Irreversible, nonselective monoamine oxidase (MAO) inhibitors have been reported adversely to interact with indirectly acting sympathomimetic amines present in many cough and cold medicines. This study investigated the safety and tolerability of concomitant administration to 12 healthy subjects of both genders (aged 19-36 years) of ephedrine and moclobemide, a reversible MAO-A inhibitor. A 2-day, randomized, crossover administration of placebo or ephedrine (two doses of 50 mg with a 4-h interval) was followed by 9 days open-label dosing with moclobemide, 300 mg b.i.d.. On the last 2 days of moclobemide dosing, the randomized crossover treatment of ephedrine and placebo was repeated. No subject was withdrawn from the study for tolerability reasons. Moclobemide treatment, however, increased the incidence of adverse events elicited by ephedrine, particularly palpitations and headache. The pharmacodynamic interaction between the two drugs was quantified by calculation of the area under the effect-time course (AUE) for systolic (SBP) and diastolic blood pressure (DBP) and heart rate (HR). The difference in AUE between monotreatment with ephedrine and placebo was statistically significant for all three vital signs. Moclobemide potentiated the effect of ephedrine by a median factor of 3.2 for SBP, 3.8 for DBP, and 0.6 for HR. Ephedrine had no significant influence on the plasma concentrations of moclobemide or its metabolites. In conclusion, the combined use of moclobemide and high doses of sympathomimetic drugs should be approached with caution.