[17] The most serious potential side effect of metformin use is
lactic acidosis; this complication is very rare,
and the vast majority of these cases seem to be related to comorbid conditions, such as impaired liver or kidney function, rather than to the metformin itself.[52]
Metformin has also been reported to decrease the blood levels of thyroid-stimulating hormone in people with hypothyroidism,[53] and, in men, testosterone.