Can my boss openly ask me if I’m on medication?

If you have a *disability*--a medical condition which is long-lasting or permanent, and which causes significant impairment of life activities--and were not acting in a way which caused problems, then the comment was likely (or at least reasonably likely) to be illegal disability-based harassment. A single incident of such harassment is not, as a practical matter, going to result in much, if any compensation, if you also did not suffer some adverse job impact (e.g. fired, suspended, denied a pending promotion, etc.), but if it is part of a pattern, you may have a viable discrimination case and should contact the federal EEOC.
If you don't have a disabilty and this simply an odd, insulting comment, it is legal: if you're not being discriminated against due to a protected condition or category (like having a disability), your employer may legally harass, insult, belittle, etc. you. Only harassment aimed at you because of a category or condition specifically protected under law is illegal.
Or if you have a disability but were acting a work in a way causing problems (e.g. falling asleep; cognition impaired; emotionally volitile) which was unusual for you, then this would be  a legitimate question--employers can inquire about potential issues affecting you at work--and there would be no liability.