hair of the dog that bit you

Whatever made you ill used as a remedy, especially alcohol as a hangover cure. For example, A little hair of the dog will cure that hangover in no time. This expression, already a proverb in John Heywood's 1546 compendium, is based on the ancient folk treatment for dogbite of putting a burnt hair of the dog on the wound. It is often shortened, as in the example.

More alcohol to counteract the effects of a hangover. Most ancient civilizations and many modern ones believed that the most effective cure for a dog bite was a bundle of the canine's hair tied inside or around the wound. On the same theory that the injury's cause could also be the cure came the metaphor for taking a drink of alcohol to lessen, if not eliminate, the discomfort of “the morning after.” There's some truth to the idea: a Bloody Mary, Screwdriver, and another alcoholic beverage will take the edge off your hangover by making you drunk (or drunker) again, but alas, another slug of the demon rum won't cure the underlying problem, and at some point the piper will have to be paid.