FAMOTIDINE 20 MG/ML

Famotidine is an H2 receptor antagonist used by veterinarians to treat stomach ulcers in dogs, cats and horses. Famotidine binds to H2 receptors in the stomach lining causing a lowered production of stomach acid which can improve conditions like acid reflux, though it is more often used to help dogs recover from stomach ulcers. These ulcers can be caused by many different things, but most commonly develop when certain gastric structures are damaged by stomach acid, or following infection with bacteria such as H. pylori.⁴ They often develop in the gut, but can also occur in the intestine, which can present marked danger due to bleeding.⁶

Unlike other H2 antagonists such as cimetidine, famotidine does not interact with any known substances, and does not effect the CYP enzyme system, giving rise to less side effects. Famotidine is also used to treat gastritis, esophagitis, and GERD.

The U.S. Food & Drug Administration (FDA) has approved famotidine for use in humans to reduce stomach acid, but it is not officially approved for use in animals. The FDA allows veterinarians to prescribe products containing this drug in different species or for other conditions in certain situations.¹

General

Famotidine has significantly fewer drug interactions than cimetidine and activity may persist longer. Famotidine leads to transient increases in serum gastrin, which returns to baseline concentrations a week after discontinuation.²

When famotidine was administered to cats over 14 days, a diminished effect on intragastric pH was observed, indicating decreased effectiveness with prolonged administration.³ In dogs, famotidine did not improve outcomes (eradication) of Helicobacter spp infections when added to triple antibiotic therapy.⁴

In cattle, famotidine significantly increased abomasal outflow fluid pH for up to 4 hours after a single dose. When famotidine was administered every 8 hours for 3 doses, tachyphylaxis appeared to occur, as the duration of increased pH progressively diminished after each dose. The total time of increased pH was ≈7 hours (30% of a 24-hour period).⁵

Famotidine: Pharmacology and Pharmacokinetics

At the H2 receptors of the parietal cells, famotidine competitively inhibits histamine, thereby reducing gastric acid output, both during basal conditions and when stimulated by food, pentagastrin, histamine, or insulin. Gastric emptying time, pancreatic or biliary secretion, and lower esophageal pressures are not altered by famotidine. By decreasing the amount of gastric juice produced, H2 blockers reduce the amount of pepsin secreted.

Famotidine is not completely absorbed after oral administration, but undergoes minimal first-pass metabolism. In humans, systemic bioavailability is ≈40% to 50%. Distribution characteristics are not well-described. In rats, only ≈15% to 20% is bound to plasma proteins; the drug does not cross the blood–brain barrier or the placenta. When the drug is administered orally, about 1/3 is excreted unchanged in the urine, and the remainder is primarily metabolized in the liver then excreted in the urine. After IV dosing, about 2/3 is excreted unchanged.

The pharmacokinetics of famotidine, ranitidine, and cimetidine have been investigated in horses.⁵ After administration of a single IV dose, elimination half-lives of cimetidine, ranitidine, and famotidine were in the 2- to 3-hour range and were not significantly different. Bioavailability of each of the drugs was low in horses (ie, famotidine, 13%; ranitidine, 13.5%; cimetidine, 30%).¹

Famotidine: Adverse Effects, Drug Interactions and Warnings

Unlike cimetidine or ranitidine, famotidine does not appear to inhibit hepatic cytochrome P450 enzyme systems, and dosage adjustments of other drugs (eg, warfarin, theophylline, diazepam, procainamide, phenytoin) that are metabolized by these metabolic pathways are usually not required.¹ However, studies have shown that taking H2 receptor antagonists for an extended period of time can raise the patient’s chances of developing kidney problems and can possibly lead to kidney failure.

The following drug interactions have either been reported or are theoretical in humans or animals receiving famotidine and may be of significance in veterinary patients. Unless otherwise noted, use together is not necessarily contraindicated, but the potential risks should be weighed and additional monitoring performed when appropriate.

AZOLE ANTIFUNGALS (eg, ketoconazole, itraconazole, fluconazole): By raising gastric pH, famotidine may decrease the absorption of these agents; if both drugs are required, the azole should be administered 1 hour before famotidine.

CEFPODOXIME, CEFUROXIME: Famotidine may decrease the absorption of these cephalosporins; separating doses by 2 hours may alleviate this effect.

IRON SALTS, ORAL: Famotidine may decrease the absorption of oral iron; iron should be administered at least 1 hour before famotidine.

