Buffy Sainte-Marie, Cod'ine
Hoyt Axton, The Pusher
Dave Van Ronk, Cocaine Blues
Velvet Underground, Heroin
Warren Zevon, Carmelita
Harry 'The Hipster' Gibson, Who Put the Benzedrine in Mrs Murphy's Ovaltine?
Dubiously classified as drug songs:
Peter, Paul, and Mary, Puff the Magic Dragon
Doors, The Crystal Ship
Tim Hardin, Red Balloon. Volume is poor, so try the Small Faces version.
Donovan, Mellow Yellow. Supposedly about cigarettes filled with dried banana peels. I tried one of these mellow yellow joints in Hollywood, Cal, in '67. It had no psychoactive effect I could discern.
Beatles, Lucy in the Sky with Diamonds. Supposedly about LSD.