Five recent studies published in the medical literature report substantial harmful effects of cannabis use in relation to development of schizophenia; depression, especially in adolescent girls; progression to drug abuse/dependence and respiratory diseases, including lung cancer.
Also, an analysis of the latest statistics on cannabis related deaths demonstrates how misleading is the claim, frequently made by advocates of cannabis law reform, that there are NO cannabis related deaths.
A study by Stanley Zammit on “Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969” was published in the British Medical Journal, November 23, 2002.
This study surveyed 50,087 Swedish conscripts from 1969-70 (97% of the country’s male population aged 18-20). Data on self-reported cannabis use prior to conscription was cross-checked against linked records for hospital admissions from 1970-1996 for schizophrenia and other psychoses. Confounding variables such as use of other psychoactive drugs and personality traits linked to social integration were controlled for.
Zammit and colleagues concluded that cannabis use is associated with an increased risk of developing schizophrenia, consistent with a causal relationship. Use of cannabis prior to age of conscription was associated with a 30% increase in risk of developing schizophrenia. Risk increased with frequency of use. Cannabis use more than 50 times prior to age of conscription was associated with a 6.7 fold increase in risk for developing schizophrenia.
A second longitudinal study was published in the same issue of the BMJ. This study on “Cannabis use in adolescence and risk for adult psychosis” by Louise Arseneault and others followed 1037 individuals born in Dunedin, New Zealand, in 1972-73 to age 26. It obtained information on psychotic symptoms at age 11 and drug use at ages 15 and 18 from self reports and assessed psychiatric symptoms at age 26.
It concluded that cannabis use is associated with an increased risk of schizophrenia even after psychotic symptoms preceding first cannabis use are controlled for. Early cannabis use confers greater risk for schizophrenia, possibly because cannabis use becomes longstanding. 10% of cannabis users by age 15 developed schizophrenia by age 26 compared to 3% of the remaining cohort.
A third report in this issue of the BMJ dealt with “Cannabis use and mental health in young people”. George Patton and his fellow researchers followed a Victoria-wide sample of 1601 students aged 14-15 for seven years to determine whether cannabis use in adolescence predisposes to higher rates of depression and anxiety in young adulthood. Daily cannabis use in young women was associated with a 5.6 fold increase in the odds of reporting state of depression and anxiety. Weekly or more frequent cannabis use in teenagers predicted a twofold increase in risk for later depression and anxiety.
Patton and colleagues concluded that frequent cannabis use in teenage girls predicts later depression and anxiety, with daily users carrying the highest risk. Given recent increasing levels of cannabis use, Patton believes that measures to reduce frequent and heavy recreational use seem warranted.
The January 22/29 issue of the Journal of the American Medical Association included a report by Michael T. Lynskey and others on the “Escalation of drug use in early-onset cannabis users vs co-twin controls”.
This study compared later drug use in a national sample of Australian twins who were discordant for early (pre-17 years of age) cannabis use. Individuals who used cannabis by age 17 years had odds of other drug use, alcohol dependence, and drug abuse/dependence that were 2.1 to 5.2 times higher than those of their co-twin, who did not use cannabis before age 17 years.
Lynskey and colleagues concluded that associations between early cannabis use and later drug use and abuse/dependence cannot solely be explained by common predisposing genetic or shared environmental factors. The association may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs.
In late 2002 the British Lung Foundation published “A smoking gun?” This report surveyed the current medical and scientific research into the direct effects of smoking cannabis on the smoker’s respiratory health.
It concluded that habitual cannabis smokers have a significantly higher prevalence of chronic and acute respiratory symptoms such as chronic cough and sputum production, wheeze and acute bronchitis episodes; that 3-4 cannabis cigarettes a day are associated with the same degree of damage to bronchial mucosa as 20 or more tobacco cigarettes; that cannabis smoking is likely to weaken the immune system; that the tar in a cannabis cigarette contain concentrations up to 50% higher of many of the known carcinogens in tobacco smoke; that cannabis cigarettes deposit four times as much tar on the respiratory tract as unfiltered tobacco cigarettes and that Benzyprene, a constituent of the tar in cannabis cigarettes, alters gene p53 which plays a role in 75% of all lung cancers.
Those in favour of cannabis legalisation often claim that there are no reported cannabis induced deaths. It is important to understand what this means. It means only that there are no reported deaths directly attributable solely and immediately to a toxic overdose of cannabis.
There are, however, cannabis related deaths. The Australian Bureau of Statistics reports a total of 184 cannabis related deaths for the five years 1997-2001. ABS breaks these figures down into: 68 Mental (i.e., mental and behavioural disorders due to psychoactive substance use); 98 Accidental (i.e., accidental poisoning by and exposure to noxious substances); 11 Suicide (i.e., intentional self-poisoning by and exposure to drugs); 7 Undetermined (i.e., drug-related deaths with undetermined intent).
Australian figures separating cannabis-related deaths where cannabis was the only drug involved are not readily available. Cannabis is often one factor in a poly-drug cocktail that causes death, including cannabis in combination with alcohol.
United States data from the Drug Abuse Warning Network, based on those counties in which medical examiners test for cannabis, indicates that out of 664 reported cannabis related deaths in 1999 some 28% or 187 involved cannabis only. (This would suggest an average of 10 deaths per year in Australia that were cannabis related only.)
United Kingdom data report some cannabis related deaths that were caused by inhalation of vomit while intoxicated only on cannabis.
Road traffic deaths
A recent report from P Swann of VicRoads Safety Department stated, “The real risks of being killed when driving whilst impaired by cannabis”, shows that cannabis intoxication leads to a relative risk of six of causing a fatal road accident. Cannabis intoxication alone (that is excluding cases where alcohol or other drugs were also present) was found to be responsible for 4.3% of driver fatalities. These cannabis related deaths are not included in the ABS figures cited above.
In the light of these studies and death statistics it would seem to require a reckless disregard for the mental and physical health of Australian young people to do anything likely to increase access to and use of cannabis.
- Richard Egan