The Use of Phentermine for Obesity in Psychiatric Patients With Antipsychotics

OBJECTIVE: Phentermine is a commonly used weight-loss agent in the United States, but there is a little information about the use of phentermine for patients with obesity taking antipsychotic medications. METHODS: We gathered 57 patients with obesity taking antipsychotic medications whose phentermine treatment was simultaneous with or after any type of antipsychotic exposure and collected data of clinical information, initial/follow-up anthropometric variables, and adverse events (AEs) for the 6-month study period. RESULTS: In total, the mean body weight reduction (BWR) was 4.45 (7.04) kg, and the mean BWR percent (BWR%) was 3.92% (6.96%) at 6 months. Based on the response to phentermine, the patients were classified into two groups: the responder (n=25; BWR% ≥5%) and nonresponder (n=32; BWR% <5%) groups. The responder group’s mean BWR and BWR% were 10.13 (4.43) kg and 9.35% (4.09%), respectively, at 6 months. The responders had higher rates of anticonvulsant combination therapy (ACT; responder, 72.0% vs. non-responder, 43.8%; p=0.033) and a shorter total antipsychotic exposure duration (responder, 23.9 [16.9] months vs. non-responder, 37.2 [27.6] months; p=0.039). After adjusting age, sex, and initial body weight, ACT maintained a significant association with phentermine response (odds ratio=3.840; 95% confidence interval: 1.082–13.630; p=0.037). In the final cohort, there was no report of adverse or new-onset psychotic symptoms, and the common AEs were sleep disturbances, dry mouth, and dizziness. CONCLUSION: Overall, phentermine was effective and tolerable for patients with obesity taking antipsychotic medications, and ACT (predominantly topiramate) augmented the weight-loss effect of phentermine.