Can metformin reduce muscle gains from training?

It can under some studied conditions. In the MASTERS randomized trial, older adults taking metformin gained less muscle with progressive resistance training than those taking placebo. This was measured over a training program, so it is stronger evidence than predicting gains from a single insulin or IGF-1 result.[1]

The participants and treatment context matter. The study does not prove the same effect size in young athletes, people taking multiple anabolic drugs, or every patient with diabetes. It also does not establish that metformin abolishes all exercise benefits. A change in one adaptation should be interpreted within the broader reason a medicine is prescribed.[1]

For someone who needs glucose-lowering treatment, this finding is a discussion point, not a reason to stop medication independently. For someone considering metformin solely as a bodybuilding or longevity add-on, it challenges the assumption that the drug can only help. The relevant balance includes glucose control, clinical need, tolerability, and training outcomes.[1]