Marilyn Bowman, in a battery of psychological tests on chronic cannabis users in Jamaica in 1972, found “no impairment of physiological, sensory and perceptual-motor performance, tests of concept formation, abstracting ability and cognitive style and tests of memory.” These Jamaicans had smoked anywhere from six to 31 years (16.6 mean average) and the average age at the first puff was at 12 years and six months.
In the 1975 study between users and non-users, no difference was found in plasma testosterone, no difference in total nutrition, slightly higher performance on the intelligence sub-tests (not statistically significant), and “a basic measure of cell-mediated immunity was no less vigorous in the users.”
Finally, “Users in our matched pair sample smoked marijuana in addition to as many tobacco cigarettes as did their partners. Yet their airways were, if anything, a bit healthier than their matches.”
“We must tentatively conclude either that marijuana has no harmful effects on such passages or that it actually offers some slight protection against the harmful effects of tobacco smoke. Only further research will clarify which, if either, is the case.”