The Health Effects of Marijuana

In this installment of a regular feature, the author attempts to present a balanced assessment of the health effects of marijuana.

069 health effects of marijuana - Fotolia - Richard Villalon

Although research into the health effects of marijuana often produced contradictory results, relatively disinterested assessments concluded the smoke irritates the throat and lungs and has as many carcinogenic compounds as tobacco.


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Dr. Tom Ferguson wrote the Medical Self Care column in this magazine in the early 1980s. This issue's column was guest authored by Michael Castleman, managing editor of Medical Self-Care magazine.   

Two years ago, investigators at the UCLA Pulmonary Research Laboratory published the abstract of a study on the respiratory effects of heavy marijuana smoking among 75 young men who consumed an average of five marijuana cigarettes, or joints, a day for two months. Among other less dramatic findings, this "Tashkin Report" suggested: "Smoking four joints a week may cause more respiratory impairment than smoking 16 cigarettes a day."  

The wire service that picked up the story dropped the word "may," and the nation learned that the lung damage caused by four joints equaled that caused by 112 cigarettes.

Later that year, Peter Bensinger (a director of the federal Drug Enforcement Agency) told a graduating class at the FBI Academy that smoking five joints a week introduced more carcinogens into the lungs than smoking a pack of unfiltered cigarettes a day. His remarks were also picked up by the wire services.

Extrapolations like these occur with disturbing frequency. Because marijuana is controversial—and illegal—the atmosphere surrounding the health effects of marijuana is highly charged, and the available data conflict at almost every turn. In fact, if all marijuana researchers are correct in their methodological criticisms of each other's work, none of the reported results can be trusted.

On the other hand, no reputable authorities argue that "grass" is completely safe, not even the legalization lobby, the National Organization for the Reform of Marijuana Laws (NORML). Clearly, lung cells are extremely delicate, and any long-term exposure to hot particulate combustion products—which contain a large number of carcinogens—can be assumed to cause some damage.

The question is, how much harm is caused by how much smoking? The research, contradictory as it is, falls into two general categories indicating either that pot is "not all that harmful" or that it's "very harmful".

"Not All That Harmful"

The best collection of papers indicating that marijuana is relatively benign—even when used to excess—is "Chronic Cannabis Use," published by the New York Academy of Sciences. The report chronicles studies in Costa Rica and Jamaica dealing with a wide range of health variables in heavy marijuana smokers as compared to non-pot-smoking controls. "Heavy" use in Jamaica would be considered nonstop smoking by United States standards, since the average user tested had consumed seven high-potency joints a day for 17.5 years.

The studies compared smokers' and nonsmokers' physical exams, electrocardiograms, blood, immunity to infection, testosterone levels, chest X-rays, lung function, and chromosome integrity .. and showed "no significant differences between the two groups in the wide range of tests administered."

"Very Harmful"

The leading researcher with a negative perspective is Gabriel Nahas—an anesthesiologist at Columbia University in New York—who summarized the case against marijuana last year in the Journal of the American Medical Association. He reported that cannabis use interfered with cell division; harmed lung tissue significantly, with the damage persisting long after cessation of smoking; lowered the lungs' defenses against bacteria "substantially more" than does tobacco; reduced sperm counts and decreased testosterone levels (although these effects were reversible after the users stopped smoking marijuana); caused birth defects; hindered brain function; impaired judgment while driving; and perhaps (this point was not considered proven) triggered epileptic seizures.

Two Official Perspectives

Two organizations have attempted to make sense of the conflicting data: NIDA (a division of the U.S. Department of Health and Human Services) and its Canadian counterpart, the Addiction Research Foundation (a branch of the Ontario Ministry of Health). Neither of the organizations can be considered a completely disinterested observer, since their funding comes from legislative bodies which have kept marijuana illegal. But despite possible bias, the reports are fairly consistent with each other and provide reasonable overviews of the current "official" consensus on the adverse effects of this drug. Here are the agencies' conclusions:

[1] Smoking pot irritates the throat and lungs and may cause bronchitis in heavy users.

[2] Careful comparison of smoke condensates shows that one joint deposits about 50% more "tar" in the lungs than does one nonfiltered cigarette, largely because joints tend to be smoked down to smaller butts, and pot smokers typically hold the smoke in their lungs longer.

[3] Marijuana's smoke has as many carcinogenic polycylic aromatic hydrocarbons (PAH's) as tobacco smoke does, although some PAH's—benzopyrene, for example—are more abundant in pot smoke, while others are less. Marijuana, however, contains neither nicotine—which is a very potent poison—nor (in most cases) the pesticide residues typically found in tobacco smoke, which may accelerate the tumor-inducing effects of PAH's.

[4] The use of marijuana increases the heart rate and may prove hazardous to those with heart diseases. It also reduces testosterone levels in men to the lower end of the normal scale, though these levels do rise again after the cessation of smoking. The drug also appears to reduce both sperm count and sperm motility. The active ingredient in marijuana, delta-9-tetrahydrocannabinol (THC), apparently stimulates these changes, because it binds to estrogen (the female hormone) receptors and exerts "a weak, but significant" estrogenic effect. (This activity may be the reason why there've been anecdotal reports of breast development, gynecomastia, in men who were heavy users.)

[5] THC crosses the placenta and has been found in the milk of lactating laboratory animals, and large doses of THC have been shown to increase the fetal death rate in rhesus monkeys.

[6] Marijuana may suppress certain immune responses in the lungs, making users more susceptible to infection, but the research is inconclusive, and this question hasn't been fully investigated in humans.

[7] Users freely admit that the drug causes disorientation, and school officials have expressed concern about what they refer to as the "antimotivational syndrome" associated with pot use.

[8] Marijuana impairs driving performance, and—when pot is used with alcohol—the user's ability to operate a vehicle deteriorates even further.