Many anti-panic drugs, administered chronically, can block pharmacologically-induced "panic attacks"; acutely they often exacerbate panic disorder. Theories of action need to account for this biphasic effect. Chronic inositol had previously shown efficacy against panic disorder. The authors investigated the effect of a single dose of 20 g inositol on an m-CPP challenge in a double-blind placebo-controlled crossover trial in panic-disorder patients. Seven patients had robust psychological, physiological and endocrine responses to 0.08 mg m-CPP i.v.; inositol had virtually no effect on these responses, although it had some acute effects during the evening before the challenge. A similar trial involving chronic inositol would be of interest.