CIMETIDINE, CIMETIDINE HCL
Chemistry - An H2- receptor antagonist, cimetidine occurs as a white to off-white, crystallinepowder. 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.
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. Cimetidineis also reported to be compatible with the following drugs: acetazolamide sodium, amikacin sulfate, atropine sulfate, carbenicillin disodium, cefoxitin sodium, chlorothiazide sodium, clindamycinphosphate, 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, nafcillinsodium, norepinephrine bitartrate, penicillin G potassium/sodium, phytonadione, polymyxin Bsulfate, 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 specificinformation (e.g., Handbook on Injectable Drugs by Trissel; see bibliography).
Cimetidine has an apparent immunomodulating effect as it has been demonstrated to reversesuppressor 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/orprophylaxis 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 beenemployed to treat hypersecretory conditions associated with gastrinomas and systemic mastocytosis. Cimetidine has also been used investigationally as a immunomodulating agent (seedoses) in dogs.
In humans, cimetidine is rapidly and well absorbed after oral administration, but a small amount ismetabolized in the liver before entering the systemic circulation (first-pass effect). The oralbioavailability 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 bufferingcapabilities of the ingesta.
Cimetidine is well distributed in body tissues and only 15-20% is bound to plasma proteins. Thedrug enters milk and crosses the placenta.
Cimetidine is both metabolized in the liver and excreted unchanged by the kidneys. More drug isexcreted by the kidneys when administered parenterally (75%) than when given orally (48%). Theaverage serum half-life is 2 hours in humans, but can be prolonged in elderly patients and in thosewith renal or hepatic disease. Peritoneal dialysis does not appreciably enhance the removal ofcimetidine 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.
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 otherdrugs (see Drug Interactions below).
Overdosage - Clinical experience with cimetidine overdosage is limited. In laboratory animals, veryhigh dosages have been associated with tachycardia and respiratory failure. Respiratory support andbeta-adrenergic blockers have been suggested for use should these symptoms occur.
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 creatinineconcentrations early in therapy. These increases are generally mild, non-progressive, and havedisappeared when therapy is discontinued. Histamine2 blockers may antagonize the effects ofhistamine and pentagastrin in the evaluation gastric acid secretion. After using allergen extractskin tests, histamine2 antagonsits may inhibit histamine responses. It is recommended thathistamine2 blockers be discontinued at least 24 hours before performing either of these tests.
Storage, Stability, Compatibility
Cimetidine products should be stored protected from light andkept at room temperature. Do not refrigerate the injectable product as precipitation may occur. Oraldosage 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. Cimetidineis also reported to be compatible with the following drugs: acetazolamide sodium, amikacin sulfate, atropine sulfate, carbenicillin disodium, cefoxitin sodium, chlorothiazide sodium, clindamycinphosphate, 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, nafcillinsodium, norepinephrine bitartrate, penicillin G potassium/sodium, phytonadione, polymyxin Bsulfate, 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 specificinformation (e.g., Handbook on Injectable Drugs by Trissel; see bibliography).
Pharmacology - CIMETIDINE, CIMETIDINE HCL
At the H2 receptors of the parietal cells, cimetidine competitively inhibits histamine, thereby reducing gastric acid output both during basal conditions and when stimulated byfood, pentagastrin, histamine or insulin. Gastric emptying time, pancreatic or biliary secretion, andlower esophageal pressures are not altered by cimetidine. By decreasing the amount of gastric juiceproduced, cimetidine also decreases the amount of pepsin secreted.Cimetidine has an apparent immunomodulating effect as it has been demonstrated to reversesuppressor 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/orprophylaxis 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 beenemployed to treat hypersecretory conditions associated with gastrinomas and systemic mastocytosis. Cimetidine has also been used investigationally as a immunomodulating agent (seedoses) in dogs.
Pharmacokinetics - CIMETIDINE, CIMETIDINE HCL
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 volumeof distribution is 1.2 L/kg.In humans, cimetidine is rapidly and well absorbed after oral administration, but a small amount ismetabolized in the liver before entering the systemic circulation (first-pass effect). The oralbioavailability 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 bufferingcapabilities of the ingesta.
Cimetidine is well distributed in body tissues and only 15-20% is bound to plasma proteins. Thedrug enters milk and crosses the placenta.
Cimetidine is both metabolized in the liver and excreted unchanged by the kidneys. More drug isexcreted by the kidneys when administered parenterally (75%) than when given orally (48%). Theaverage serum half-life is 2 hours in humans, but can be prolonged in elderly patients and in thosewith renal or hepatic disease. Peritoneal dialysis does not appreciably enhance the removal ofcimetidine 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 dosagesgenerally used. Potential adverse effects (documented in humans) that could be seen, include mentalconfusion, 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 otherdrugs (see Drug Interactions below).
Overdosage - Clinical experience with cimetidine overdosage is limited. In laboratory animals, veryhigh dosages have been associated with tachycardia and respiratory failure. Respiratory support andbeta-adrenergic blockers have been suggested for use should these symptoms occur.
Drug Interactions
Cimetidine may inhibit the hepatic microsomal enzyme system and therebyreduce the metabolism, prolong serum half-lives, and increase the serum levels of several drugs. Itmay also reduce the hepatic blood flow and reduce the amount of hepatic extraction of drugs thathave 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 creatinineconcentrations early in therapy. These increases are generally mild, non-progressive, and havedisappeared when therapy is discontinued. Histamine2 blockers may antagonize the effects ofhistamine and pentagastrin in the evaluation gastric acid secretion. After using allergen extractskin tests, histamine2 antagonsits may inhibit histamine responses. It is recommended thathistamine2 blockers be discontinued at least 24 hours before performing either of these tests.