Discussion on answer by Daniel: What can I say to coworkers after taking a day off for mental health, if I don't wish to reveal the mental aspect of it?

@Daniel then you have sensible co-workers and you are lucky! (I do too.) But some people take this sort of thing as a challenge - "have you tried chiropractic? Have you tried a gluten-free diet?" and if OP 's aren't willing to leave it at "I was sick", they may be the latter kind.