Increased circulating levels of liver enzymes emerging during treatment with psychotropic drugs are frequently encountered and, in general, attributed to drug metabolism or toxic effects. Because obesity was shown to be associated with elevated liver enzyme levels in different non-psychiatric study samples, we hypothesized that drug-induced weight gain might be an additional causative factor. We tested this hypothesis in 67 inpatients who received psychopharmacological treatment across five weeks. Stepwise linear regression was used to predict changes in the serum levels of aspartate-amino transferase (ASAT) and alanine-amino transferase (ALAT) by changes in the body mass index (BMI), by changes in other biological parameters related to body weight (tumor necrosis factor-alpha [TNF-alpha], soluble TNF receptors [sTNF-R], interleukin-6 [IL-6], leptin plasma levels) and by the respective liver enzyme baseline level. BMI changes from baseline to endpoint were significantly associated with the changes in ALAT and ASAT levels across five weeks of treatment and with ALAT and ASAT levels at the end point of the study. The baseline levels of ALAT and ASAT also had a significant impact on these liver enzyme level changes, whereas all other variables had not. These results suggest that weight gain-associated metabolic changes occurring during treatment with psychotropic drugs have consistent and clinically relevant effects on the liver.