A quarter-century after the U.S. Food and Drug Administration approved the first prescription drugs based on the main psychoactive ingredient in marijuana, additional medicines derived from or inspired by the cannabis plant itself could soon be making their way to pharmacy shelves, according to drug companies, small biotech firms and university scientists.

A British company, GW Pharma, is in advanced clinical trials for the world’s first pharmaceutical developed from raw marijuana instead of synthetic equivalents — a mouth spray it hopes to market in the U.S. as a treatment for cancer pain. And it hopes to see FDA approval by the end of 2013.

Sativex contains marijuana’s two best known components — delta 9-THC and cannabidiol — and already has been approved in Canada, New Zealand and eight European countries for a different usage, relieving muscle spasms associated with multiple sclerosis.

FDA approval would represent an important milestone in the nation’s often uneasy relationship with marijuana, which 16 states and the District of Columbia already allow residents to use legally with doctors’ recommendations. The U.S. Drug Enforcement Administration categorizes pot as a dangerous drug with no medical value, but the availability of a chemically similar prescription drug could increase pressure on the federal government to revisit its position and encourage other drug companies to follow in GW Pharma’s footsteps.

“There is a real disconnect between what the public seems to be demanding and what the states have pushed for and what the market is providing,” said Aron Lichtman, a Virginia Commonwealth University pharmacology professor and president of the International Cannabinoid Research Society. “It seems to me a company with a great deal of vision would say, ‘If there is this demand and need, we could develop a drug that will help people and we will make a lot of money.’”

Possessing marijuana still is illegal in the United Kingdom, but about a decade ago GW Pharma’s founder, Dr. Geoffrey Guy, received permission to grow it to develop a prescription drug. Guy proposed the idea at a scientific conference that heard anecdotal evidence that pot provides relief to multiple sclerosis patients, and the British government welcomed it as a potential way “to draw a clear line between recreational and medicinal use,” company spokesman Mark Rogerson said.

In addition to exploring new applications for Sativex, the company is developing drugs with different cannabis formulations.

“We were the first ones to charge forward and a lot of people were watching to see what happened to us,” Rogerson said. “I think we are clearly past that stage.”

Lisa Leff, Associated Press