Drug Interaction Report

ADJUST DOSING INTERVAL: Systemic antihistamines may diminish the response to the trichophyton diagnostic skin test and lead to falsely insignificant or false-negative results. The proposed mechanism is inhibition of histamine release in the skin leading to suppression of the wheal and flare response. This interaction has been documented in numerous studies examining allergenic extracts and different antihistamines, but clinical data with the trichophyton diagnostic skin test are not readily available.

MANAGEMENT: The labeling for the trichophyton skin test recommends avoiding use of antihistamines for at least 24 hours prior to skin testing, though longer may be required depending on the specific antihistamine in question. Consultation with the labeling of the individual antihistamine as well as any relevant local and/or national guideline(s) is advisable.

H2 antagonists may reduce the clearance of nicotine. Cimetidine, 600 mg given twice a day for two days, reduced clearance of an intravenous nicotine dose by 30%. Ranitidine, 300 mg given twice a day for two days, reduced clearance by 10%. The clinical significance of this interaction is not known. Patients should be monitored for increased nicotine effects when using the patches or gum for smoking cessation and dosage adjustments should be made as appropriate.