Cimetidine

Overview

Cimetidine (Zitac®, Tagamet®) is a histamine H2-receptor antagonist used in veterinary medicine to reduce gastric acid secretion. By blocking H2 receptors on gastric parietal cells, it decreases histamine-stimulated gastric acid production and helps protect the gastrointestinal mucosa from acid-related injury.

Cimetidine is commonly used in dogs and cats for management of acid-related gastrointestinal disorders such as gastritis, gastric or duodenal ulceration, and esophagitis. It may also be used in hypersecretory conditions associated with diseases such as gastrinoma, mast cell tumors, or short bowel syndrome.

Mechanism of Action (MOA): Cimetidine competitively inhibits histamine at H2 receptors in gastric parietal cells, thereby suppressing gastric acid secretion. This reduces gastric acidity and promotes healing of ulcerated or inflamed gastrointestinal tissues.

The drug is rapidly absorbed following oral administration and has relatively high bioavailability. It undergoes hepatic metabolism and is primarily excreted via the kidneys. The plasma elimination half-life is approximately 2 hours. Compared with newer H2 blockers and proton pump inhibitors, cimetidine is generally considered less potent in reducing gastric acidity.

Indications

Cimetidine is used in dogs and cats to reduce gastric acid secretion in a variety of gastrointestinal disorders associated with excessive acid production or mucosal irritation. It may also be used in conditions associated with increased gastric acid secretion.

• Erosive gastritis: Used in the management of idiopathic, uremic, or drug-associated erosive gastritis to decrease gastric acid irritation and promote mucosal healing.
• Gastric and duodenal ulcers: Administered to reduce gastric acidity and support healing of ulcerated gastric or duodenal mucosa.
• Esophagitis: Used to decrease gastric acid exposure and help reduce esophageal inflammation associated with reflux disease.
• Hypersecretory gastric disorders: May be used in conditions associated with increased gastric acid secretion such as gastrinoma, mast cell neoplasia, or short bowel syndrome.

Dosage (Reference)

Dog

In dogs, cimetidine is used to reduce gastric acid secretion in acid-related gastrointestinal disorders. The drug may be administered orally or by parenteral routes depending on the clinical situation.

Cat

In cats, cimetidine is used similarly to dogs for the management of gastric acid–related gastrointestinal disorders. The dosing interval is typically longer than that used in dogs.

Warnings & Precautions

Cimetidine should be used cautiously in certain patient populations because of its effects on hepatic drug metabolism and the potential for altered drug clearance in compromised animals.

• Hypersensitivity: Contraindicated in animals with known hypersensitivity to cimetidine or other components of the formulation.
• Renal or hepatic impairment: Use cautiously in patients with significant renal or hepatic dysfunction. Dose adjustments may be necessary because the drug undergoes hepatic metabolism and renal excretion.
• Geriatric patients: Older animals may be more susceptible to adverse central nervous system effects, particularly when organ function is reduced.
• Acid-suppressive therapy risks: Long-term suppression of gastric acid may increase susceptibility to infections such as pneumonia, as reported in human patients.
• Cytochrome P450 inhibition: Cimetidine inhibits hepatic CYP450 enzymes and may alter the metabolism of other drugs, increasing the potential for drug interactions.

Drug Interactions

Cimetidine inhibits hepatic microsomal cytochrome P450 enzymes and can slow oxidative drug metabolism. As a result, concurrent administration with certain medications may increase their plasma concentrations and prolong their pharmacologic effects.

• Beta-blockers (e.g., propranolol): Plasma concentrations may increase due to reduced hepatic metabolism.
• Calcium-channel blockers (e.g., verapamil): Cimetidine may increase circulating drug levels by inhibiting hepatic metabolism.
• Benzodiazepines (e.g., diazepam): Metabolism may be slowed, potentially prolonging sedative effects.
• Lidocaine: Reduced hepatic metabolism may increase plasma concentrations and the risk of toxicity.
• Metronidazole: Concurrent use may increase serum concentrations due to inhibition of oxidative metabolism.
• Opioid analgesics (e.g., pethidine): Plasma levels may increase when used with cimetidine.
• Theophylline: Cimetidine may decrease hepatic clearance, increasing the risk of toxicity.
• Leucopenia-inducing drugs: Concurrent use with drugs that may cause leucopenia could increase the risk or severity of this adverse effect.
• Sucralfate: May reduce the oral bioavailability of cimetidine; administering sucralfate at least 2 hours before cimetidine is recommended.
• Antacids, digoxin, itraconazole, or maropitant: Oral doses should be staggered by approximately 2 hours to minimize potential interactions affecting absorption.

Side Effects & Overdose

Side Effects

Adverse effects associated with cimetidine are generally uncommon and typically mild when they occur. Most reported reactions are reversible and resolve after discontinuation of therapy.

• Thrombocytopenia: Decreased platelet counts have been reported in dogs receiving cimetidine therapy.
• Mammary gland swelling: Transient and self-resolving enlargement of the mammary glands may occur in female dogs.
• Endocrine effects: Because of weak anti-androgenic activity, high doses have been associated with hormonal effects such as reduced libido in humans.
• Central nervous system effects: In people, headache and confusion have been reported, particularly in elderly patients or those with impaired organ function.
• Rare organ toxicity: Hepatotoxicity and nephrotoxicity have been reported rarely in human patients.

Overdose

Clinical experience with cimetidine overdose in veterinary patients is limited. Experimental data from laboratory animals suggest that severe toxicity may involve cardiovascular and respiratory complications.

• Tachycardia: Elevated heart rate has been observed in laboratory animals following toxic doses.
• Respiratory failure: Severe overdose may lead to respiratory compromise requiring supportive care.
• Management: Treatment is primarily supportive and may include respiratory support and administration of beta-adrenergic antagonists (e.g., atenolol or propranolol) if clinically indicated.
• Dialysis: Peritoneal dialysis does not significantly enhance elimination of cimetidine from the body.

Key Notes

Practical clinical considerations that may help optimize the use of cimetidine in dogs and cats in everyday veterinary practice:

• Relative potency: Cimetidine is generally less effective at reducing gastric acidity compared with newer H2 blockers and proton pump inhibitors, so alternative agents may be preferred in severe acid-related disease.
• Rebound acid secretion: Abrupt discontinuation may result in rebound gastric acid secretion; gradual tapering of therapy is recommended when stopping treatment.
• Combination therapy: Concurrent use with gastroprotectants such as sucralfate may provide additional protection for the gastrointestinal mucosa in ulcerative conditions.
• Administration considerations: When administered intravenously, the drug should be given slowly (over approximately 30 minutes) to reduce the risk of cardiovascular complications.
• Prokinetic activity: Cimetidine has minimal prokinetic effects and should not be relied upon to improve gastrointestinal motility.