LOS ANGELES: Californian laws legalising medical marijuana and permitting collectives to cultivate the plant have had some unexpected consequences: one is the challenge local growers are posing to the profits of Mexican drug barons; another is an explosion in the number of marijuana stores, or dispensaries, in Southern California.

Law enforcement is arguing that most are for-profit enterprises that violate the 1996 Compassionate Use Act and the 2003 collective cultivation law.

Dispensations ... a man holds a placard at a clinic at Venice Beach, Los Angeles. Critics say some doctors will prescribe marijuana for people who are not ill. Credit:AFP/Mark Ralston

The Los Angeles County District Attorney, Steve Cooley, has announced he will prosecute medical marijuana dispensaries for over-the-counter sales.

''The vast, vast, vast majority - about 100 per cent - of dispensaries in Los Angeles County and the city are operating illegally; they are dealing marijuana illegally,'' he said. ''The time is right to deal with this problem.''