The drug will be used to treat pain from multiple sclerosis. Medicinal marijuana advocates see decision as furthering their cause in the U.S.

Canada became the first nation Tuesday to approve a pharmaceutical prescription spray derived from the cannabis plant, a move that could shift the medical marijuana debate in the U.S.

The drug, called Sativex, is being produced by GW Pharmaceuticals of Britain and is expected to be available in Canadian pharmacies within weeks, principally for the treatment of pain from multiple sclerosis.

"I think the Canadian approval will change an awful lot of things," said Geoffrey Guy, GW Pharmaceuticals executive chairman. In particular, it should help "create momentum" for approval in other countries, including the U.S., he said.

The company isn't expected to apply in the United States until late this year. An examination of the drug's merits could take three to five years.

But the Canadian approval of Sativex, announced by GW Pharmaceuticals at the opening of the London stock exchange, is causing ripples in the U.S.

Bush administration officials declined to comment but have privately said approval of a prescription form of cannabis in the U.S. might draw a bright line between its use by patients and by recreational users.

Some medical marijuana activists, meanwhile, see approval of Sativex as proof that cannabis is a worthy medicine.

"Sativex is for all practical purposes liquid marijuana, so the question of whether marijuana is medicine has been settled," said Bruce Mirken of the Marijuana Policy Project. "The only question is what form people use, and that's best left to doctors and patients."

Meanwhile, a few activists have promised to travel north of the border to get Sativex instead of waiting for the drug's approval in the U.S.

"It's not the fault of MS patients that the U.S. is so far behind in medical marijuana research and development," said Steph Sherer of Americans for Safe Access, a Berkeley-based medicinal marijuana advocacy group.

California and nine other states allow medical marijuana, but the federal government maintains strict prohibitions. The U.S. Supreme Court is expected to soon decide a case involving two California women who smoke marijuana to alleviate their illnesses.

Guy and his colleagues began research into prescription forms of marijuana in 1997 at the behest of the British government, reacting to the emotional and heavily publicized arrests of multiple sclerosis patients medicating themselves with illegal cannabis.

In a remote corner of England, GW Pharmaceuticals set up high-security greenhouses capable of producing 30 tons of cannabis a year. Sativex gives physicians more consistent quality and the ability to set standardized dosages.

"I think physicians will feel a lot more comfortable with this," said Dr. David Bearman, a Santa Barbara internist who specializes in medical marijuana. "One of the reasons cannabis fell out of favor was a lack of standardization."

The spray, laced with a peppermint flavoring, contains no carcinogenic smoke, Guy said, and patients in trials reported that they could avoid the drug's intoxicating effects once they discovered what dose worked best for them. Patients are given Sativex through a spray under the tongue or on the inside of the cheek.

Although his fledgling pharmaceutical company expected to win approval in Britain last year, government officials said in December that they wanted more evidence about the benefits of the medicine. GW Pharmaceuticals has launched a new series of tests and expects approval by late this year or in 2006.

Guy said the company, which has teamed up with Bayer to market Sativex in Canada, hopes for approval in Europe, China, Southeast Asia and the U.S. within five years.

"The U.S. has one of the best regulatory systems in the world, but it requires a lot of effort and the generation of an awful lot of data," Guy said.

To help win U.S. approval, the firm has hired U.S. drug experts as well as Andrea Barthwell, former medical director for federal drug czar John Walters.

"Having this product available will certainly slow down the dash to make the crude plant material available to patients across the country," said Barthwell, an addiction medicine specialist.

Some medical marijuana activists suggested that Sativex could help spur efforts to legalize medicinal use of leafy marijuana.

"In practical terms," said Mirken, of the Marijuana Policy Project, "Sativex is to marijuana as a cup of coffee is to coffee beans."

Barthwell drew a different comparison.

"Comparing crude marijuana to Sativex is like comparing a raging forest fire to the fire in your home's furnace," she said. "While both provide heat, one is out of control."