Several studies demonstrate that abuse rates for cannabis are lower than rates for other common drugs. Cannabis use is usually not problematic use and cannabis users usually have no social problems which can be attributed to cannabis. The abuse potential of cannabis is insufficient to justify prohibition of medical use.

In a sample of 10,641 Australians aged 18 years and older, 2.2% of adults were diagnosed with DSM-IV cannabis use disorder, comprising cannabis dependence (1.5%) and cannabis abuse (0.7%) (Swift et al. 2001). In this sample, 21% of cannabis users met criteria for cannabis dependence and 10.7% for abuse. Thus, there was a considerable number of cannabis users in this sample with substance use disorders without being dependent. In this sample, cannabis dependence was twice as likely to occur as cannabis abuse.

Most cannabis use is not problematic even for adolescents. In a survey of 2641 UK school students aged 15-16 years, 201 students reported having used cannabis 40 times or more. They were examined using cluster analysis and also compared to other students.

"Three clusters of heavy cannabis users emerged. The smallest was largely distinguished by antisocial behaviour. Another cluster were clearly unhappy, with little support from parents and friends, high levels of depressed mood and low levels of self-esteem. The largest cluster were 'ordinary' and had little to distinguish them apart from a belief that their environment was stable and predictable and that society's rules should be obeyed. Although clear relationships emerged between heavy cannabis use and heavy use of other substances, the 'ordinary' cluster of heavy cannabis users were less likely than the others to have used other illicit drugs. It is therefore concluded that teenage heavy cannabis users have varied motivations and contexts for their usage. They should not be seen as a homogeneous group and many do not appear to use other illicit drugs" (Miller and Plant 2002).

Often cannabis users are treated as a homogeneous group, usually when attempting to analyze a correlation with the use of other drugs, with mental illnesses (depression, schizophrenia), or to find predictors for a certain development (e.g. Griffin et al. 2002, Degenhardt et al. 2001a). Degenhardt et al. (2001a) analyzed relationships between alcohol, cannabis and tobacco and indicators of mental health problems. Alcohol users had lower rates of affective and anxiety disorders than non-users of alcohol, while those meeting criteria for alcohol dependence had the highest rates. Tobacco and cannabis use were both associated with increased rates of all mental health problems examined. However, after controlling for demographics, neuroticism and other drug use, cannabis was not associated with anxiety or affective disorders. Alcohol dependence and tobacco use remained associated with both of these indicators of mental health. All three types of drug use were associated with higher rates of other substance use problems, with cannabis having the strongest association. It should be noted that researchers differentiated alcohol use and alcohol dependence and found very different results, while no such differentiation was made for cannabis.

It is well established that most users of legal drugs, notably alcohol, tobacco and caffeine, control their use and are not abusing the drug. It appears from cluster analyses that this is also the case with cannabis and that studies which do not use cluster analyses and do not distinguish use from problematic use will overlook relevant information.

The associations that are found with cannabis have also been found with legal drugs. Degenhardt and Hall (2001) examined the comorbidity between tobacco use, substance-use disorders and mental health problems among Australian adults aged 18 years and over. DSM-IV diagnoses of substance use, anxiety, and affective disorders were derived using the Composite International Diagnostic Interview (CIDI). Other measures included a screener for psychosis and measures of psychological distress and disability. Researchers found that current tobacco use was strongly associated with abuse/dependence upon alcohol, cannabis, and other substances, and with higher rates of anxiety and affective disorders. Current smokers were more likely to screen positively for psychosis and reported greater psychological distress and disability than non-smokers and persons who had never smoked. These higher rates of other problems were not explained by differences in demographic characteristics, neuroticism scores, or by use of other drugs. The authors concluded:

"Current tobacco use is associated with a range of other substance-use and mental health problems. These are likely to reduce the success of attempts to quit smoking. The presence of these other problems needs to be considered when considering smoking-cessation treatment, and further research may provide information on more effective treatment strategies for persons with co-existing substance-use and mental health problems."

Degenhardt et al (2001b) found that psychosis in a sample of 6,722 Australian adults were associated with the regular use of tobacco, alcohol, cannabis and opiates.

"Ninety-nine persons (1.4%) screened positively for psychosis. Regular tobacco, alcohol and cannabis use were much more common among persons screening positively, as were alcohol, cannabis and other drug use disorders. Among alcohol and cannabis users, psychosis 'cases' were much more likely to be dependent. Ordinal logistic regressions revealed that regular tobacco use, cannabis and alcohol dependence, and opiate abuse were predictors of psychosis scores."

