Yes you can drink the leftover baby formula as an adult.
However when I tried doing that the formula was so disgusting that I could not get more than a few sips of the baby formula down.
If the baby had better taste buds and could taste that formula they would refuse to drink it because it's nasty tasting stuff.
So you would likely not want to even drink that leftover formula because formula doesn't taste like regular milk does because of the ingredients that are put into the formula to help a growing baby get the needed nutrients to grow and survive.
I also had a stomach ache for the rest of the day after drinking a bit of baby formula so I will never try drinking any baby formula again.