The Health Effects of Marijuana on Humans

by Marc Anderson

June, 1992


Marijuana has been used as a drug since the beginning of time, yet there are still many mysteries about its health effects on humans. Marijuana, or cannabis sativa, is a preparation of the crushed flowers and buds of female hemp plant. The existence of the plant has been reported as early as 1500-1200 BC. in China, and cannabis has been described as an analgesic as early as 200 AD (Walton, 1938). Since then, an overwhelming number of studies have attempted to explain the physical and psychological effects of cannabis on humans.

Physical Effects

Physically, cannabis is relatively harmless. Studies have observed interesting results, including that it causes structural changes in the brain, depresses male sperm counts, causes chromosome damage, lowers testosterone levels, and damages the lungs. Most of these claims, however, have been unreplicated in humans or have been contradicted by other work. This section will address each of these reported negative side-effects.

Various studies have claimed that cannabis destroys brain cells (Landfield et al., 1988; Haper et al., 1977; Meyers and Heath, 1979; Heath et al., 1980). However, several other studies found no structural or neurochemical atrophy in the brain at all (Cabral et al., 1991; Paule et al., 1992; Co et al., 1977; Kuehnle, 1977). Furthermore, it should be noted that Heath's work was sharply criticized for avoiding safeguards of bias and reporting "changes" that occur normally in the mammalian brain (Natl. Acad. of Sciences, Inst. Medicine, 1982).

Wu et al. (1988) found a correlation between cannabis use and low sperm counts in human males. This is misleading because a decrease in sperm count has not been shown to have a negative effect on fertility and because the sperm count returns to normal after cannabis use has stopped. (Natl. Acad. Sciences, Inst. Medicine, 1982)

Another claim made was that cannabis causes chromosome breakage. The primary source for this are studies that were conducted by Dr. Gabriel Nahas in the early 1980s. Nahas observed abnormalities in somatic (not sex) cells of rhesus monkeys in vitro (i.e., in test tubes and petri dishes) and then made the unjustified conclusion that these changes would occur in human bodies in vivo (in the body). Nahas' work was criticized by his colleagues and, in 1983, he backed away from his own conclusions.

A widely held claim has also been that cannabis lowers male testosterone levels (Kolodny, 1974). This theory has been challenged by several studies (Block, 1991; Mendelson et al., 1974; Coggins et al., 1976) that found no correlation at all. Marijuana and Health (Natl. Acad. Sciences, Inst. Medicine, 1982), also, after reviewing literature at that time, concluded that "Due to conflicting and incomplete evidence, it is not possible to conclude at the present time whether marijuana smoking has a significant effect upon gonadotropic and testosterone concentrations in humans."

The most serious physical danger of using cannabis is in smoking it. Inhaling any sort of burnt plant matter is not very good for the lungs. Tashkin et al. (1990) reports decreased gas exchange capacity and the existence of particle residue in the lungs of marijuana smokers several times greater than for tobacco smokers. Wu et al. (1988) noted that marijuana is several times more carcinogenic than tobacco. These findings, though, must be interpreted with caution. In both studies, smoked marijuana was not filtered, while smoked tobacco was. Tashkin et al. notes that, "these differences could largely account for more than twofold greater tar yield from marijuana than tobacco that was measured using syringe-simulated puffs of similar volume and duration." Smoking cannabis through a water-pipe will filter out water soluble carcinogens and will also greatly cool down the smoke. Furthermore, cannabis need not be smoked: In Middle Eastern countries, it has been consumed through teas and food for centuries, avoiding the carcinogenicity of smoke altogether.

Despite cannabis' known negative effects to lung function, it has never been reported to cause a single instance of lung cancer. Tobacco, though, is expected to kill 400,000 people this year (Glenn, 1992). If cannabis is so much more dangerous to a user's lungs than tobacco and is so much more carcinogenic, why aren't there stacks of reports of cannabis-induced lung cancer? One interesting theory is that it's because tobacco tars are significantly radioactive, while marijuana tars aren't at all. Winters et al. (1982) found that a pack-and-a-half-a-day smoker of tobacco is exposed to 8000 mrem of radiation a year, equal to the dose of 300 chest x-rays. A more recent study indicates that a pack-and-a-half-a-day smoker receives 16000 mrem of radiation more than a non-smoker, annually (NCRP Report #95, 1987). It could also be noted that the mere contents of carcinogenic chemicals doesn't necessarily indicate an extreme health hazard. For example, roasted coffee contains 800 volatile chemicals, of which only 21 have been tested on rodents, and of those, 16 were carcinogenic (Ames, 1990). Coffee has never been considered a great cancer-causing substance, though.

