http://www.deadiversion.usdoj.gov/schedules/schedules.htmSchedule I drugs are all illegal
There are five schedules, ranked in decreasing potential for abuse and therapeutic potential. Heroin is Schedule I. Oxycontin and Cocaine are Schedule II. OTC narcotic cough syrups and Lomotil are Schedule V. Two LSD precursors, lysergic acid and lysergic acid amide, are in Schedule III although bringing the amide out to diethylamide moves the product to Schedule I. Phentermine is schedule IV and codeine-based cough syrups with no more than a certain amount of codeine in them are schedule V (and OTC at that). It would be hard to get busted for possession of a Schedule V drug, but you better tell HR you're on it if you're in a Drug-Free Workplace.
When the National Lampoon did a drug-war parody on Lomotil, I thought it was a riot--the gist was that people were abusing Lomotil so they wouldn't have to waste time on the toilet, and one of the signs of Lomotil abuse was tickets to Mexico on your kid's nightstand. Little did I know it really is a drug of concern.