QT-PROLONGING MEDICATIONS (eg, cisapride, fluoxetine): Concurrent use of famotidine with other QT prolonging medications may increase the risk for arrhythmias.¹

Famotidine is contraindicated in patients with known hypersensitivity to the drug. Rapid IV infusion may cause bradycardia. Other H2 blockers have been demonstrated to be relatively safe and exhibit minimal adverse effects. Agranulocytosis has been reported rarely in humans.¹

There have been rare anecdotal reports of famotidine causing intravascular hemolysis when given IV to cats. A retrospective study evaluating IV famotidine in 56 hospitalized cats did not show any evidence of hemolysis; the authors concluded that IV famotidine appeared safe in cats when administered over 5 minutes.⁶

Famotidine is generally tolerated well by both dogs and cats, but in some animals it may cause:

• Vomiting
• Diarrhea
• Decreased or loss of appetite
• Dry mouth or skin¹

These symptoms are not cause for alarm, but if they become severe, companion animal owners should contact their veterinarian.

Dosages

Famotidine is available in several forms, including tablets, injectable and oral suspension. The most common forms are famotidine 10mg tablets for dogs, famotidine 20mg for dogs, famotidine 10mg tablets for cats and famotidine 20mg for cats.

Suggested dosages per Plumb’s Veterinary Drugs, are as follows:

Dogs and Cats:

Reduce gastric acid production (extra-label): Dosage recommendations vary. Recommended dosage is at least 1 mg/kg PO, SC, or IV every 12 hours and is likely subtherapeutic. Rounding doses to the nearest 5 mg increment (eg, 5 mg per cat) is reasonable. Once-daily administration is most often recommended in patients with significantly diminished renal function. IV injection should be given over at least 5 minutes. Alternatively, a loading dose of famotidine 1 mg/kg IV followed by 8 mg/kg/day as a constant-rate infusion (CRI)of 8 mg/kg/day may provide the best antacid efficacy in dogs.⁷

Small Mammals:

Ferrets - Stress-induced ulcers (extra-label): 0.25 – 0.5 mg/kg PO or IV every 24 hours⁸

In combination with antibiotics for Helicobacter spp treatment (extra-label): 0.25 – 0.5 mg/kg PO or IV every 24 hours⁹

Rabbits: Stress-induced ulcer prevention once critically ill animal has stabilized (extra-label): 1 mg/kg IV every 24 hours¹⁰

Horses:

NOTE - Racing Commissioners International (ARCI) has designated famotidine as a UCGFS CLASS 5 DRUG.

Adjunct in ulcer treatment (extra-label): 0.23 mg/kg IV every 8 hours or 0.35 mg/kg IV every 12 hours; 1.88 mg/kg PO every 8 hours or 2.8 mg/kg PO every 12 hours¹¹

Cattle:

Reduce gastric acid production (extra-label): From a pharmacokinetic study in 4 adult steers: 0.4 mg/kg IV every 8 hours increased abomasal outflow fluid pH for 3 hours, 2 hours, and 1 hour after each respective dose.⁵ Further studies are needed to support clinical efficacy.¹

Where to buy Famotidine

Famotidine is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. Famotidine 20 mg/ml oral suspension by NexGen suspension aids in the healing of stomach ulcers and stomach ulcer formation, and is used to treat gastritis, esophagitis, and GERD.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹Plumb’s Veterinary Drugs.

²Parente NL, Olivier NB, Refsal KR, Johnson CA. Serum concentrations of gastrin after famotidine and omeprazole administration to dogs. J Vet Intern Med. 2014;28(5):1465-1470.

³Golly E, Odunayo A, Daves M, et al. The frequency of oral famotidine administration influences its effect on gastric pH in cats over time. J Vet Intern Med. 2019;33(2):544-550.

⁴Leib MS, Duncan RB, Ward DL. Triple antimicrobial therapy and acid suppression in dogs with chronic vomiting and gastric Helicobacter spp. J Vet Intern Med. 2007;21(6):1185-1192.

⁵Balcomb CC, Heller MC, Chigerwe M, Knych HK, Meyer AM. Pharmacokinetics and efficacy of intravenous famotidine in adult cattle. J Vet Intern Med. 2018;32(3):1283-1289.

⁶de Brito Galvao JF, Trepanier LA. Risk of hemolytic anemia with intravenous administration of famotidine to hospitalized cats. J Vet Intern Med. 2008;22(2):325-329.

⁷Hedges K, Odunayo A, Price JM, Hecht S, Tolbert MK. Evaluation of the effect of a famotidine continuous rate infusion on intragastric pH in healthy dogs. J Vet Intern Med. 2019.

⁸Williams BH. Therapeutics in ferrets. Vet Clin North Am Small Anim Pract. 2000;3(1):131-153.

⁹Fisher P. Ferret medicine I. Paper presented at: Western Veterinary Conference; 2005.

¹⁰Johnston M. Clinical monitoring of the critically ill rabbit. Paper presented at: IVECCS; 2006.

¹¹Duran SH, Ravis WR. Comparative pharmacokinetics of H2 antagonists in horses. Paper presented at: 11th Annual Veterinary Medical Forum; 1993; Washington, DC.