Cannabis has also been known for its many therapeutic uses, including the treatment of open angle glaucoma, asthma, and the nausea associated with chemotherapy. It has also been described as a tumor retardant, an antibiotic, a sleep-inducer, and a muscle relaxant (Cohen, 1980).

Psychological Effects

The psychological effects of cannabis use have been described quite many years before the physical effects, yet are as accurate today as they were 100 years ago. Following is an early account of its intoxicating properties made by Dr. John Bell in 1857:

"I had taken the drug with great skepticism as to its reputed action, or at any rate with the opinion that it was grossly exaggerated, and I accordingly made up my mind not to be 'caught napping' in this way again, and to keep a careful watch over my thoughts. But while enforcing this resolution as I supposed, I found myself, to my own astonishment, waking from a reverie longer and more profound than any previous. From skepticism, to the fullest belief of all I had read on the subject, was but a step. Its effects so far surpassed anything which words can convey, that I began to think I was on the verge of narcotic poisoning; yet, strange to say, there was not the slightest feeling of inquietude on that account. I resolved to walk into the street. While rising from the chair, another lucid interval showed that another dream had come and gone. While passing through the door, I was aware of having wandered again, but how or when I had permitted myself to fall into the reverie I was perfectly unconscious, and knew only that it seemed to have lasted an interminable length of time." (Bell, 1857)

The user of cannabis feels the onset of the "high" between 7 seconds (when smoking) and up to 30 minutes (after eating). This involves a relaxed and peaceful, yet sometimes euphoric state of mind. At high doses, it can cause hallucinations. The effects last from 2 to 4 hours after the drug is ingested, and it usually leaves the user in a relaxed state for several hours after the high. One of the main intoxicating properties is that short term memory is inhibited for the duration of the high. Thoughts may seem unclear, and it might be difficult for a user to concentrate on logical-complicated concepts like mathematics.

Long-term effects have been argued for many years. There are claims of an "amotivational syndrome" where users are said to withdraw from society and lose ambition. In reviewing evidence for and against the theory of this "syndrome," however, Marijuana and Health (Nat. Acad. Sciences, Inst. Medicine, 1982) concluded that:

"Such symptoms have been known to occur in the absence of marijuana. Even if there is an association between this syndrome and the use of marijuana, that does not prove that marijuana causes the syndrome. Many troubled individuals seek an 'escape' into use of drugs; thus, frequent use of marijuana may become one more in a series of counterproductive behaviors for these unhappy people."
Other studies have found another interesting correlation: Shedler et al. (1990) reported these results in a longitudinal survey of adolescents:
"Adolescents who engaged in some drug experimentation (primarily with marijuana) were the best adjusted in the sample. Adolescents who used drugs frequently were maladjusted, showing distinct personality syndrome marked by interpersonal alienation, poor impulse control, and manifest emotional distress. Adolescents who, by age 18, had never experimented with any drug were relatively anxious, emotionally constricted, and lacking in social skills."
Among other findings, Utah Power and Light spent $215.00 per year less on health insurance benefits for drug users than on the control group, and employees who tested positive for cannabis at Georgia Power Co. had a higher promotion rate than the company average, and were absent 30 percent less (Morris, 1991).


Whether cannabis use causes permanent physical or psychological changes in its users is still under question. The most serious concern is its effects on the pulmonary system, yet, studies have often used poor controls (i.e., no filtration) and their results can mislead an uncareful reader. Smoking the drug with a different apparatus or ingesting it without smoking at all could greatly effect the results of these studies.

The acute psychological effects of cannabis that cause its intoxicating properties are no mystery, as any user can report. Long term effects of cannabis use could possibly lead to the so-called "amotivational syndrome," but scientific evidence is lacking.

