Famotidine

Mechanism

Mechanism of action

Histamine released basally or stimulated by enterochromaffin cells (ECL) binds to the H₂ receptor coupled to the G_{α s} protein subunit. This stimulates adenylate cyclase catalyzing the conversion of ATP to cAMP. Famotidine competitively prevents this binding, drastically reducing the production of intracellular cAMP and the consequent activation of Protein Kinase A (PKA).

In addition, by selectively blocking the cAMP pathway, it indirectly reduces the sensitivity of the parietal cell to calcium-mediated stimuli (gastrin and acetylcholine), achieving a global decrease in basal, nocturnal, and food-induced acid secretion.

Mechanism of Drug Tolerance (Tachyphylaxis)

Unlike PPIs, H₂ antagonists induce a rapid loss of clinical efficacy known as tachyphylaxis after 3 to 5 days of continuous treatment. This physiological phenomenon is due to the development of compensatory hypersensitivity of unblocked secretory pathways (adaptive increase in circulating gastrin levels and increase in the density of CCK₂ and muscarinic receptors in the parietal cell), limiting its usefulness in long-term suppressive therapies.

Pharmacokinetics

Pharmacokinetics

• Absorption: Rapid but incomplete absorption after oral administration. Bioavailability of 40% to 45% (not affected by the presence of food in the tract).
• Distribution: Volume of distribution of 1.1 to 1.4 L/kg. Low plasma protein binding (~15% to 22%). It crosses the blood-brain barrier to a limited extent.
• Metabolism: Moderate hepatic (~30% to 35%) via microsomal oxidation to famotidine S-oxide. It does not interfere with the cytochrome P450 system, unlike cimetidine.
• Excretion: Predominantly renal (65% to 70% of systemic clearance is excreted unchanged in urine through glomerular filtration and active tubular secretion). The rest is eliminated through the feces.
• Half-life: 2.5 to 3.5 hours in adults with normal kidney function. It is exponentially prolonged in advanced stages of kidney disease.

Indicators and dose

Approved and Off-label indications

• Treatment and maintenance of benign active duodenal/gastric ulcer.
• Gastroesophageal reflux disease (GERD): Symptomatic relief of heartburn and mild esophagitis.
• Prevention of acid aspiration (Mendelson syndrome): Preanesthetic administration in emergency surgery (Off-label).

Dosage and Clinical Adjustment

Adults:

• Active ulcer (duodenal or gastric): 40 mg orally at night at bedtime, or 20 mg twice a day for 4 to 8 weeks.
• GERD: 20 mg orally twice a day for up to 6 weeks.
• Prevention of heartburn: 10 mg to 20 mg orally 15-60 minutes before consuming trigger foods.

Adjustment in Renal Failure: The dosage should be modified strictly according to the estimated glomerular filtration rate (eGFR):

• eGFR 50 mL/min/1.73m²: Standard dose unchanged.
• eGFR 10 - 49 mL/min/1.73m²: Reduce the dose to 20 mg once daily or extend the dosing interval to 48 hours.
• eGFR < 10 mL/min/1.73m² (or dialysis): Reduce dose to 10 mg or 20 mg every 48-72 hours. Hemodialysis does not significantly remove the drug.

Adjustment in Hepatic Insufficiency: Does not require dose modifications in mild to moderate hepatic insufficiency.

Security

Contraindications

• Absolute: Hypersensitivity to famotidine or other H₂ receptor antagonists (risk of cross-reactivity).
• Relative: Moderate to severe renal failure (creatinine clearance < 50 mL/min) without dose adjustment, due to the high risk of central nervous system toxicity.

Adverse effects (ADRs)

• Frequent (>1/100): Headache, dizziness, constipation, diarrhea.
• Uncommon / Rare: Dry mouth, nausea, vomiting, arthralgia, fatigue.
• Effects on the Central Nervous System (severe): Reversible encephalopathy, mental confusion, motor agitation, visual or auditory hallucinations and lethargy. These effects occur predominantly in the elderly and patients with advanced renal failure, in whom decreased clearance facilitates drug accumulation and greater passage of the blood-brain barrier.
• Hematological (very rare): Immune-induced thrombocytopenia, agranulocytosis or pancytopenia.

Drug interactions

• Due to pH alteration: Drastic decrease in the oral absorption of azoles, atazanavir and cefpodoxime.
• Exclusion of CYP interactions: Unlike cimetidine (which nonspecifically inhibits CYP1A2, CYP2C9, CYP2D6 and CYP3A4), famotidine does not inhibit the CYP450 oxidase system, therefore it does not alter the metabolism of theophylline, phenytoin, warfarin or clopidogrel.

Pregnancy and Breastfeeding

FDA Category B: Easily crosses the placenta. Animal reproductive toxicity studies have not revealed fetal harm. It is considered a safe alternative to PPIs in gestational reflux refractory to behavioral measures.

Breastfeeding: Caution. It is excreted in active human milk. The milk/plasma concentration ratio is greater than 1. The infant should be monitored for the appearance of irritability, vomiting, food refusal or sedation.