Since 1990 a reported 20.5 million people have used marijuana in an average year. From 1990 to 2005 annual usage was at its greatest reported level in 2002 at 25.9 million and its lowest level in 17.4 million in 1992. (See Table 1.)

The National Survey on Drug Use and Health and its predecessor, the National Household Survey on Drug Abuse, are among the most professional, sophisticated, and reliable population surveys conducted. Nonetheless, for both practical and methodological reasons they do not provide a complete accounting of drug use in the United States. For example in 2002 the National Survey on Drug Use and Health (NSDUH) revised its data collection procedures and increased their estimate of annual marijuana users from 21.1 million (as reported in the 2001 survey results) to 25.7 million.(9) NSDUH is a very extensive survey, and in 2002 respondents were paid to complete the entire survey. While this improved data collection, it also calls attention to incomplete data collection in prior years. At best, NSDUH provides data on the minimum number of drug users in the country.

A report by ONDCP on drug consumption in the United States includes this explanation why surveys likely underreport drug use:

"These estimates may be low. Users are likely to under report socially disapproved behaviors, even when those behaviors are legal. They would seem to have even more incentive to under report illegal behaviors. Given under reporting rates for tobacco and alcohol use, it might be reasonable to inflate marijuana estimates by about one-third."(10)

A recent study issued by the Substance Abuse and Mental Health Services Administration (SAMHSA) provides additional data on this trend. Comparing self-reporting of marijuana use within the past month with urine testing of the same subjects indicated that 40% of the individuals who tested positive for marijuana use had declined to accurately report their marijuana use prior to urine testing.(11)

Despite marginal changes in annual data, marijuana use in the United States has remained fundamentally unchanged in the last decade and a half. Since the beginning of annual surveys on drug use, now called the National Survey on Drug Use and Health, in 1990 the average level of annual marijuana use has been 9.3% (± 1%) of the population age 12 and over. In 1990 10.2% of this population used marijuana in the last year, and in 2005 annual usage was at 10.5%. (See Table 2.)

During this period the average monthly use of marijuana averaged 5.1% (± .6%). In 1990 monthly marijuana usage was at 5.1%; in 2005 monthly marijuana usage was reported by 6% of this population. During this period monthly use of marijuana by adolescents age 12 to 17 averaged 6.9% (± 1.6%). In 1990 monthly marijuana use was reported by 5.2% of this age group; in 2005 this age group reported monthly marijuana use by 6.8%. (See Table 4.)

Since the NSDUH was revised and improved in 2002 the number of annual users has remained essentially unchanged at 25 million plus per year. (See Table 1.)

If, as suggested by ONDCP and SAMSHA, illicit drug use is under-reported, then it is reasonable to inflate reports of marijuana use by two-thirds (reflecting the 40% under-reporting of use in the SAMSHA report), then the number of annual marijuana users in the United States is closer to 41 million than 25 million. This will be a key factor in accounting for the consumption of the available marijuana supply (see below).

In the early 1990s the National Household Survey on Drug Abuse collected data on the amount of marijuana consumed and the frequency of use by individuals who reported marijuana use in the last month. Table 5 is based on data derived from the Household Survey by Kevin Chen, Denise Kandel and Mark Davies in an article in the journal Drug and Alcohol Dependence.(12) It shows that over 20% of monthly marijuana users consumer more than 3 marijuana joints (cigarettes) per day.

There is little reliable research on the average size of a marijuana cigarette. A 2001 ONDCP report utilizes a benchmark of approximately .4 grams per joint.(13) A training manual prepared by the California Commission on Peace Office Standards and Training estimates a slender "matchstick" type of marijuana cigarette 3/16th inches in diameter to be .34 grams in weight, a typical homemade 5/16th inches in diameter cigarette to be .50 grams, and a tobacco cigarette-refilled with marijuana would contain approximately .90 grams of marijuana.(14) Marijuana cigarettes produced for research purposes are designed to replicate the size and use of marijuana cigarettes used by consumers. The marijuana provided by the federal government for research and a limited number of legal patients is grown at the University of Mississippi, and processed into standardized cigarettes by the Research Triangle Institute in North Carolina. As described in one study, "these cigarettes were approximately 85 mm (length) by 25mm (circumference), weighed from 750 to 900 mg."(15)

Aside from frequency of use, another reason marijuana users consume seemingly large quantities of the drug is due to the development of tolerance, as explained in the following comment from a landmark article explaining the phenomenon's neurological basis by way of changes in the levels of cannabinoid receptors in the brain that accompany heavy use:

"[E]xperienced users are capable of consuming enormous quantities of the drug with few or no obvious ill effects. Scores in cognitive tasks, both in human and non-human primate studies, show a paucity of measurable effects associated with chronic use . . . tolerance to most psychoactive and physiological effects does occur in humans when high doses are administered daily."(16)

"[Indications of receptor regulation in other neuronal systems] stand in stark contrast to the massive and homogenous changes in cannabinoid receptor levels found in the present [animal] study. The magnitude of the present effect, like the striking behavioral tolerance, may stem in part that, unlike other psychoactive agonist drugs, cannabinoids can be administered in very high doses. It is ironic that the magnitude of both tolerance (complete disappearance of the inhibitory motor effect) and receptor down-regulation (78% loss . . .) is so large, whereas cannabinoid dependence and withdrawal phenomena are minimal."(17)

Reports of heavy marijuana use are well documented in the scientific literature, particularly in articles about the effects of marijuana use on the lungs and articles about treatment strategies for dependence that develops among some frequent users. For example:

"Subjects were eligible if they . . .had a history of smoking or otherwise using the equivalent of 10 'joints' of marijuana per week for at least 5 years. . .Of the 1,163 volunteers, 396 met all the eligibility criteria . . ."(18)

"Subjects used marijuana on an average of 78.71 of the 90 days before testing. Almost all the subjects (93%) used marijuana more than once on a typical day of use, and nearly 50% used it four or more times per day."(19)

"Subjects were excluded 'because they had used marijuana fewer than 50 times in the past 90 days . . .'"(20) pg 92

"Marijuana use was measured on a four point scale "that included once (1), 2-3 times (2), 4-5 times (3), and 6 or more times (4) per day."(21)

These reports indicate that among many users marijuana use is both frequent and substantial. The prevalence of use reported in national surveys, frequency of use, the amount used per day, the amount consumed per day, tolerance, and the recognition of the characteristics of heavy marijuana use by researchers are all factors that help explain how a minimum of 25 million annual marijuana users in the United States can consume the estimated supply of marijuana available each year.