First of all, it should be said that, in all of recorded history, no documentable deaths attributable to marijuana use have ever been recorded. This can be said for very few substances. Indeed, every major study of marijuana use has come to the same conclusion: Relative to legal drugs like alcohol and tobacco, even chronic, long-term marijuana use is fairly safe.
That said, there is some evidence that heavy use of marijuana can cause certain problems. For instance, chronic marijuana use has been shown to cause "pre-cancerous lesions" in the large airways of the lungs, similar to the effects of tobacco use on the small airways. However, these lesions have not been shown to develop into cancer, as they sometimes do in tobacco users. A DEA-funded study in 1975 showed that not only does marijuana use not cause cancer, it shows powerful anti-tumor activity in certain cancers. That study was immediately de-funded.
Although the "amotivational syndrome" theory has been pretty well disproven, marijuana use has been shown to trigger 'psychotic episodes' in already mentally-unbalanced people. There have been, however, no documented reports of the "crazed lunatics" that Harry Anslinger warned us about.
Marijuana use has been linked to certain respiratory problems, the most common being bronchial asthma. It seems that, in some asthma sufferers more than others, marijuana smoke irritates bronchial passages. On the other hand, one hit from a 'joint' has been known to stop the onset of a full-blown asthma attack.
A number of studies have reported low birth weight and other physical problems among babies exposed to marijuana in utero. However, when other factors known to affect pregnancy outcomes were controlled for--for example, maternal age, socio-economic class, and alcohol and tobacco use--the association between marijuana use and adverse fetal effects became tenuous. Moreover, numerous other studies have failed to find serious negative impacts from marijuana exposure.
Also used as evidence of marijuana-induced fetal harm are two longitudinal studies, in which the children of marijuana users were examined repeatedly. On close examination, the effects of marijuana appear to be quite minimal, if existent at all. After finding a slight deficit in visual responsiveness among marijuana-exposed newborns, no differences were found at 6 months, 12 months, 18 months, or 24 months. At age 3, the only difference (after controlling for confounding variables) was that children of "moderate" smokers had superior psycho-motor skills. At age 4, children of "heavy" marijuana users (averaging 18.7 joints/week) had lower scores on one subscale of one standardized test of verbal development. At age 6, these same children scored lower on one computerized task--that measuring 'vigilance'. On dozens of others scales and subscales, no differences were ever found. Although it is sensible to advise pregnant women to abstain from using most drugs--including marijuana--the weight of scientific evidence indicates that marijuana has few adverse consequences for the developing human fetus.
Confusing? Yes! We told you it was. This is the nature of the research into marijuana use. The bottom line is this:
In all of recorded history, there has not been a single, documentable death that could be attributed to marijuana use alone. Every major study on the subject in the last 100 years confirms the relative safety of even chronic, long-term marijuana use.
Therefore, we can safely assume that the laws against marijuana aren't there to protect us from harm.