There is some evidence from human studies that long-term marijuana use can have adverse effects, although it seems that the spectrum of these effects is narrower than previously thought and mainly centres around cognitive (intellectual) function.
A paper in the Journal of the American Medical Association (JAMA) vol. 287 No. 9, March 6, 2002, pp. 1123-1131) found that long-term cannabis users performed less well than shorter-term users and controls (people who didn’t use cannabis) in tests of memory and attention. The long-term users averaged 23.9 years of use, compared with 10.2 years in the shorter-term users. You can find an abstract at the American
Another study published in a Canadian medical journal (CMAJ 2002 Apr 2;166(7):887-91) found that:
“Current marijuana use had a negative effect on global IQ score only in subjects who smoked 5 or more joints per week. A negative effect was not observed among subjects who had previously been heavy users but were no longer using the substance. We conclude that marijuana does not have a long-term negative impact on global intelligence.”
This study compared IQ scores of individuals aged 9-12 years before they started using cannabis, with their scores at 17-20 years. Light users, former users and non-users showed a gain in IQ over the same period.
One reason for the decreased IQ and memory in consistent heavy users could be the fact that long-term cannabis use has been found to decrease blood flow to the front areas of the brain.
A study published in Public Health and Nutrition, June 2001, vol. 4 pp. 781-786 (Dietary intake and nutritional status of US adult marijuana users: results from the Third National Health and Nutrition Examination Survey. By Smit E, Crespo CJ. of the Department of Social and Preventive Medicine, SUNY School of Medicine, University at Buffalo, Buffalo, NY 14214, USA.) found that marijuana users have lower levels of carotenoids in the serum. The authors concluded this was associated with different dietary habits:
“We found higher cigarette-smoking rates and higher consumption of sodas and alcohol, specifically beer, among marijuana users than among non-current marijuana users. Marijuana users also consumed more sodium, fewer fruits, and more pork, cheese, and salty snacks.”
Carotenoids are important anti-oxidants which help to prevent cancer and other diseases. An increased risk of these diseases might be due not so much to marijuana use itself as to the lifestyle led by many who use it. Better attention to nutrition, in order to ensure a good intake of fresh fruit and vegetables, particularly the red, yellow and orange ones, will help to boost antioxidant status
A review of various studies came to the conclusion that:
“Regular marijuana use can lead to extensive airway injury and alterations in the structure and function of alveolar macrophages, potentially predisposing to pulmonary infection and respiratory cancer.”