What's the evidence for metformin in the absence of diabetes?

Dr Michael Tam details the case of a 25-year-old  patient with well-controlled bipolar affective disorder

A 25-year-old man with bipolar affective disorder saw me recently. 

His symptoms were well controlled with quetiapine (Seroquel), but I noticed his psychiatrist had prescribed metformin. The patient had neither diabetes nor pre-diabetes.  

I wondered about the evidence that supported this practice. 

Clinical question 

What is the effect of metformin on cardiovascular disease (CVD) in people without diabetes on long-term antipsychotic medication? 

The research evidence

Step 1: The