Overview
People who smoke cannabis but not cigarettes rarely experience lung problems. Yet the potential for marijuana-induced pulmonary troubles remains. Conclusive proof of marijuana’s negative impact on the lungs of humans will require decades of research. Inhaled particles, gases, and heat take time to create disease. Comparable challenges arose in the quest to prove that cigarettes caused lung problems, a task that required nearly 40 years of work. More people die from smoking tobacco than any other single cause. Tobacco smoke and cannabis smoke are very comparable, suggesting that the potential for both to contribute to lung disease is very high (Iversen, 2000). Widespread marijuana smoking did not appear in some countries until fairly recently. Many chronic cannabis smokers are still too young to experience severe pulmonary problems. Nevertheless, chronic users of cannabis do show adverse respiratory symptoms, includ- ing cough, phlegm, wheezing, and bronchitis. They also show changes in
their bronchial cells that parallel those seen in the early stages of lung cancer (Zimmer & Morgan, 1997).
Respiratory Illness
Current studies have found little evidence of marijuana-induced increases in respiratory illnesses. A review of a large sample of hospital records revealed that 36% of daily marijuana smokers saw a physician for colds, flu, or bronchitis in a six-year period. Only slightly fewer (33%) of the nonsmokers sought treatment for these same problems (Polen, 1993). These data suggest that cannabis consumption does not create meaningful increases in the rates of respiratory illnesses. Nevertheless, other work reveals more symptoms of bronchitis, including chronic cough and phlegm production, in heavy marijuana smokers. A study of daily can- nabis users who did not smoke cigarettes showed that they had a higher rate of these symptoms than nonsmokers. Tobacco smokers and people who smoked both substances showed more of these symptoms, too (Tashkin et al., 1987). Another study confirmed these results but found that people who smoked both tobacco and marijuana were more likely to develop bronchitis than those who smoked only one or the other (Bloom, Kaltenborn, Paoletti, Camilli, & Leibowitz, 1987). Thus, chronic, heavy use of cannabis can create respiratory problems compa- rable to those that tobacco creates.
Lung Function
Research concerning marijuana’s impact on lung function has produced mixed results. One study of chronic obstructive pulmonary disease (COPD), a disorder of the lung airways, found no difference between marijuana smokers and nonsmokers. This work relied on a measure of the volume of air that people can expel from their lungs in one second. People who can force more air from their lungs have fewer obstructions in their respiratory tracts. Consumers of tobacco cigarettes invariably show more and more obstructions each year that they smoke, suggesting blocked airways. Yet people who had smoked 2 to 3 marijuana joints per day for 15 years did not differ significantly from others who did not smoke at all (Tashkin, Simmons, Sherrill, & Coulson, 1997). These re- sults suggest that cannabis use may not lead to emphysema.
day revealed significant impairment in lung function (Bloom et al., 1987). There were no obvious differences between these two samples to account for the different effects. Thus, marijuana may or may not meaningfully impair the functions of the lungs. Further work can help illuminate the impact of the drug on chronic obstructive pulmonary disease.
Lung Airway Problems
A number of problems with the airways of the lung can appear during examinations with a bronchoscope. This assessment technique can reveal damage that occurs prior to obvious deficits in lung function. Visual in- spections of the lungs revealed that people who smoked 5 joints a week for 2 years had more redness, swelling, and mucous. People who smoked both cannabis and tobacco had particularly bad symptoms (Roth et al., 1998). Thus, even without creating emphysema, marijuana can alter the bronchial tract.
Biopsies taken from some of these people revealed that marijuana smokers had more abnormal cells in their lungs. For example, many lung cells normally have cilia, small hairs that help clear the lungs of particles. In cannabis smokers, many of these ciliated cells had transformed into cells more similar to skin. The changes were particularly common among people who smoked both cannabis and tobacco. These sorts of cellular transformations are particularly alarming because they may be the begin- nings of the development of lung cancer.
