Depends on the context
The greek letter Mu µ is often used to indicate micro or 10^-6. Due to the difficulty of printing the letter µ often a lower case u is often substituted. While mg is milligram ug µg or would indicate microgram.
U can mean unit as in units of insulin.
U can part of an Latin abbreviation such as U. D. for "ut dictum" or "as directed.
That is all I can think of off the top of my head anyway. There may be other things U stands for in medicine.
It's just a u, nothing else?
