Sounds like a
ritalin generic.
Methylphenidate is the generic name for Ritalin. I take these for ADD. The pill is a disc shape with the top/bottom surfaces tapered on all four sides front and back. "M" is on one side, "10" is on the other side, and there are two dashes at the top and bottom side with the "10."
These pills are a controlled substance because they are a stimulant. It is common for students in high school or college to buy or sell these pills to their classmates for academic performance enhancement, increased energy, or weight loss (one of the common side effects is decreased appetite).
I hope this helps. Good luck!