The alleged interaction between
MAOIs and most
antihistamines appears to be based on a single animal study, and is probably more theoretical than real. The exception seems to be cyproheptadine and promethazine (see below).
The UK manufacturers of most of the sedating
antihistamines (alimemazine, brompheniramine, chlorphenamine, diphenhydramine) state that
MAOIs may intensify the antimuscarinic effect of
antihistamines, and many contraindicate or caution concurrent use.