Background and aim: Young women are at high risk of weight gain but few weight management interventions have been investigated in this group. This study aimed to determine the effect of metformin on body weight, body composition, metabolic risk factors and reproductive hormone levels in overweight or obese young women compared to placebo and comprehensive lifestyle intervention. Methods and results: 203 overweight or obese young women (BMI 33.2+/-0.3 kg/m2, age 28+/-0.3 years) were randomised to 1500 mg/day metformin (M) plus general lifestyle advice, placebo (P) plus general lifestyle advice or comprehensive lifestyle intervention including structured diet, exercise and behavioural therapy (L) for 12-weeks. At 12-weeks, linear mixed models found that L group had greater weight loss (-4.2+/-0.4 kg) compared to M (-1.0+/-0.4 kg) and P groups (-0.2+/-0.3 kg) (P < 0.0001). Weight loss between M and P groups were not significantly different. Attrition rate was 48% for L, 34% for M and 29% for P (P = 0.08). Intention-to-treat analysis showed that 10% (8/79) of the subjects in P group had gained weight (>3%), compared to 3% (2/65) from M group and none (0/59) from L group (P < 0.001). The L group had the greatest decrease in waist circumference (-5.2+/-0.7 cm) and fat mass (-5.4+/-0.7 kg) compared to the other groups (P < 0.05). No significant time-by-group effects were seen in plasma lipids, SHBG, testosterone, blood pressure, serum folate, serum ferritin and serum vitamin B12. Conclusion: Lifestyle intervention was more effective in reducing body weight and improving body composition compared to metformin among healthy overweight or obese young women.