TORONTO, Ontario (AP) -- A cannabis-based painkiller for multiple sclerosis patients went on sale Monday in Canada, the first country to approve the spray derived from the marijuana plant.

Sativex can now be obtained by prescription through Canadian pharmacies, Bayer HealthCare announced Monday. Bayer markets the drug in Canada for British drug company GW Pharmaceuticals, which developed the drug.

Health Canada, the federal agency that oversees medical care for Canadians, announced in April it had approved Sativex, made from components derived from the cannabis plant that have been shown to ease pain.

Medical professionals welcomed the availability.

"Effective pain control and management are extremely important in a disease like MS," said Dr. Allan Gordon, a neurologist and director of the Wasser Pain Management Center at Mount Sinai Hospital in Toronto. "The availability of Sativex addresses the great demand for an effective treatment option in the field of neuropathic pain in MS."

Many people with multiple sclerosis, an autoimmune disease that affects the central nervous system, treat their pain by smoking marijuana. But the dose is hard to regulate and the drug is difficult to obtain legally.

About 2.5 million are believed to have MS worldwide, of which about 50,000 are Canadian, according to the U.S. National Multiple Sclerosis Society. About half of MS patients say they suffer from chronic pain, the society said.

Sativex is administered through a spray pump under the tongue or on the inside of the cheek.

In 2001, Canada became the first country to adopt a system regulating the medicinal use of marijuana for people suffering from terminal illnesses and chronic conditions.

In the United States, the federal government has classified marijuana as a drug that is as dangerous as heroin, although 10 states have passed laws that allow its use under medical supervision.