Cimetidine dosage

Seaman JJ, Randolph WC, Peace KE, Frank WO, Dickson B, PutteimanK, Young MD. Effects of two cimetidine dosage regimens on serum theophylline levels. Postgrad Med Custom Comm (1985) 78, 47-53. 

When carbamazepine is administered with primidone, decreased primidone levels and higher carbamazepine serum levels may result. Cimetidine administered with carbamazepine may result in an increase in plasma levels of carbamazepine that can lead to toxicity. Blood levels of lamotrigine increase when the agent is administered with valproic acid, requiring a lower dosage of lamotrigine... 

The effects of buspirone are decreased when the drug is administered with fluoxetine Increased serum levels of buspirone occur if the drug is taken with erythromycin or itraconazole Should any of these combinations be required, the dosage of buspirone is decreased to 2.5 mg BID, and the patient is monitored closely. Venlafaxine blood levels increase with a risk of toxicity when administered witii MAOIs or cimetidine There is an increased risk of toxicity when trazodone is administered with the phenothiazines and decreased effectiveness of trazodone when it is administered with carbamazepine Increased serum digoxin levels have occurred when digoxin is administered with trazodone There is a risk for increased phenytoin levels when phenytoin is administered witii trazodone... 

Factors that may decrease theophylline clearance and lead to reduced dosage requirements include advanced age, bacterial or viral pneumonia, heart failure, liver dysfunction, hypoxemia from acute decompensation, and use of drugs such as cimetidine, macrolides, and fluoroquinolone antibiotics. 

Only a few well-documented drug combinations with phenytoin may necessitate dosage adjustment. Coadministration of the following drugs can result in elevations of plasma phenytoin levels in most patients cimetidine, chloramphenicol, disulfiram, sulthiame, and isoniazid (in slow acetylators). Phenytoin often causes a decline in plasma carbamazepine levels if these two drugs are given concomitantly. 

Inhibitors of the renal cation secretion mechanism, eg, cimetidine, prolong the half-life of dofetilide. Since the QT-prolonging effects and risks of ventricular proarrhythmia are directly related to plasma concentration, dofetilide dosage must be based on the estimated creatinine clearance. Treatment with dofetilide should be initiated in hospital after baseline measurement of the rate-corrected QT interval (QTC) and serum electrolytes. A baseline QTC of > 450 ms (500 ms in the presence of an intraventricular conduction delay), bradycardia of < 50 bpm and hypokalemia are relative contraindications to its use. 

The risk of development of tolerance and dependence with extended use of zolpidem appears to be less than with the use of hypnotic benzodiazepines. Zolpidem is rapidly metabolized to inactive metabolites by the liver via oxidation and hydroxylation. The elimination half-life of the drug is 1.5-3.5 hours, with clearance decreased in elderly patients. Dosage reductions are recommended in patients with hepatic dysfunction, in elderly patients, and in patients taking cimetidine. Rifampin, an inducer of hepatic cytochrome P450, decreases the half-life of zolpidem. 

This test may be used with the chemical, tablet, or liquid dosage form containing cimetidine or cimetidine hydrochloride. 

Cimetidine Potentiation (144) not documented for phenprocoumon (145) Stereoselective inhibition of warfarin metabolism (146) Adjust dosage... 

As the induction of hepatic microsomal oxidative activity by a lipid-soluble drug (e.g. phenobarbitone) or xenobiotic could decrease the duration of action of therapeutic agents that are mainly eliminated by microsomal oxidation, the effect of induction would be considered a form of biochemical antagonism. Drug-induced inhibition of microsomal oxidative activity, without adjustment of dosage of a concomitantly administered therapeutic agent that undergoes extensive hepatic metabolism, could lead to toxicity. Cimetidine, ketoconazole and chloramphenicol inhibit hepatic microsomal enzyme activity. 

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