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 is also a boon for small U.K. drug developers, which have struggled in recent years to win investor confidence—and funding—because of setbacks in clinical trials of their experimental medicines.

Sativex was approved Friday by the U.K. Medicines and Healthcare products Regulatory Agency, or MHRA, as a treatment for spasticity in multiple- sclerosis patients who aren't benefiting from other treatments. It had been in development for 11 years.

Sativex's approval follows successful clinical trials. The drug will be marketed in the U.K. by Germany's Bayer AG , which said Monday it will cost the country's state-run National Health Service roughly £11, or about $16, a day for each patient.