Plasma fenfluramine levels, weight loss and side effects: a failure to find a relationship.

Thirty-six women were treated for 16 weeks with fenfluramine and behavior therapy. There was a strong correlation between drug dosage and plasma levels of fenfluramine. In contrast to two previous reports, however, there was no correlation between plasma levels of fenfluramine and its metabolite norfenfluramine and weight loss, nor was there any correlation between these levels and side effects. If weight loss is not related to drug levels, as suggested by this report, there is no merit in increasing drug levels to tolerance as is the current practice; modest dosage to produce modest drug levels is indicated.