Elvis' fondness for prescription drugs had begun back in the early Sixties (although at least one confidant claims Elvis began by stealing diet pills from Gladys, his mother). Facing a punishing work schedule set up by his manager, "Colonel" Tom Parker, Presley began to use "uppers" to get him going in the morning and "downers" to help him relax and sleep at night. By the early Seventies, Elvis had come to rely on these pills as necessary equipment for his hectic career, especially since Parker's schedule now had him working like a dog: an average of one show every other day from 1969 until June 1977 and a three-album-a-year schedule for RCA.
In order to get these prescriptions, Elvis needed doctors, and there were many in Los Angeles, Vegas, Palm Springs, and Memphis who were happy to help the wealthy star out. When he visited a doctor (or dentist), Elvis would almost inevitably talk him into a prescription, usually for painkillers. Eventually, Elvis took to carrying around a copy of The Physician's Desk Reference -- an encyclopedia of legal drugs and their uses -- so that he knew just what to ask for and, when necessary, which symptoms to fake.
Elvis actually had near-fatal overdoses at least twice in the Seventies and was admitted to hospitals for "exhaustion" -- that is, detoxification.