LONDON—A drug manufactured from cannabis went on sale in the U.K. Monday as a treatment for symptoms of multiple sclerosis.

Sativex, which is made from cannabis plants grown at secret locations in England by developer GW Pharmaceuticals PLC, is the first prescription drug made from cannabis to officially go on sale anywhere in the world. It offers legal access to the beneficial effects of an illegal drug that thousands of multiple-sclerosis sufferers have smoked in an attempt to relieve their pain.

It...