
Q3: What are the common side effects of famotidine?
A: Headache, dizziness, constipation, diarrhea.
Q4: Is famotidine safe in pregnancy?
A: Yes, generally considered safe.
Q5: Can famotidine be used in children?
A: Yes, with appropriate dosage adjustments based on age and weight.
Q6: What are the signs of an allergic reaction to famotidine?
A: Rash, itching, swelling (especially of the face, tongue, throat), severe dizziness, trouble breathing. Seek immediate medical attention.
Q7: What should I do if I miss a dose of famotidine?
A: Take the missed dose as soon as you remember. If it is almost time for your next dose, skip the missed dose and continue your regular dosing schedule. Do not double the dose to catch up.
Q8: How does famotidine differ from proton pump inhibitors (PPIs)?
A: While both reduce gastric acid, PPIs irreversibly inhibit the proton pump, offering more potent and prolonged acid suppression compared to famotidine’s reversible H2-receptor blockade. PPIs may be more effective for certain conditions, such as erosive esophagitis.
Q9: Can I take famotidine with antacids?
A: While combination products exist, spacing administration by 1-2 hours is generally recommended, as antacids can slightly reduce famotidine absorption.
Q10: What is the role of famotidine in stress ulcer prophylaxis?
A: Famotidine is used to prevent stress ulcers in critically ill patients at risk due to factors like mechanical ventilation, coagulopathy, or major trauma.
