IIRC metformin may also cause fertility problem in men, after prolonged use (which is required to get slow-aging effect).
You will lose your appetite and it will be hard to want to eat. You will tend to lose weight and eat just enough.
It can make you nauseous and you will probably find the bitter taste and fishy (amine) smell of the pills to be repulsive.
It is not an easy drug to take, and like you said, it may also affect male fertility.
But it is well-studied and safe, and offers various benefits.