For marijuana, even simple associations between an undifferentiated group of users and commonly believed attributes, for example that cannabis users are not ambitious in sports or at work, cannot generally be established. The French Monitoring Centre for Drugs and Drug Addictions (OFDT) conducted a national school survey on the relationship between sporting activities and alcohol, cigarette and cannabis use among adolescents (Peretti-Watel et al. 2002). Respondents were asked confidentially by self-administered questionnaire (pen and paper) about their use of licit and illicit drugs and life-style (including sporting activities outside school: hours per week, registration in a club, type of sport).

"FINDINGS: The U-shaped curve between the intensity of physical activities and licit and illicit drug use appeared not to be systematic. It depended mainly on the product and the level of use. It only remained significant for boys and heavy smoking once gender and age effect were taken into account.

CONCLUSION: The results stress the need to control for age and gender when the survey participants are teenagers. The relationship between drug use and sporting activity also depends on the type of sport" (Peretti-Watel et al. 2002).

One criteria of substance abuse deals with the "failure to fulfill major role obligations at work, school, or home." There are several studies dealing with the effects of cannabis use on school and work performance, with conflicting results.

McDaniel (1988) analyzed the relationship between pre-employment drug use and on-the-job performance. He found only a small positive correlation. Blank and Fenton (1989) found a positive association between positive pre-employment testing for marijuana and later dismissals. On the other side, Parish (1989) did not find any relation between pre-employment drug testing result and performance at work. Normand et al. (1990) did not find any association between drug test results and subsequent change in employment. Zwerling et al. (1990) noted a positive association between cannabis use and change of occupation, absenteeism and discipline related problems at work. One year later they reassessed the same cohort and found that there was no longer an association between cannabis use and absence from work, while discipline-related problems had decreased (Ryan et al. 1992). These results from studies that all relied on results from pre-employment drug testing suggests that only a minor sub-set of cannabis users suffers from problems at work.

A recent study by Braun et al. (2000) demonstrated that the cannabis effect is modulated by cultural aspects. This was a nearly population based study on the prospective interrelationship of smoking, alcohol intake, marijuana use, and educational and occupational attainment of black and white young adults. Researchers used data from the U.S. CARDIA study (Coronary Artery Risk Development in Young Adults) which involved 5,115 persons 18-30 years of age during the 1985-86 period, who were reevaluated in 1987/88, 1990/91, 1992/93 and 1995. At the start of the study, 28.0% stated that they had used cannabis in the past month. In the following 10 years, cannabis use was negatively associated with completion of college. However, this negative association was only found in the younger sub-set aged 18-24 years at the start of the study, while in the older sub-set there was only a negative assocation between tobacco use and college completion. Associations of substance use with occupational measures were dependent on skin colour.

"In Whites, marijuana use was associated with less prestigious occupations and lower family income, while smoking was unrelated and moderate daily drinking was positively associated. In Blacks, marijuana use was generally unrelated to occupational measures, while smoking and daily alcohol consumption were negatively associated" (Braun et al. 2000)

Another criteria of substance abuse deals with "recurrent substance use in situation in which it is physically hazardous (e.g. driving an automobile or operating a machine when impaired by substance use)." Culpability studies provide the best data on the problems cannabis can cause in the context of driving. This method studies crashes post hoc based upon information (usually from coroners and/or police data) about the causative factors of a crash and blood analyzes on drugs. Examination of these causative factors permits the researchers to apportion a score for each crash-involved driver to determine culpability for the crash. Although there are some differences between studies, these scores classify each driver as "culpable", or "not culpable" for the crash. The cases are then divided into groups according to the results of the blood analysis. Those drivers who had no detectable drugs in blood constitute the control group. A recent analyzes summarizes:

"To date (September 1999), seven studies using culpability analysis have been reported, involving a total of 7,934 drivers. Alcohol was detected as the only drug in 1,785 drivers and together with cannabis in 390 drivers. Cannabis was detected in 684 drivers and in 294 of these was the only drug detected. (…)
Using the culpability analysis method the dominant role of alcohol in motor vehicle accidents is clearly demonstrated, confirming the results with the case-control method. Indeed, in three of the studies outlined in Table 28.2 the concentration-dependence of alcohol was exhibited. At BAC ?0.1 the culpability ratios were significant, whereas BAC <0.1 did not achieve significance.

The results to date of crash culpability studies have failed to demonstrate that drivers with cannabinoids in blood are significantly more likely than drug-free drivers to be culpable in road crashes" (Chesher and Longo 2002).

