Elephant specific information, if available, is in blue.

Chemistry – An H2- receptor antagonist, cimetidine occurs as a white to off-white, crystalline powder. It has what is described as an “unpleasant” odor and a pKa of 6.8. Cimetidine is sparingly soluble in water and soluble in alcohol. Cimetidine HCl occurs as white, crystalline powder and is very soluble in water and soluble in alcohol. It has a pKa of 7.11 and the commercial injection has a pH of 3.8-6.

 

Storage/Stability/Compatibility – Cimetidine products should be stored protected from light and kept at room temperature. Do not refrigerate the injectable product as precipitation may occur. Oral dosage forms should be stored in tight containers.

 

The cimetidine injectable product is compatible with the commonly used IV infusions solu­tions, including amino acid (TPN) solutions, but should be used within 48 hours of dilution. Cimetidine is also reported to be compatible with the following drugs: acetazolamide sodium, amikacin sulfate, atropine sulfate, carbenicillin disodium, cefoxitin sodium, chlorothiazide sodium, clindamycin phosphate, colistimethate sodium, dexamethasone sodium phosphate, digoxin, epinephrine, erythromycin lactobionate, furosemide, gentamicin sulfate, heparin sodium, insulin (regular), isoproterenol HCl, lidocaine HCl, lincomycin HCl, methylprednisolone sodium succinate, nafcillin sodium, norepinephrine bitartrate, penicillin G potas­sium/sodium, phytonadione, polymyxin B sulfate, potassium chloride, protamine sulfate, quinidine gluconate, sodium nitroprusside, tetracycline HCl, vancomycin HCl, verapamil HCl, and vi­tamin B complex (w/ or w/o C).

 

The following drugs are reported to be either incompatible with cimetidine or data are conflicting: amphotericin B, ampicillin sodium, cefamandole naftate, cefazolin sodium, cephalothin sodium, and pentobarbital sodium. Compatibility is dependent upon factors such as pH, concentration, temperature and diluents used. It is suggested to consult specialized references for more specific information (e.g., Handbook on Injectable Drugs by Trissel; see bibliography).

 

Pharmacology – At the H2 receptors of the parietal cells, cimetidine 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 cimetidine. By decreasing the amount of gastric juice produced, cimetidine also decreases the amount of pepsin secreted.

 

Cimetidine has an apparent immunomodulating effect as it has been demonstrated to reverse suppressor T cell-mediated immune suppression. It also possesses weak anti-androgenic activity.

 

Uses/Indications – In veterinary medicine, cimetidine has been used for the treatment and/or prophylaxis of gastric, abomasal and duodenal ulcers, uremic gastritis, stress-related or drug-induced erosive gastritis, esophagitis, duodenal gastric reflux and esophageal reflux. It has also been employed to treat hypersecretory conditions associated with gastrinomas and systemic mas­tocytosis. Cimetidine has also been used investigationally as a immunomodulating agent (see doses) in dogs.

 

Pharmacokinetics –  Pharmacokinetic data for veterinary species is limited for this agent. In dogs, the oral bioavailability is reported to be approximately 95%, serum half-life is 1.3 hours and volume of distribution is 1.2 L/kg.

 

In humans, cimetidine is rapidly and well absorbed after oral administration, but a small amount is metabolized in the liver before entering the systemic circulation (first-pass effect). The oral bioavailability is 70-80%. Food may delay absorption and slightly decrease the amount absorbed, but when given with food, peak levels occur when the stomach is not protected by the buffering capabilities of the ingesta.

 

Cimetidine is well distributed in body tissues and only 15-20% is bound to plasma proteins. The drug enters milk and crosses the placenta.

 

Cimetidine is both metabolized in the liver and excreted unchanged by the kidneys. More drug is excreted by the kidneys when administered parenterally (75%) than when given orally (48%). The average serum half-life is 2 hours in humans, but can be prolonged in elderly patients and in those with renal or hepatic disease. Peritoneal dialysis does not appreciably enhance the removal of cimetidine from the body.

