San Francisco will take centre stage in a passionate debate over medical marijuana when a federal judge decides whether to send the city's "ganja guru" to jail for doing something that is legal in California.

A federal court jury last January convicted Ed Rosenthal, 58, the author of many books on marijuana and a columnist for High Times magazine, on three counts of growing the weed - a violation of federal law.

Rosenthal had wanted to argue that he was growing the drug for medical use, which is legal and voter approved in California, although barred by federal law. But Federal District Court Judge Charles Breyer did not allow Rosenthal's lawyers to present this background.