Obesity-related genetic variants, including TMEM18 (rs6548238), SH2B1 (rs7498665), and GNPDA2 (rs10938397), have been shown to be associated with obesity in the general population. Our study aimed to test whether these genetic variants are associated with metabolic profiles and metformin treatment response in clozapine-treated schizophrenic patients. We recruited 107 clozapine-treated patients who were genotyped and measured their metabolic profiles. Fifty-five subjects, who had at least 1 metabolic abnormality in a range of measures, were subsequently randomized to a 24-week trial of metformin (n = 28) or placebo (n = 27). We examined the associations between TMEM18, SH2B1, GNPDA2 genetic variants and metabolic profiles at baseline in all patients and metabolic changes in the trial groups. We found a significant association between SH2B1 and blood pressure at baseline in all patients. In the metformin group, TMEM18 minor allele carriers had a greater reduction in insulin levels (P = 0.04). A significantly higher proportion of TMEM18 and GNPDA2 minor allele carriers (60% and 40%) lost more than 7% of their body weight after metformin treatment as compared with their homozygous counterparts (21.7% and 15.4%, P = 0.02 and 0.004, respectively).There were trends toward favorable metabolic changes in minor allele carrier groups. In the placebo group, no association between genetic variants and changes in metabolic profiles was found. In conclusion, the study results suggest that these genes might be associated with metabolic abnormalities and response to metformin in clozapine-treated patients with schizophrenia.