"Determining exactly how many patients use medical marijuana with state approval is difficult. According to a 2002 study published in the Journal of Cannabis Therapeutics, an estimated 30,000 California patients and another 5,000 patients in eight other states possessed a physician’s recommendations to use cannabis medically.67 More recent estimates are much higher. The New England Journal of Medicine reported in August 2005, for example, that an estimated 115,000 people have obtained marijuana recommendations from doctors in the states with programs.68
"Although 115,000 people may be approved medical marijuana users, the number of patients who have actually registered is much lower. A July 2005 CRS telephone survey of the state programs revealed a total of 14,758 registered medical marijuana users in eight states.69 (Maine and Washington do not maintain state registries, and Rhode Island, New Mexico, and Michigan had not yet passed their laws.) This number vastly understates the number of medical marijuana users, however, because California’s state registry was in pilot status, with only 70 patients so far registered."
Eddy, Mark, "Medical Marijuana: Review and Analysis of Federal and State Policies," Congressional Research Service (Washington, DC: March 31, 2009), p. 19.