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Cimetidine

Cimetidine is a histamine H₂-receptor antagonist that inhibits the production of acid in the stomach. It is largely used in the treatment of heartburn and peptic ulcers. It is marketed by GlaxoSmithKline under the trade name Tagamet®, and was approved by the Food & Drug Administration for prescriptions starting January 1, 1979. more...

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History and development

Cimetidine was the prototypical histamine H₂-receptor antagonist from which the later members of the class were developed. Cimetidine was the culmination of a project at Smith, Kline & French (SK&F; now GlaxoSmithKline) to develop a histamine receptor antagonist to suppress stomach acid secretion.

At the time (1964) it was known that histamine was able to stimulate the secretion of stomach acid, but also that traditional antihistamines had no effect on acid production. In the process, the SK&F scientists also proved the existence of histamine H₂-receptors.

The SK&F team used a rational drug-design structure starting from the structure of histamine - the only design lead, since nothing was known of the then hypothetical H₂-receptor. Hundreds of modified compounds were synthesised in an effort to develop a model of the receptor. The first breakthrough was N^α-guanylhistamine, a partial H₂-receptor antagonist. From this lead the receptor model was further refined and eventually led to the development of burimamide - the first H₂-receptor antagonist. Burimamide, a specific competitive antagonist at the H₂-receptor 100-times more potent than N^α-guanylhistamine, proved the existence of the H₂-receptor.

Burimamide was still insufficiently potent for oral administration and further modification of the structure, based on modifying the pKa of the compound, lead to the development of metiamide. Metiamide was an effective agent, however it was associated with unacceptable nephrotoxicity and agranulocytosis. It was proposed that the toxicity arose from the thiourea group, and similar guanidine-analogues were investigated until the ultimate discovery of cimetidine.

Shortcomings

Cimetidine is a known inhibitor of many isozymes of the cytochrome P450 enzyme system (specifically CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4). This inhibition forms the basis of the numerous drug interactions that occur between cimetidine and other drugs. For example, cimetidine may decrease metabolism of some drugs, such as oral contraceptives.

Adverse drug reactions were also found to be relatively common with cimetidine.

The development of longer-acting H₂-receptor antagonists with reduced adverse effects such as ranitidine proved to be the downfall of cimetidine and, whilst it is still used, it is no longer amongst the more widely used H₂-receptor antagonists.

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Cimetidine and hot flashes
From Nursing, 2/1/00 by Shuster, Joel

WHY DID THIS MAN

DEVELOP NU990PAUSAL SYMPTOM?

A 31-YEAR-OLD MAN with end-stage renal disease started taking 400 mg of cimetidine (Tagamet) daily without his physician's knowledge. Some days, he took a second 400-mg dose of cimetidine. The man had been on hemodialysis for 9 years and also took calcium carbonate.

About a month after starting the cimetidine, he noticed episodes of fluttering, racing heartbeats followed by hot flashes, increased nervousness, sweating, agitation, and shortness of breath. These episodes happened during warm weather and usually lasted 5 to 10 minutes each, although some episodes lasted hours. The frequency ranged from 2 to 20 times per day, When he stopped taking cimetidine, the episodes stopped.

What went wrong?

A similar case of symptoms in a perimenopausal woman on cimetidine has been reported. This is the first report of such a problem in a male patient and in a patient on renal dialysis.

Antacids such as calcium carbonate can interfere with cimetidine absorption, so the two drugs should be taken at least an hour apart. Hemodialysis can reduce blood levels of cimetidine, so it should be taken after dialysis. It's not clear when the patient in this report was taking the drug in relation to his dialysis treatments and his other medications.

In patients with renal dysfunction, cimetidine's half-life is increased from the normal 2 hours. Also, the drug is distributed to many body tissues, where it may accumulate in patients with impaired renal function.

What precautions can you take?

Tell patients to inform their health care providers of all over-the-counter medications they're taking.

Patients on dialysis should take a lower dosage of cimetidine-300 mg P.O. or IN. every 8 to 12 hours, after dialysis. Patients with renal failure also should take a lower dose to prevent the drug from accumulating and to maintain steady blood levels of the drug.

The dose may need to be reduced further if the patient also has hepatic failure.

Sources: "Cimetidine-induced Climacteric Symptoms in a Young Man Maintained on Chronic Hemodialysis." Nephrology, 18:538-540, B. Bastani, et aL, 1998. Nursing2000 Drug Handbook, Springhouse Corp., 2000. Nurse Practitioner's Drug Handbook, 2nd edition. Springhouse Corp., 1998.

Dr. Shuster is clinical associate professor, Temple University, and clinical pharmacist, Medical College of Pennsylvania Hospital, both in Philadelphia, Pa. Send examples of adverse drug reactions to: Joel Shuster, PharmD, The Institute for Safe Medication Practices, 300 W. Street Rd., Warminster, PA 18974. Fax: 215-956-9266. If we publish your item (anonymously), we'll pay you $25.

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