 

Contraindications/Precautions – Cimetidine is contraindicated in patients with known hypersensitivity to the drug.

 

Cimetidine should be used cautiously in geriatric patients and in patients with significantly impaired hepatic or renal function. In humans meeting these criteria, increased risk of CNS (confusion) effects may occur; dosage reductions may be necessary.

 

Adverse Effects/Warnings – Adverse effects appear to be very rare in animals at the dosages generally used. Potential adverse effects (documented in humans) that could be seen, include mental confusion, headache (upon discontinuation of the drug), gynecomastia and decreased libido. Rarely, agranulocytosis may develop and if given rapidly IV, transient cardiac arrhythmias may be seen. Pain at the injection site may be noted after IM administration.

 

Cimetidine does inhibit microsomal enzymes in the liver and may alter the metabolic rates of other drugs (see Drug Interactions below).

 

Overdosage – Clinical experience with cimetidine overdosage is limited. In laboratory animals, very high dosages have been associated with tachycardia and respiratory failure. Respiratory support and beta-adrenergic blockers have been suggested for use should these symptoms occur.

 

Drug Interactions – Cimetidine may inhibit the hepatic microsomal enzyme system and thereby reduce the metabolism, prolong serum half-lives, and increase the serum levels of several drugs. It may also reduce the hepatic blood flow and reduce the amount of hepatic extraction of drugs that have a high first-pass effect. The following drugs may be affected: beta-blockers (e.g., propranolol), lidocaine, calcium channel blockers (e.g., verapamil), diazepam (and other ben­zodiazepines), ethanol, metronidazole, phenytoin, quinidine, theophylline, and warfarin. Dosage adjustment or increased therapeutic monitoring may be necessary.

Cimetidine may decrease the renal clearance of procainamide. Cimetidine may exacerbate leukopenias when used with other agents that can cause this problem. Stagger doses (separate by 2 hours if possible) of cimetidine with antacids, metoclopramide, sucralfate, digoxin, and ketoconazole.

 

Drug/Laboratory Interactions – Cimetidine may cause small increases in plasma creatinine concentrations early in therapy. These increases are generally mild, non-progressive, and have disappeared when therapy is discontinued. Histamine2 blockers may antagonize the effects of histamine and pentagastrin in the evaluation gastric acid secretion. After using allergen extract skin tests, histamine2 antagonsits may inhibit histamine responses. It is recommended that histamine2 blockers be discontinued at least 24 hours before performing either of these tests.

 

Doses –

Horses:

For foals:

a)   1000 mg divided bid or tid PO, IV or IM (Robinson 1987)

b)   300 – 600 mg PO or IV 4 times a day (Clark and Becht 1987)

 

Monitoring Parameters –

1)   Clinical efficacy (dependent on reason for use); monitored by decrease in symptomatology, endoscopic examination, blood in feces, etc.

2) Adverse effects if noted

 

Client Information – To maximize the benefit of this medication, it must be administered as prescribed by the veterinarian; symptoms may reoccur if dosages are missed.

 

Dosage Forms/Preparations/FDA Approval Status/Withholding Times –

 

Veterinary-Approved Products: None

 

Human-Approved Products:

Cimetidine Tablets 100 mg, 200 mg, 300 mg, 400 mg, 800 mg; Tagamet®  HB (SKBeecham) (OTC); Tagamet® (SK-Beecham) (Rx); generic (Rx)

 

Cimetidine HCl Liquid 300 mg (as HCl) per 5 ml; Tagamet® (SK-Beecham) (Rx); Cimetidine Oral Solution® (Barre-National) (Rx)

 

Cimetidine HCl for Injection 150 mg/ml in 2 ml vials and 8 ml multiple-dose vials and 8 ml vials; 300 mg (as HCl) in 50 ml 0.9% sodium chloride in premixed single-dose containers; Tagamet® (SK-Beecham) (Rx);Cimetidine® (Endo) (Rx)