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: Concurrent use of ethanol and phenothiazines may result in additive CNS depression and psychomotor impairment. Also, ethanol may precipitate dystonic reactions in patients who are taking phenothiazines. The two drugs probably act on different sites in the brain, although the exact mechanism of the interaction is not known.

MANAGEMENT: Patients should be advised to avoid alcohol during phenothiazine therapy.