“Marijuana” refers to the dried material (leaves, stems, seeds) of the hemp plant, Cannabis sativa; this is the form used most commonly in the United States. “Hashish” refers to the dried resin made from hemp flowers. “Hash oil” is a liquid extracted from the plant material, and this form is the most potent. The active ingredient in all forms of marijuana is delta-9-tetrahydrocannabinol (THC).

The most common route of administration for marijuana is smoking, either as a rolled cigarette (“joint” or “blunt”) or through a pipe (“bong”). It can be taken orally, which results in a slower onset but longer duration of action. It is rarely used intravenously due to the risk of complications from injection of undissolved plant material.

THC and other cannabinoids are rapidly absorbed from the lungs and bind to endogenous cannabinoid receptors in the central nervous system. This binding is responsible for the psychoactive properties that users seek. Two endogenous cannabinoid receptors have been identified in the central nervous system.

This topic review discusses marijuana use in adults. Abuse of other substances is discussed separately.

EPIDEMIOLOGY — Marijuana is the most frequently used illicit drug in the United States. Its use is on the rise, especially among junior high and high school students. Among adults in the United States, the prevalence of use has stayed relatively steady at 4 percent since the early 1990s. However, the prevalence of marijuana abuse and dependence (as defined by DSM-IV ) has increased significantly.

Data from a survey of over 40,000 US adults found the 12 month prevalence of marijuana abuse and dependence to be 1.1 and 0.3 percent respectively; risk factors for abuse and dependence included being male, Native American, widowed/separated/divorced, and living in the West. There was a strong association between Axis I and II disorders and abuse or dependence on marijuana.

The “gateway” theory of development of drug abuse describes sequential stages of progression in drug involvement from adolescence into adulthood, starting with legal drugs such as alcohol or cigarettes, followed by marijuana, illicit drugs other than marijuana, and abuse of prescription drugs. As an example, in a cross-sectional survey of 311 monozygotic and dizygotic same-sex twin pairs who were discordant for early cannabis use, those who used cannabis by age 17 had odds of other drug use or alcohol dependence that were 2.1 to 5.2 times higher than their twin who did not use cannabis. The authors hypothesized that this may represent a causal effect, but an alternative hypothesis is that similar genetic and environmental risk factors lead both to the use of marijuana and to the later use of other substances.

The gateway theory is also supported by a study of boys who transitioned from licit alcohol and tobacco use, to illicit marijuana use between the ages of 10 to 22 years. The gateway sequence is not absolute, as 22 percent of boys who used marijuana had no prior history of licit drug use. A study of school-age children in Spain identified specific factors associated with onset of marijuana consumption: use of alcohol and tobacco, antisocial behavior, low academic performance, and leisure time patterns. This study supports the gateway theory, although it does not eliminate the possible existence of a common factor underlying initiation of alcohol, tobacco, and marijuana.

Whether the association is causal (ie, whether marijuana is a “gateway” drug) or reflects shared risk factors, marijuana use appears to be the best predictor of later use of “harder drugs” like cocaine or heroin. Marijuana use also predicts later ecstasy use.

According to an advance report of the Community Epidemiology Working Group in December 1999, a number of factors may be responsible for the overall trend in increased marijuana use:

 
1. Marijuana has become more available and is relatively cheap.  
2. Marijuana use is considered less risky than other drug use.  
3. Law enforcement agents have been focusing more attention upon other drugs (eg heroin, cocaine, and methamphetamine).  
4. More potent marijuana, such as sinsemilla, is available.  
5. Marijuana is being packaged for use in larger quantities.  
6. Marijuana is being used as a delivery medium for other psychoactive substances such as crack cocaine and phencyclidine (PCP). Marijuana joints containing crack are known as “fireweed.”  
7. Marijuana use is spreading to all social strata.  
8. Marijuana may be easier to sell than other drugs because profit margins are good and the penalties are relatively small.

The most common unit of purchase of marijuana in a US study was a “nickel bag,” which costs $5 for an unknown quantity of marijuana. Friends are the main source of marijuana acquisition, and it is often obtained for free through sharing. A more potent “designer” marijuana has become available in some US cities, grown indoors from special strains, and selling for up to ten times the cost of commercial marijuana.