If urine instead of blood is analyzed, predominantly drivers with regular cannabis use will be found and not those actually impaired since cannabis use can be detected for some weeks in the urine of heavy users. In a U.S. study with 414 injured drivers, 32 of the urine samples were positive for at least one potentially impairing drug (Lowenstein and Koziol-McLain 2001). Marijuana was detected most frequently (17%), surpassing alcohol (14%). Compared with drug- and alcohol-free drivers, the odds of crash responsibility were higher in drivers testing positive for alcohol alone (odds radio [OR] = 3.2) and in drivers testing positive for alcohol in combination with other drugs (OR = 3.5). Marijuana alone was not associated with crash responsibility (OR = 1.1). In a multivariate analysis, controlling for age, gender, seat belt use, and other confounding variables, only alcohol predicted crash responsibility. Researchers concluded:

"Alcohol remains the dominant drug associated with injury-producing traffic crashes. Marijuana is often detected, but in the absence of alcohol, it is not associated with crash responsibility" (Lowenstein and Koziol-McLain 2001).

The first controlled, population based study on accidents on cannabis users compared to non-users was conducted by Braun et al. (1998) in the U.S. Researchers compared 4,462 individuals with different cannabis use status (never, former, current use) with regard to frequency of injuries within three years. Participants were randomly selected from 64,862 patients of a health maintenance program aged 15 to 49 years. All injuries independently of cause and severity were included. A total of 2,524 accident victims were treated outpatient, 22 were treated inpatient and 3 were fatalities. There was no association between cannabis use and injuries.

The abuse potential of a certain substance can also be determined from the variation in the intensity of use over the course of several years. A high variability in intensity indicates a weak potential for dependency and abuse. Von Sydow et al. (2001) determined incidence and patterns of the course of cannabis use and disorders as well as cohort effects in a community sample of adolescents and young adults (n=2,446) aged 14-24 years at the outset of the study. Patterns of cannabis use, abuse and dependence (DSM-IV) were assessed using the Composite International Diagnostic Interview (M-CIDI). They reported the following results: (1) Cumulative lifetime incidence for cannabis use (at second follow-up): 47%; 5.5% for cannabis abuse, 2.2% for dependence. (2) Men used and abused cannabis more often than women. (3) The majority of the older participants (18-24 years at baseline) had reduced their cannabis use at follow-up, while younger participants (14-17 years at baseline) more often had increased their use and developed abuse or dependence. (4) The younger birth cohort (born 1977-1981) tended to start earlier with substance (ab)use compared to the older birth cohort (1970-1977). (5) Cannabis use was associated with increasing rates of concomitant use of other licit and illicit drugs. The authors concluded:

"Cannabis use is widespread in our sample, but the probability of developing cannabis abuse or dependence is relatively low (8%). The natural course of cannabis use is quite variable: about half of all cannabis users stopped their use spontaneously in their twenties, others report occasional or more frequent use of cannabis" (Von Sydow et al. 2001)

Felder and Glass (2001) explain that the abuse potential of cannabis is not sufficient to preclude its medical use. Their assessment of the relative abuse potential of cannabis suggests that it does not have the high potential for abuse required for Schedule I or II status..

Much of the political and public objection to the use of [Delta]9 THC or marijuana as a therapy centers around its abuse potential and the belief by some that it serves as a "gateway" drug leading users to "harder" drugs of abuse. Many therapeutic drugs have abuse potential, yet this does not invalidate their role in current therapies. While there is some preliminary evidence for cannabinoids activating the reward pathways in the brain (Tanda et al. 1998), most investigators have failed to find addictive or reinforcing effects of cannabinoids in animal models. Unlike cocaine or heroin, cannabinoid agonists produce conditioned place aversion even at low doses (McGregor et al. 1996; Parker and Gilles 1995) and anxiogenic effects (Onavi et al 1990). Furthermore, animals will not self-administer cannabinoids (Harris et al 1974; Leite and Carlina 1974; Cocoran and Amit 1974), and a lack of cross-sensitization between cocaine (McGregor et al 1995) or amphetamines (Takahashi and Singer 1981) and cannabinoids has also been demonstrated. (Felder and Glass 1998, 192)

A considerable number of cannabis users suffer from problems that meet the criteria for abuse. However, the large majority of cannabis users do not experience any relevant problems related to their use. When compared to legal drugs, abuse problems with cannabis are generally less severe. The abuse of cannabis does not preclude its medical use. Relative to other scheduled drugs cannabis does not have a high potential for abuse.

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