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Marijuana (Cannabis) and the Body

by Reid K. Hester, Ph.D.

What harm does marijuana do to the body? Is it worst for teens?

We probably know more about the health effects of marijuana than those of any other drug, including alcohol, yet there are still unanswered questions and varying interpretations of what we do know. Those who wish a more in depth discussion than I can provide in a brief response may want to read "Cannabis and Health" by Reese T. Jones, in the 1983 Annual Review of Medicine or "Exposing Marijuana Myths" by Lynn Zimmer and John Morgan which is available here.

The first reference tends to be biased toward finding harm while the latter is biased in the opposite direction, but both are responsible and stick to the facts. The Zimmer and Morgan analysis finds that the research "generally supported the idea that marijuana was a relatively safe drug -- not totally free from potential harm, but unlikely to create serious harm for most individual users or society."

Laboratory and clinical studies have found a variety of potential harms related to marijuana, often occurring only at dosages vastly disproportionate to any real world levels of exposure. I draw my conclusions primarily from epidemiological investigations which have examined real populations of actual users.

Most marijuana users are familiar with coughing and throat irritation as acute effects of marijuana smoking, so it was natural for many of us to expect marijuana to be associated with damage to the respiratory system. The evidence does show minor changes in large airway function which may increase susceptibility to bronchitis, but small airway changes such as those that are associated with lung cancer are not characteristic of marijuana use. Epidemiologic investigations indicate that marijuana use does not contribute to lung cancer, emphysema, or to diminished pulmonary function.

Marijuana smoking, however, may be adding to the greater harm done by tobacco smoking in persons who smoke both marijuana and tobacco, the contribution of marijuana smoking might well be masked due to the smaller number of marijuana cigarettes a typical user will smoke per day.

At one time, I predicted that marijuana would prove to be more damaging to the heart than tobacco smoking; I was wrong, there is no evidence of cardiovascular damage due to marijuana. It may, however, be dangerous for persons who suffer from severe hypertension or atherosclerosis.

Reports of chromosome damage, damage to the reproductive system, breast enlargement in males, and suppressed immune function all have been discredited. Earlier reports of brain damage have likewise been discredited, but some new studies suggest that marijuana may produce very subtle lasting effects in the nervous system, these studies, however, seem to have serious methodological weaknesses and may also prove spurious.

Finally, it has been suggested that there may be a fetal marijuana syndrome similar to fetal alcohol syndrome. Evidence has shown that the effects of maternal marijuana smoking on the unborn child are, at most, about the same as those of maternal tobacco smoking, and diminish rapidly as the infant matures, disappearing entirely within a few years.

There is a great deal of speculation, but little evidence, on the question of whether the harms are greater for teens than for adults. There is no particularly good reason to think that this would be so. The greater plasticity of neurological and other development during adolescence generally make youths less vulnerable to toxic exposures of most sorts. Most of our evidence is in fact drawn from populations in which the bulk of the exposure to marijuana was in middle to late adolescence, so my guess is that the answer will prove to be no -- but I have been wrong before.

About the Author:

Reid K. Hester, Ph.D., Director, Research Division, Behavior Therapy Associates, 3810 Osuna Rd. NE Suite 1, Albuquerque, NM 87109. Phone: 505.345.6100.

Originally published 01/01/99
Revised 8/19/08 by Marlene M. Maheu, Ph.D.
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