Objective: To investigate the efficacy and safety of increasing doses of cathine (nor-pseudoephedrine) as a weight-lowering agent in patients with obesity. Methods: Overweight and obese patients (n = 241, mean BMI 34.6 ± 3.4 kg/m²) were randomly allocated to one of three doses of cathine (16 mg, 32 mg, 53.3 mg) or placebo in addition to a multimodal lifestyle intervention program in a multicenter, double-blind, controlled, dose-finding study for 24 weeks. Primary outcome was weight loss. Results: Treatment with the 3 doses of cathine resulted in a significantly greater weight loss compared to placebo over 24 weeks: 6.5 ± 4.2 kg for 16 mg cathine, 6.2 ± 4.7 kg for 32 mg cathine, and 9.1 ± 5.4 kg for 53.3 mg cathine versus 2.4 ± 4.4 kg for placebo (each p < 0.01, ANCOVA). The percentage of patients losing > 5% / >10% of initial body weight was significantly greater for all doses of cathine than for placebo (each p < 0.01, chi-square test). Heart rate increased dose-dependently (by 1.2 bpm under 16 mg, 5.8 bpm under 32 mg, and 6.2 bpm under 53.3 mg cathine), but no suspected unexpected serious adverse reactions were noted. The overall dropout rate was 24.9%, with the highest rate in the placebo group (42.3%). Conclusion: Cathine appears to be an effective weight-lowering agent for adjunct treatment of obesity, but additional clinical studies on its efficacy and safety are required.