From the Guidelines

Yes, metformin mitigates antipsychotic-induced weight gain.

Metformin has been shown to counteract the effects of weight gain-promoting agents, particularly antipsychotics, with a mean difference of −3.27 kg (95% CI, −4.66 kg to −1.89 kg) 1.
The medication is associated with approximately 3% weight loss, and approximately 25% to 50% of participants achieve at least 5% weight loss in RCTs and prospective studies 1.
Metformin's pleiotropic effects include decreased inflammation, increased insulin and leptin sensitivity, and decreased hunger and ghrelin levels, especially with twice-daily dosing 1.

A total daily dose of 1000 mg of metformin is effective in mitigating antipsychotic-induced weight gain 1.
Doses greater than 1500 mg are associated with the greatest weight loss 1.

Metformin is widely available and inexpensive, making it a viable option for patients with antipsychotic-induced weight gain 1.
Although not FDA approved for this indication, metformin is frequently offered to patients to mitigate weight gain due to antipsychotic medication 1.

From the FDA Drug Label

The drug label states: "Mean change in body weight from baseline to week 29 was 0.9 lbs, -0.7 lbs, and -8.4 lbs in the metformin hydrochloride tablets/glyburide, glyburide, and metformin hydrochloride tablets arms, respectively."
It also states: "Mean change in body weight from baseline to week 16 was -3.3 lbs and -2.0 lbs in the metformin hydrochloride tablets and placebo arms, respectively."
Metformin may help with weight loss or mitigate weight gain as shown by the negative change in body weight in the metformin arms. The FDA drug label does not directly answer the question regarding antipsychotic-induced weight gain 2.

Metformin has been shown to significantly attenuate weight gain in patients receiving antipsychotics, with a mean weight change of -3.32 kg [95% confidence interval (CI): -4.57 to -2.07] 3.
The use of metformin resulted in a marked decrease in body mass index (-1.24 kg/m2, 95% CI: -1.70 to -0.77) 3.
Metformin was found to be effective in treating antipsychotic-induced weight gain in patients with schizophrenia or schizoaffective disorder, with a mean change in weight of -3.27 kg (95% CI: -4.66 to -1.89) 4.
Co-commencement of metformin with antipsychotic medications was found to be effective in attenuating weight gain, with a mean weight change of -3.12 kg (95% CI: -4.22 to -2.01 kg) 5.

Metformin has been shown to improve metabolic parameters, including fasting glucose levels, total cholesterol, and total triglyceride levels 5.
The use of metformin was found to have a favorable safety profile, with no significant difference in adverse drug reactions (ADR) incidence between metformin and placebo groups 5.
Metformin may be considered as a secondary or tertiary intervention for patients with a personal and/or family history of obesity or metabolic dysfunction, and in subjects who have rapid weight gain early in antipsychotic treatment 6.
Further long-term studies are needed to extend the observations and improve the strategy of using metformin to mitigate antipsychotic-induced weight gain 7.

Metformin may be beneficial if started early in antipsychotic drug treatment 7.
Metformin has been shown to prevent or delay the onset of type 2 diabetes mellitus in high-risk individuals from the general population 7.
Antipsychotic drug-treated patients who might benefit from metformin therapy include those with a high risk of weight gain and metabolic problems 7.