In previous separate studies, dexfenfluramine (DF) and ephedrine/caffeine (EC) have been shown to promote weight loss in obese patients as compared with placebo. In order to compare the efficacy and safety of these two anorectic drugs, 103 patients with 20-80% overweight were included in a 15-week double-blind study in general practice. Patients were randomized to either 15 mg DF twice daily (n = 53), or 20 mg/200 mg ephedrine/caffeine three times a day (n = 50), supplementary to a 5 MJ/day diet. Forty-three patients from the DF group and 38 from the EC group completed the study. After 15 weeks of treatment, the DF group (n = 43) had lost 6.9 +/- 4.3 kg and the EC group (n = 38) had lost 8.3 +/- 5.2 kg (mean +/- s.d., P = 0.12). In the subgroup of patients with BMI > or = 30 kg/m2 (n = 59), the mean weight loss was 7.0 +/- 4.2 kg in the DF group (n = 29) and 9.0 +/- 5.3 kg in the EC group (n = 30), P < 0.05. Both systolic and diastolic blood pressures were reduced similarly during both treatments. Twenty-three patients in the DF group (43%) and 27 in the EC group (54%) complained of side-effects. Central nervous system side-effects, especially agitation, were more pronounced in the EC group (P < 0.05), whereas gastro-intestinal symptoms were more frequent in the DF group (P < 0.05). The side-effects declined markedly during the first month of treatment in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)