FAMOTIDINE, CALCIUM CARBONATE, AND MAGNESIUM HYDROXIDE
Clinical safety rating: safe
Animal studies have demonstrated safety
Famotidine is a competitive histamine H2-receptor antagonist that inhibits gastric acid secretion by blocking H2 receptors on parietal cells. Calcium carbonate and magnesium hydroxide act as antacids, neutralizing gastric acid via chemical neutralization.
| Metabolism | Famotidine: minimally metabolized in liver (CYP450 not significantly involved); excreted unchanged in urine. Calcium carbonate and magnesium hydroxide: not metabolized; react with gastric acid to form salts. |
| Excretion | Famotidine: renal (65-70% unchanged), biliary/fecal (30-35%). Calcium carbonate: feces (unabsorbed calcium), urine (absorbed). Magnesium hydroxide: feces (unabsorbed magnesium), urine (absorbed). |
| Half-life | Famotidine: 2.5-3.5 hours (prolonged in renal impairment, up to 20 hours when CrCl <10 mL/min). |
| Protein binding | Famotidine: 15-22% (plasma proteins). Calcium and magnesium: minimal binding (ionized forms). |
| Volume of Distribution | Famotidine: 1.1-1.4 L/kg (widely distributed, not extensively tissue bound). |
| Bioavailability | Famotidine: oral 40-50% (first-pass metabolism). Calcium carbonate: absorption 30-40% (variable). Magnesium hydroxide: absorption 15-30%. |
| Onset of Action | Famotidine: oral 1 hour; calcium carbonate and magnesium hydroxide: oral 0.5-1 hour (antacid effect). |
| Duration of Action | Famotidine: 10-12 hours (acid suppression); calcium carbonate and magnesium hydroxide: 1-3 hours (neutralizing effect). |
1 tablet (famotidine 10 mg, calcium carbonate 800 mg, magnesium hydroxide 165 mg) orally once or twice daily as needed for heartburn; maximum 2 tablets in 24 hours.
| Dosage form | TABLET, CHEWABLE |
| Renal impairment | No specific adjustment for this combination product; however, for famotidine alone: CrCl <50 mL/min: reduce dose to 20 mg orally once daily or extend interval to 36-48 hours. Calcium carbonate and magnesium hydroxide may accumulate; avoid in severe renal impairment (CrCl <30 mL/min). |
| Liver impairment | No specific adjustment; famotidine is minimally metabolized. Use with caution in severe hepatic impairment due to potential for ammonia accumulation from antacids. |
| Pediatric use | Not recommended for children under 12 years. For ages 12 and older, same as adult dose: 1 tablet orally once or twice daily as needed. |
| Geriatric use | Use lowest effective dose; consider reduced renal function. No specific dose adjustment, but monitor for constipation (calcium carbonate) and hypermagnesemia (magnesium hydroxide) especially with prolonged use. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Can decrease the absorption of many other drugs Can cause diarrhea and abdominal cramping.
| Breastfeeding | Famotidine: Excreted in breast milk in low amounts; M/P ratio approximately 1.78. Calcium and magnesium: Normal dietary components; excess may cause maternal hypercalcemia or hypermagnesemia. Compatible with breastfeeding; use caution in infants with renal impairment. |
| Teratogenic Risk | Famotidine: FDA Category B. No evidence of teratogenicity in animal studies; insufficient human data. Calcium carbonate and magnesium hydroxide: Generally considered safe; no known teratogenic risk. First trimester: No increased risk of major malformations. Second/third trimester: Use only if clearly needed; monitor for maternal electrolyte imbalances. |
■ FDA Black Box Warning
None
| Common Effects | dyspepsia |
| Serious Effects |
["Hypersensitivity to famotidine or any component","Severe renal impairment (CrCl <10 mL/min) for prolonged use"]
| Precautions | ["Renal impairment: reduce famotidine dose; accumulation possible","Aspiration risk: use cautiously in patients with gastrointestinal obstruction or regurgitation","Drug interactions: antacids may alter absorption of other oral medications (e.g., tetracyclines, iron); separate dosing by at least 2 hours","Hypercalcemia: avoid excessive calcium intake in patients with renal disease or hypercalcemia","Hypophosphatemia: chronic use of magnesium hydroxide may cause phosphate depletion"] |
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| Fetal Monitoring | Monitor maternal serum calcium, magnesium, and renal function periodically, especially in prolonged therapy or renal insufficiency. Fetal monitoring not routinely required; watch for signs of maternal electrolyte disturbances. |
| Fertility Effects | No known adverse effects on human fertility. Animal studies with famotidine showed no impairment. Calcium and magnesium at therapeutic doses are unlikely to affect reproduction. |