Cancer
Currently, no data reveal definitive increases in rates of lung cancer among people who smoke marijuana but not tobacco. A retrospective study of over 64,000 patients showed no increases in risk for many types of cancer once alcohol and cigarette use were controlled (Sidney, Que- senberry, Friedman, & Tekawa, 1997). Nevertheless, a few lines of re- search suggest that cases of cannabis-induced lung cancer may appear in the years ahead. THC is not carcinogenic itself. Yet when isolated cells are exposed to marijuana smoke, they change in ways that parallel the early stages of cancer (Leuchtenberger, 1983). Biopsies taken from the lung tissue of cannabis users reveal cellular changes that could lead to tumors (Roth et al., 1996). A number of reports suggest considerable marijuana use among young people with cancers of the lung, oral cavity,
and esophagus (IOM, 1999). These data are comparable to early studies of tobacco and cancer and suggest that cannabis smoke is capable of damaging the bronchial system in ways that may lead to tumors.
Pulmonary Harm Reduction
Drug lore suggests that certain strategies may minimize marijuana’s po- tential harm to the lungs. These include ingesting the drug orally, using water pipes or vaporizers, refraining from holding smoke down in the lungs for extended periods, and smoking stronger cannabis. Research sug- gests that results for some of these strategies are mixed. Obviously, eating marijuana or hashish will have no impact on the respiratory system. Wa- ter pipes that cool the smoke will decrease the negative effects of heat. Yet despite popular belief, water pipes do not appear to decrease the amount of tar and particles in smoke (Doblin, 1994). In addition, these pipes may filter out some of the THC, leading users to smoke more cannabis than they might without a pipe. Smoking more may create in- creased deposits of tar and particles in the lungs. Thus, the water pipe is not a panacea for all cannabis-induced respiratory problems. Its cooling properties may help limit lung damage caused by heat, but other effects are limited.
Another gadget designed to lower exposure to carcinogens is the va- porizer. This pipe uses a hot plate to heat marijuana to the point where cannabinoids vaporize. This temperature should be below the level where carcinogenic hydrocarbons burn. Users can then inhale the vapor, which ideally would contain more THC with fewer contaminants and less tar. Counter to the intentions of the inventors of this machine, it creates a vapor with an unusually low amount of the psychoactive THC and a high amount of the less active cannabinol. An unfiltered joint ac- tually provides a better ratio of THC to tar. Although the benefits of cooler smoke remain, the vaporizer is not the ideal preventer of marijuana-induced lung problems either (Gieringer, 1996).
An additional strategy for reducing lung damage associated with smok- ing concerns the length of time users keep smoke inside their bodies. The common habit of holding smoke in the lungs for long periods likely in- creases tar deposits, which undoubtedly add to respiratory problems. Al- though many experienced users swear by this habit, two studies show that holding “hits” longer does not appear to lead to greater changes in mood (Zacny & Chait, 1989, 1991). Holding one’s breath without in-
haling any smoke at all can certainly lead to a light-headed, dizzy expe- rience of consciousness easily confused with marijuana’s effects. For ex- ample, one study revealed that holding one’s breath for a long time altered cognitive abilities even when smoking a placebo (Block, Farin- pour, & Braverman, 1992). Thus, users who are committed to holding their breath might find that exhaling cannabis smoke beforehand could lead to comparable changes in mood with markedly less risk of lung damage. Otherwise, exhaling soon after inhaling should produce identical subjective experiences with markedly less potential for respiratory injury. A final approach to preventing harm to the lungs concerns smoking potent marijuana. Cannabis with higher amounts of THC can provide the same subjective experience with a reduced intake of smoke and ac- companying deposits of tar. One study showed that 10 regular users deposited less tar when smoking marijuana that was approximately 4% THC than they did while smoking marijuana that was approximately 2% THC. These results suggest that stronger cannabis may actually decrease the risk of respiratory problems (Matthias et al., 1997). British police have confiscated marijuana with THC concentrations around 20% (House of Lords, 1998). Perhaps cannabis this strong could create sub- jective effects with little exposure to damaging tars. The potential to smoke less while maintaining the same subjective state will undoubtedly help decrease lung and respiratory troubles.
