I don't think most drug legalization types intend to advocate for unrestricted access to antibiotics, but it's certainly worth discussing.

antibiotics are less effective for everyone the more they are used, so you can conceptualize the effectiveness of antibiotics as a common resource that can be spent down. if I abuse antibiotics, I am directly harming you.

there are arguments that drug abuse also harms others, but I find them somewhat less compelling. usually they follow the pattern of "I feel compelled to help you in X situation, so you are harming me by putting yourself in X situation". not an entirely illegitimate argument, but much more debatable imo. there's also the common "we have to protect people from their own poor choices" which I consider incompatible with free society.