Sir, - Your correspondent Joel Conroy asks me "to prove this so called fact" about the damaging effect of cannabis on the lungs and heart as well as on learning and memory.
In 1982 the Institute of Medicine in the US published a report on marijuana and health, which included a detailed comparison of cannabis and tobacco smoke. This comparison clearly showed that cannabis smoke contains 70 per cent more carcinogens than tobacco smoke. The "tar" from cannabis smoke has been shown to be carcinogenic by a number of research groups, e.g. Leuchtenberger et al.
Tashkin's group reported in the American Review of Respiratory Diseases (1987) that "relatively young, habitual heavy marijuana smokers have a high prevalence of abnormal airway appearance and histologic findings (squamous metaplasia, hyperplasia of basal and goblet cells), irrespective of concomitant tobacco use.
Their findings were supported by Bloom et al, in the British Medical Journal, who concluded that the effect of marijuana on pulmonary function in male non-tobacco smokers was greater than the effect of tobacco cigarette smoking. Finally, Donald in presenting details of cases of advanced head and neck cancer in young patients who were regular marijuana users (Otolaryngology - Head and Neck Surgery, 1986) drew attention to the numerous carcinogens, as well as respiratory irritants, in marijuana smoke.
In relation to the heart and blood vessels, a recent Australian Government report listed individuals with cardiovascular disease as being at increased risk from cannabis. This warning is based on the long recognised effects of THC from cannabis on the cardiovascular system including increased heart rate, low blood pressure and aggravation of angina (Benowitz and Jones, 1975).
With regard to learning and memory, my conclusions on the damaging effect of cannabis were influenced by the Australian National Drug Strategy Monograph No, 25, "The Health and Psychological Consequences of Cannabis Use", which stated that cognitive impairment, especially of attention and memory, was a major acute adverse psychological effect of cannabis. In addition, a leaflet from The Netherlands explicitly warns that "concentration will diminish when using hash and weed. So do not use at school, at work or if you still have to study".
The facts upon which those conclusions are based include research papers by the following Schwartz et al., "Short term Memory Impairment in Cannabis Dependent Adolescents" in American Journal of Diseases of Children, 1989; Tart C.T. "Marijuana Intoxication Common Experiences" In Nature 1979; Pope H.G. "Residual Cognitive Effects of Heavy Marijuana Use in College Students" in JAMA 1996. Particularly noteworthy is the work of Lundqvist in two articles published in Life Sciences, 1995. He reported that "in the course of clinical treatment of cannabis users, we have noticed that the use of cannabis more often than about every six weeks for approximately two years leads to changes in cognitive functioning." QED? - Yours, etc.,
Dept. of Pharmacognosy,
School of Pharmacy,
Trinity College,
Dublin 2.