A boy or girl who is smoking marijuana at 13 is likely to earn less money as a young adult than peers who aren't abusing the drug. An adolescent who smokes less marijuana than a friend but enjoys the experience more is likelier to be addicted to the drug at 21. These are findings from two recent studies that looked at adult outcomes associated with marijuana abuse in adolescence.

Dr. Phyllis L. Ellickson and colleagues at the RAND Corporation in Santa Monica, California, surveyed 5,800 adolescents from 30 schools in California and Oregon about their marijuana use between ages 13 and 23. A statistical analysis of the responses revealed four distinct patterns related to marijuana abuse.

Relationship Between Number of Positive Responses to Marijuana And Later Addiction
Positive Responses	Marijuana-Addicted Subjects
0	5.2%
1	8.5%
2	13.3%
3	19.9%
4	28.7%
5	39.1%
Source: Fergusson, D.M.; Horwood, L.J.; Lynskey, M.T.; and Madden, P.A.F. (2003).

Approximately 45 percent did not smoke marijuana. The researchers called these youths Abstainers. Of those who did smoke marijuana:

• Some 5 percent were considered Early High Users—teens who smoked marijuana from once a week to monthly at age 13, decreased their abuse by age 18, and as young adults smoked 3 to 10 times a year.
• About 17 percent were Stable Light Users—teens who smoked infrequently at age 13 and never abused the drug more than 10 times a year, on average.
• Occasional Light Users made up 53 percent of the marijuana-abusing population. They were similar to Stable Light Users but started after age 13 and abused lower amounts than Stable Light Users throughout the study period.
• Steady Increasers, approximately 25 percent of the abusers, started after age 13 and increased their usage during the study period.

The researchers found that the Early High Users lagged behind all other groups in earnings and education when resurveyed at age 29. Their average yearly earnings were $20,940, compared with about $32,000 for the Occasional Light Users and Abstainers and $28,140 for the Steady Increasers. Both groups that initiated marijuana abuse by age 13 reported less schooling than Abstainers and those who first smoked after age 13: Early High Users and Stable Light Users did not usually go to college, while Steady Increasers completed on average one year of college, Occasional Light Users almost two years of college, and Abstainers, almost three years of college.

"The bad news is that if you start marijuana use by age 13, even if you eventually decrease your usage, you are likely to have a lower income and lower level of schooling by age 29," Dr. Ellickson says. "The good news is that 45 percent of the youths in our sample did not use marijuana between adolescence and emerging adulthood. We need to understand what helped those kids abstain over time." Dr. Ellickson says although her findings show an association between marijuana abuse and reduced income and educational performance, they do not prove that marijuana contributes causally to those outcomes, which result from multiple factors. The results also suggest that delaying initiation of marijuana abuse does not necessarily guarantee better outcomes, especially if the late starters escalate abuse. Youths who started after age 13, but steadily increased their marijuana use during and after high school, abused marijuana more than any other group as young adults, smoking 3 to 10 times during the past month on average. In young adulthood, the late starters who intensified their use of marijuana also abused other illegal drugs at rates similar to those of the early starters, with 32 percent admitting to past-year abuse of illegal drugs besides marijuana.

The patterns of abuse revealed by the study suggest that interventions against marijuana abuse may be important from primary school through to high school graduation, says Dr. Kathleen Etz, of NIDA's Division of Epidemiology, Services and Prevention Research.

"At each stage, the goal will be to prevent children who are at risk from initiating use, and to persuade any who have already started to reduce or quit," Dr. Etz says. "The counseling ideally would educate children about the potential problems of marijuana abuse and address the broad goal of self-awareness leading to healthy life choices."

Positive Initial Response Linked to Later Dependence

In the second study, led by Dr. David Fergusson of the Christchurch School of Medicine and Health Sciences in Christchurch, New Zealand, researchers found that positive emotional responses to initial marijuana abuse predicted later addiction to the drug better than did the amount of marijuana smoked. The researchers worked with data from the Christchurch Health and Development study, a 21-year longitudinal project that followed a group of children born in the city from birth through adolescence. Dr. Fergusson's analysis included 1,011 of these youths.

When the study participants were 15 or 16, they answered a series of interview questions about their cannabis exposure in the preceding year. These included their frequency of abuse as well as feelings about their most recent experience: Did they get "really high," feel happy, feel relaxed, do silly things, laugh a lot, feel ill or dizzy or frightened, or pass out?

In this first interview, 20 percent of those later included in Dr. Fergusson's study had abused marijuana. Study participants were interviewed again at ages 18 and 21. By then, approximately 9 percent reported signs of being addicted to marijuana; the prevalence among those who started before age 16 was 21.7 percent. Almost half (46.7 percent) of those who had reported five or more pleasurable responses to the drug in the first interview reported being addicted by age 21, compared with 3.9 percent of those reporting no positive responses.

The results indicate that the pleasure of early use, not how much marijuana is used, determines the likelihood of later addiction. "There are people smoking a lot of marijuana who aren't addicted to it," Dr. Fergusson explains. "Others are smoking less but are addicted or headed for addiction."

"At each stage, the goal will be to prevent children who are at risk from initiating use."

Dr. Fergusson says his findings parallel those of studies on cigarette smoking and alcohol use among adolescents, which suggest that initial subjective response to a drug might be a behavioral marker for biological addiction-vulnerability. This growing body of research indicates that vulnerable individuals seem to experience enhanced sensitivity to a drug's positive effects, diminished sensitivity to its negative effects, or a combination of the two responses.

Dr. Wilson Compton, director of the Division of Epidemiology, Services and Prevention Research at the National Institute on Drug Abuse, says that research in animals has revealed genetic differences between those that prefer alcohol and those that do not. Similar research might identify the neurobiological factors underlying differences in the response to marijuana, perhaps eventually leading to risk-reduction treatments, says Dr. Compton.

Sources

Ellickson, P.L.; Martino, S.C.; and Collins, R.L. Marijuana use from adolescence to young adulthood: Multiple developmental trajectories and their associated outcomes. Health Psychology 23(3):299-307, 2004.
[Abstract]

Fergusson, D.M.; Horwood, L.J.; Lyndskey, M.T.; and Madden, P.A.F. Early reactions to cannabis predict later dependence. Archives of General Psychiatry 60(10):1033-1039, 2003.
[Abstract]

Volume 19, Number 5 (January 2005)