It inhibits muscle growth. Also depletes folate, vitamin B12, etc. Not something you'd want to take unless you are diabetic. Use natural alternatives like cinnamon, Alpha Lipoic Acid (R-ALA form), Berberine, etc. However, anytime you preferentially activate the AMPK pathway, muscle growth will be inhibited to some degree. Depends on if you are trying to maximize fat loss or muscle growth.
"Metformin activates the AMPK pathway through LKB1, eventually causing inhibition of the mTOR pathway and thus a reduction in protein synthesis and cellular proliferation. Metformin also appears to indirectly reduce Akt activation, through AMPK- mediated phosphorylation of IRS-1, causing inhibition of mTOR pathway."