This is a vague imprint - the pill could be anything small.
I'm looking at a small round white tablet imprinted 100 on one side - nothing is on the backside. It's smaller in diameter than a regular M&M (sorry, I don't know how big a mini M&M is) and it's the same thickness all over the pill - it's not skinny on the edges and fatter in the middle like an M&M.
My tablet is an obscure brand of caffeine 100 mg - it's called Jet-Alert and is made by Bell Pharmaceuticals in Maine. If you aren't in the US, this is probably not your pill. And if you are in the US, it still might not be your pill!
Some of these imprints, especially when they only have a number, are very difficult to identify.
I'm not a pharmacist or a medical doctor. This message is not medical advice nor is it an offer to provide medical advice. All drug identifications should be validated by a licensed MD or pharmacist.