I used to work with a guy who bought veterinary antibiotics, cut them in half until they were a human dose, and took them whenever he felt a sniffle, and stopped when he felt better.

And he completely understood the concept of antibiotic resistance. He just gave a sheepish grin when I confronted him like "Aw, shucks, ain't I a scamp". He was a really nice guy, except for this case where he was a sociopathic asshole.