I'm not sure it's that easy to notice that the drug is doing damage to the mitochondria. Metformin is a hugely popular drug for Type 2 diabetes, yet of the 10-12 patients on this forum who mentioned trying it, none are still taking it. All but one stopped it due to it making them feel worse.
You can add me to that list. I have bad circulation in my left ankle after the titanium implant on my bone. My new doc wanted me back on metformin as my blood results were suddenly much worse. I was reluctant, explained there might be problems, but took two, one a day, before problems arose.
After the second one my left ankle got really itchy. I rubbed it, not really scratching it. A few hours later a dozen blisters formed, then the skin fell off, leaving a dozen fingernail sized ulcers. That was a month ago. They did not start to heal till I stopped taking the metformin. They are slowly healing, but its not even half done yet.
Metformin works in part by decreasing energy production including via pyruvate dehydrogenase, though I have not looked deeply at the literature on this. Its almost like doubling down on ME pathology. Its also known to slow wound healing.
If you have ME and are prescribed Metformin be prepared to stop taking them if you get side effects.