Metronidazole
"I was told not to use drink alcohol with Flagyl, of course, but I also found I cannot use Purell (ethanol-based) or isopropyl rubbing alcohol with it. These led to terrible stomach pain and headache. One pharmacist told me this couldn't happen, not enough alcohol would get in, but another said Yes, it happens, and some people even have a reaction to alcohol in the slight fumes you get while pumping gasoline that includes alcohol. When I avoid Purell and isopropyl, I don't get those severe stomach pains."