The Twilight Zone - Season 3 Episode 7

An alcoholic, Miley Judson, is approached by a doctor who offers to provide him with a unique cure. Desperate, Judson agrees and is secretly given a "hellgramite" insect. The insect in his stomach will keep him sober no matter how much he drinks, and will feed on any alcohol he does drink. The more Judson drinks, the larger it will get until it is fatal. Judson can stop drinking, but if he does the insect will force him through terrible withdrawal pains until it goes into hibernation. Judson manages to make it, and offers a card for the doctor to another alcoholic buddy.