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.

GENERALLY AVOID: Some beverages such as tonic water contain varying amounts of quinine. Coadministration of a single 430 mg dose of quinine has been shown to increase plasma concentrations of astemizole and its metabolite, desmethylastemizole. Elevated levels of these agents may cause a prolongation of the electrocardiographic QT interval and potentially fatal ventricular arrhythmias. Although pharmacokinetic data have indicated that the amounts of quinine in beverages (up to 80 mg quinine in 32 oz of tonic water) are not sufficient to produce a significant effect, the potential for an interaction exists if large amounts of tonic water are ingested. Also, grapefruit juice has been shown to inhibit CYP450 enzymes, which may lead to increased serum astemizole concentrations. The risk of life-threatening ventricular arrhythmias may be increased.

MANAGEMENT: Patients should be counseled to limit consumption of quinine-containing beverages and avoid grapefruit juice while they are taking astemizole.