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

The launch of Sativex, which is made from cannabis plants grown at secret locations in England by developer GW Pharmaceuticals, follows the first fully-fledged marketing authorization for a drug made from "weed" anywhere in the world, and offers legal access to the beneficial effects of an illegal drug thousands of multiple sclerosis sufferers have smoked in an attempt to relieve their pain.

It is also a shot in the arm 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 Friday approved by the U.K. Medicines and Healthcare products Regulatory Agency (MHRA) as a treatment for relieving spasms and cramps in multiple sclerosis patients who are not benefiting from other treatments.

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

Pam Macfarlane, Chief Executive of the MS Trust, a charity that has campaigned for a licensed medicine derived from cannabis to be made available to multiple sclerosis sufferers, said the launch of Sativex represents a milestone in treating the disease.

The charity has long known cannabis helped patients' symptoms, Macfarlane said. GW Pharmaceuticals Managing Director Justin Gover told Dow Jones Newswires roughly a third of multiple sclerosis patients are believed to have tried it to get relief, with some surveys suggesting the proportion is as high as 43 percent.

Bayer said it estimates about 11,500 people in the U.K. will be eligible for treatment with Sativex and about half of these will get a good response.

Spain is expected to be the next country to approve Sativex for sale and GW and its Europe-wide marketing partner, Almirall SA, will be seeking approval in countries including Germany, Italy and France later this year, Gover said.

KBC Peel Hunt analyst Paul Cuddon said he estimates Sativex sales could reach £50 million at their peak in Europe.

At 9:49am London time, shares listed in the U.K. for GW, which gained 56 percent since the beginning of the year and were 80 percent up on year-ago levels, were down five pence or 3.6 percent at 136 pence.

Nomura Code Securities analyst Samir Devani said the product's approval and launch was well publicized. He said Sativex's prospects in treating multiple sclerosis patients was limited, and added trials of the drug as a painkiller for cancer patients — potentially a bigger market opportunity — were much more risky.

Click here to read more from NewsCore.