California's patient population can be estimated from data from other medical marijuana states where patients are required to register, shown in the table below. The top two of these are Colorado and Montana, which, like California, have a well developed network of cannabis clinics and dispensaries, and which report usage rates of 2.5% and 3.0%, respectively. Other states, where medical marijuana is less developed, report lower rates of 1% and less. However, California is likely to be on the high side because it has the oldest and most liberal law in the nation. Significantly, California is the only state that permits marijuana to be used for any condition for which it provides relief - in particular, psychiatric disorders, such as PTSD, bipolar disorder, ADD, anxiety and depression, which account for some 20%-25% of the total patient population. Adjusting for this, usage in California could be as much as 25% to 33% higher than in Colorado and Montana, which would put it well over 3% of the population (1,125,000).

A 2%+ patient population estimate is supported by data from the Oakland Patient ID Center, which has been issuing patient identification cards to its members since 1996. The OPIDC serves patients from all over the state, but especially the greater Oakland-East Bay area of Northern California, where its cards are honored by law enforcement. As of 2010, the OPIDC had issued ID's to 19,805 members from five East Bay cities (Oakland, Berkeley, Alameda, Hayward and Richmond), amounting to 2.4% of the local population. Because the cards were issued over a period of 14 years, they include numerous patients who have lapsed, moved, or deceased. On the other hand, they do not include many other local patients who have current recommendations but never registered with the OPIDC.