Reproduction
Several interesting lines of research suggest that the cannabinoids play an important role in the function of sex hormones and sperm. This work has led to growing concerns about marijuana’s impact on fertility and birth defects. Unfortunately, some enthusiastic efforts to prevent drug use have overstated the data, implying that consumption can lead to permanent infertility or drastic birth defects. (My junior high school health teacher suggested that the children of marijuana smokers would be born with one eye in the center of their foreheads.) Extremely large doses of cannabis could, in theory, decrease fertility in humans. Never- theless, research has yet to show a definitive decrease in reproductive function in people who smoke marijuana. The drug also has not been linked to human birth defects.
doses of the drug can decrease hormones central to menstruation, in- cluding follicle-stimulating hormone, luteinizing hormone, and proges- terone. These effects require an injected dose of 2.5 mg of THC per kilogram of body weight, a dose comparable to a 130-pound woman smoking 7 joints per day. Monkeys who received this dose for 18 con- secutive days did not ovulate. Nevertheless, with chronic administration of the drug for up to a year, monkeys developed tolerance to this effect and began ovulating again. Comparable results appear for rats and rab- bits. Thus, large doses can disrupt female sex hormones, but functions return to normal with time (Smith, Almirez, Scher, & Asch, 1984).
Research on marijuana’s impact on fertility in human women offers mixed results. One study showed increased rates of marijuana use in women who are infertile. In the infertile group, 61% of the women had used marijuana. In the fertile group, only 53% had smoked cannabis. Nevertheless, infertile women did not use marijuana more often or for a longer period than those who were fertile. The use of cocaine had a much larger impact on fertility. Once the investigators controlled for cocaine consumption and other factors that contribute to infertility, the impact of cannabis decreased (Mueller, Daling, Weiss, & Moore, 1990). In con- trast, a later study found that women who used marijuana regularly con- ceived more quickly than women who did not use the drug (Joesof, Beral, Aral, Rolfs, & Cramer, 1993). Perhaps these data say more about mari- juana’s alleged functioning as an aphrodisiac than anything about a direct impact on fertility.
Animal and human research also reveals that THC may interfere with the production of sperm. Extensive studies of sea urchin sperm reveal that cannabinoids decrease their capacity to fertilize an egg (Schuel et al., 1999). In addition, THC lowers the motility of bull sperm in a petri dish (Shahar & Bino, 1974). THC treatments of 5 mg per kilogram of body weight (18 joints per day for a 160-pound man) created twice as many abnormal sperm as usual in mice (Zimmerman, Zimmerman, & Raj, 1979). Anyone intending to breed mice, cows, or sea urchins should keep them away from large doses of cannabinoids. In human research, men who smoked an average of 8 joints per day for one month showed significant drops in sperm count and motility (Hembree, Nahas, Zeiden- berg, & Huang, 1979). Nevertheless, their sperm count and functions did not fall to abnormal levels and returned to normal after the study ended. Despite all this work, no studies show an actual decrease in fertility for men who use cannabis.
Pregnancy
The behavior of pregnant women has become a giant public health con- cern that may mirror attitudes about sex and sexism (Stoltenberg, 1988). Some states may prosecute pregnant women for child abuse if they test positive for drugs. These statutes may lead pregnant women to avoid prenatal health care, dramatically increasing the chances of problematic deliveries. Laws like this assume that illicit drugs damage the fetus. Re- search designed to assess the impact of marijuana on the fetus is often compounded by polysubstance abuse. Investigators often must limit their work to countries like Jamaica, where women who smoke cannabis are not particularly likely to use other drugs. Alternatively, researchers can estimate the effects of other drugs like alcohol and cocaine and then see if marijuana use contributes to additional problems.