References Cited

  1. Ames, B.N., Gold, L.S. Too many rodent carcinogens: Mitogenesis increases mutagenesis. Science. Vol 149. Pg. 971. 1990.
  2. Bell, J. On the haschisch or cannabis indica. The Boston Medical and Surgical Journal. Vol LVI, No. 11. April 16, 1857.
  3. Cabral, G. et al. Chronic Marijuana Smoke Alters Alveolar Macrophage Morphology and Protein Expression. Fundamental and Applied Toxicolgy. 17:321-32. 1991.
  4. Coggins, W.J., Swenson, E.W., Dawson, W.W., et al. Health status of chronic heavy cannabis users. Ann. N.Y. Acad. Sci. 282:148-161. 1976.
  5. Co, B.T., Goodwin, D.W., Gado, M., Mikhael, M., and Hill, S.Y. Absence of cerebral atrophy in chronic cannabis users. JAMA. 237:1229-1230. 1977.
  6. Cohen, S. Therapeutic Aspects. Marijuana Research Findings: 1980. NIDA Research Mongraph 31. 1980.
  7. Glenn, Reed. Daily Camera. Jan. 2, 1992.
  8. Harper, J.W., Heath, R.G., and Myers, W.A. Effects of cannabis sativa on ultrastructure of the synapse in monkey brain. J. Neurosci. Res. 3:87-93. 1977.
  9. Heath, R.G., Fitzjarrell, A.T., Garey, R.E., and Myers, W.A. Chronic marihuana smoking: Its effects on function and structure of the primate brain. In Nahas, G.G. and Paton, W.D.M. (eds) Marihuana: Biological Effects. Analysis, Metabolism, Cullarlar Responses, Reproduction and Brain. Pergamon Press: Oxford. 1979.
  10. Heath, R.G., Fitzjarrell, A.T., Fontana, C.J., and Garey, R.E. Cannabis sativa: Effects on brain function and ultrastructure in Rhesus monkeys. Biological Pschiatry. 15:657-690. 1980.
  11. Kolodny, R.C., Masters, W.H., Kolodner, R.M., and Toro, G. Depression of plasma testosterone levels after chronic intensive marijuana use. NEJM. 290:872-874. 1974.
  12. Kuehnle, J., Mendelson, J.H., Davis, K.R., and New, P.F.J. Computed tomographic examination of heavy marijuana smokers. JAMA. 237:1231-1232. 1977.
  13. Landfield, P., Cadwallader, L. B., and Vinsant, S. Quantitative changes in hippocampal structure following long-term exposure to delta-9-tetrahydrocannabinol: possible mediation by glucucorticoid systems. Brain Research. Vol 443. 1988.
  14. Mendelson, J.H., Kuehnle, J. Ellingboe, J., and Babor, T.F. Plasma testosterone levels before, during, and after chronic marihuana smoking. NEJM. 291:1051-1055. 1974.
  15. Morris, D. Saint Paul Pioneer Press. May 6, 1991.
  16. National Academy of Sciences, Institute of Medicine. Marijuana and Health. National Academic Press: Washington D.C. 1982.
  17. NCRP Report #95. Radiation Exposure of the U.S. population from consumer products and miscellaneous sources. National Council on Radiation Protection and Measurement. Dec 30, 1987.
  18. Paule, M. et al. Chronic marijuana smoke exposure in the rhesus monkey II: Effects on progressive ratio and conditioned position responding. Journal of Pharmacology and Experimental Therapeutics. 260: 210-22. 1992.
  19. Shedler, J., Block, J. Adolescent drug use and psychological health: A longitudinal inquiry. American Psychologist. Vol 45(5) 612-630. 1990.
  20. Tashkin, D.P., Fligiel S., Wu, T-C., Gong, H. Jr., Barbers, R.G., Coulson, A.H., Simmons, M.S., Beals, T.F. Effects of habitual use of marijuana and/or cocaine on the lung. Research Findings on Smoking of Abused Substances. NIDA Monograph 99. 1990.
  21. Walton, R.P. Marihuana: America's New Drug Problem. J.B. Lippincott: Philadelphia. 1874. [sic.]
  22. Winters, T.H., Franza, J.R. Radioactivity in Cigarette Smoke. NEJM. 306(6): 364-365. 1982.
  23. Wu, T-C., Tashkin, D.P., Djahed, B., and Rose, J.E. Pulmonary hazards of smoking marijuana as compared with tobacco. NEJM. 1988.

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