It has been observed that those who use large amounts of cannabis frequently use other drugs as well, especially alcohol. This can create a potential synergistic effect, resulting in increased harms [1–4]. Economic research has looked at the substitution and complimentarity of particular substances by modelling the effects of price fluctuation on use, although the limits of such research have been noted [5]. When considering youth, Pacula has found cannabis and alcohol to be compliments. As beer prices rose, cannabis use declined [6]. This could potentially be because the introduction of alcohol into an adolescent environment increases the likelihood of other substance being brought into that environment; once the presence of alcohol decreases, the presence of other substances might decrease as well. Among adults, amphetamine has been found to be a substitute for those who's drug of choice is alcohol, and alcohol as a substitute for those who cannot obtain MDMA and cocaine [7, 8]. This research suggests that through various patterns, individuals are making personal decisions about alcohol and drug substitution.

For the purposes of this study, substitution was operationalized as the conscious choice to use one drug (legal or illicit) instead of, or in conjunction with, another due to issues such as: perceived safety; level of addiction potential; effectiveness in relieving symptoms; access and level of acceptance. The substitution of cannabis for alcohol and other drugs has been observed among individuals using cannabis for medical purposes. Medical cannabis patients are regular cannabis users with a stable supply, and their access to cannabis not granted under a standardized prescription system, yet still legitimized by a doctor's recommendation (self-medication). This, in addition to the legal protection given to patients in California, increases the freedom of choice regarding the use of cannabis as a substitute among this population. A survey of 11 medical cannabis doctors in California found that all doctors had seen patients who were using cannabis as a substitute for alcohol. Furthermore, one said that over half of her patients reported preferring cannabis to alcohol, and another reported that 90% of his patients reduced their alcohol use after beginning the use of medical cannabis [4]. The dual use of alcohol and cannabis has been observed in several research studies on medical cannabis patients. First, previous alcohol abuse was reported in 59 of 100 medical cannabis users in a University of California, San Francisco study. Furthermore, 16 of 100 subjects reported previous alcohol dependence [9].

Beyond the population of medical cannabis patients, substituting cannabis or other drugs for alcohol has been described as a radical alcohol treatment protocol. If alcohol negatively affects a person's level of functioning, cannabis or another drug might be an alternative for the user. Charlton has suggested that the radical approach of substitution with substances such as benzodiazepine might be used to address heavy alcohol use in the British Isles by incorporating the idea of self-medication into his discussion by his assertion that "the drug-substitution strategy is based on the assumption that most people use lifestyle (recreational) drugs rationally for self-medication purposes" (p. 457). It is posited that people might substitute a safer drug with less negative side-effects if it were socially acceptable and available [10].

The first cannabis substitution study was a single subject study conducted by Tod Mikuriya in 1970, in which a female (age 49) who was an alcoholic was instructed to substitute cannabis for alcohol. The subject was also administered Antabuse to assist in her abstention from alcohol. The subject reported increased ego strength, useful behaviour, ability to control cannabis intake, euphoria and tranquilization. In addition, there were improvements in concentration, disposition, physical health, ability to revisit social situations and ability to appropriately express anger [11]. The issue was revisited in 2001 with a study of 104 medical cannabis patients in California who used cannabis in an effort to stop the use of other drugs, in particular alcohol. For example, participants may have been previous alcoholics who have replaced their alcohol use with a daily regimen of cannabis. Demographic data were collected as well as information on family alcohol history and alcohol and cannabis usage patterns. The authors included both descriptive statistics and excerpts from interviews. With respect to family alcohol history, 55% of participants reported having one or two alcoholic parents. Most of the participants (90%) listed alcohol as their primary drug of choice, although a few participants had also had addiction issues with heroin, cocaine, amphetamine and other drugs. One interesting finding in this study is that 45% of patients reported using cannabis to relieve pain that they suffered as a result of an alcohol related injury [12].

Cannabis substitution has also been discussed as part of a harm reduction framework. A record review of 92 medical cannabis patients who used marijuana as a substitute for alcohol was conducted with the goal of describing these patients and determining the reported efficacy of treatment. Fifty-three percent of participants reported being raised by at least one alcoholic/addict parent. Concerning reported health problems, 64% of the sample identified alcoholism or cirrhosis of the liver as their presenting problem. Thirty six percent identified themselves as alcohol abusers but listed another health problem as their primary concern. As in Mikuriya's 2001 study, 21% of the sample reported having been injured in an alcohol related incident. When addressing the efficacy of cannabis as a substitute for alcohol, all participants reported cannabis substitution as very effective (50%) or effective (50%). Ten percent of the patients reported being abstinent from alcohol for more than a year and attributed their success to cannabis. Twenty one percent of patients had a return of alcoholic symptoms when they stopped using cannabis. Reasons for stopping the cannabis use ranged from entering the armed forces to being arrested for using cannabis [13].

Previous alcohol use, treatment, and substitution were also documented in a sample of 130 medical cannabis patients in the San Francisco Bay Area. Twenty four had reported previous alcohol treatment. Half of the sample reported using cannabis as a substitute for alcohol, 47% for illicit drugs and 74% using it as a substitute for prescription drugs. The most common reason reported for using cannabis as a substitute was fewer side effects from cannabis and better symptom management from cannabis [14].

The personal health practice of substitution among medical cannabis patients can provide information concerning non-traditional and alternative means used by individuals to personally address their health issues without official involvement in the health care system. Furthermore, examining substitution among this population might translate into the development of more effective, client-centred treatment practices within the field of addiction.