The limited available research suggests that marijuana may have little effect on offspring when they are young. Problems related to prenatal marijuana exposure may not appear until children reach the age of 4 or older. These might include an increase in problems related to attention and delinquency. Yet the women who choose to use cannabis during pregnancy may have behavioral problems themselves, making it unclear if their children develop troubles because of marijuana exposure, genet- ics, or poor parenting. Animal studies can avoid this problem by admin- istering the drug to a random sample while ensuring that another group receives no drugs. These experiments show that extremely large doses of THC can lower birth weights, as well as increase spontaneous abortions and deformities, but generalizing these data to humans requires consid- erable caution (Zimmer & Morgan, 1997).
Research on extremely young children exposed to marijuana prena- tally shows few effects. A study of more than 12,000 newborns found no link between cannabis use and gestation, birth weight, or malforma- tions (Linn et al., 1983). Other studies found statistically significant re- sults in large samples, but these often have little practical meaning. For example, research on 583 women showed shorter gestation periods for those who smoked cannabis 6 times per week while pregnant. Yet their babies were born an average of only 6 days early, once investigators con- trolled for the effects of alcohol and nicotine. Other work has found no link between marijuana use and the length of gestation (Witter & Niebyl,
1990). Cannabis use had no impact on birth weight, either (Fried, Wat- kinson, & Willan, 1984).
Other studies of very young children prenatally exposed to marijuana also show few meaningful effects. At 3 days and at 1 month of age, the offspring of mothers who smoked cannabis in Jamaica seemed no differ- ent from those born of mothers who never touched the drug. In fact, children of heavy users appeared less irritable, as well as more alert and stable (Dreher, Nugent, & Hudgins, 1994). In addition, the cognitive abilities of Jamaican children age 4 or 5 appeared unharmed by prenatal marijuana exposure (Hayes, Lampart, Dreher, & Morgan, 1991). These studies may provide some of the best information on the impact of mar- ijuana exclusively because the use of other drugs is less common in Ja- maica. Several North American studies also showed no effect of prenatal marijuana use on a few different measures. For example, American chil- dren exposed to marijuana prenatally showed no deficits on gross motor skills at age 3 (Chandler, Richardson, Gallagher, & Day, 1996) and no differences in total growth at age 6 (Day, Richardson, Geva, & Robles, 1994).
Although the studies above suggest little negative consequence for smoking marijuana during pregnancy, research that follows the children for a longer period reveals some potentially disturbing links to cognitive abilities and behavior problems. For example, children exposed to mar- ijuana prenatally showed problems with a sustained attention task when they reached age 6 (Fried, Watkinson, & Gray, 1992). Another com- mendable longitudinal study followed over 600 mothers through preg- nancy until their children reached age 10 (Goldschmidt, Day, & Rich- ardson, 2000). Prenatal exposure to marijuana predicted several behavioral problems in this sample. Mothers who smoked cannabis while pregnant reported that their children were more impulsive and hyper- active and had more trouble paying attention. In addition, these chil- dren’s teachers rated them as more delinquent. Mothers who did not smoke cannabis during pregnancy had half the rate of delinquency in their children as the mothers who smoked one joint per day. These ef- fects remained even when the researchers statistically controlled for other contributors to these problems, including the mother’s use of other drugs, her depression, and her hostility.
These data suggest that prenatal exposure to cannabis can increase troubles many years later. Nevertheless, the authors caution that they did
not take the mother’s own behavioral problems into account. Perhaps inattentive, hyperactive, impulsive, delinquent women are more likely to use cannabis during pregnancy. These problems might also have a genetic component. Thus, the children might have inherited these troubles from their mothers regardless of marijuana exposure. Alternatively, mothers with these qualities may serve as poor parents, leading their children to develop problems.
In addition, the researchers performed dozens of analyses only to re- veal a few significant effects. They did not correct their statistics for the number of analyses conducted. Therefore, some of these findings may have occurred simply by chance. As a result, a clear, confirmed link be- tween prenatal exposure to cannabis and later problems remains elusive. Nevertheless, pregnant women would probably do well to abstain from all drugs, as their long-term impact on offspring is often negative or un- known.