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Does Marijuana Cause Cancer?

Smoked marijuana and smoked tobacco are chemically very similar.

Key points

  • Smoking pot results in five times the amount of absorption of carbon monoxide and four to five times more tar being retained in the lungs.
  • Careful scientific analyses have identified at least 6,000 of the same chemicals in marijuana smoke as are present in tobacco.
  • Because marijuana smoke is very similar to cigarette smoke, the long-term effects may be the same, but it has not been proven.

The greatest health danger of marijuana is due to smoking, like cigarettes. While THC is a relatively safe drug, smoked marijuana is hazardous. Careful scientific analyses have identified at least 6,000 of the same chemicals in marijuana smoke as are present in tobacco (Iversen, 2008). The main difference between the two is THC in marijuana and nicotine in tobacco. Otherwise, smoked marijuana and smoked tobacco are chemically very similar.

Furthermore, one of the most potent carcinogens in tobacco smoke, benzo(α)pyrene, is present in even greater amounts in marijuana smoke. As marijuana smokers frequently inhale and hold the smoke in their lungs, this increases the amount of tar deposited in the respiratory system by about a factor of four. No wonder research shows that approximately 20% of regular pot smokers (and it only takes 3 to 4 joints a day) complain of chronic bronchitis, coughing, and excess mucus (Tashkin, 2005).

A burning tobacco cigarette can be thought of as a "miniature chemical factory" (Iverson, 2008). The process of combustion creates extra chemicals on top of those already present in the plant material.

Smoke consists of two components. They are minute droplets in the particulate phase and volatile gases in the vapor phase. Experienced pot smokers inhale more deeply and hold their breath longer when compared to cigarette smokers. Wu et al. (1988) compared the amount of particulate matter (tar) and carbon monoxide absorbed in volunteers who smoked both pot and tobacco. Compared to tobacco, smoking pot resulted in five times the amount of absorption of carbon monoxide and four to five times more tar being retained in the lungs.

Some studies indicate that when pot with a higher THC content is smoked, users take smaller drags and don't hold it in as long (Matthias et al. 1997). Experienced users know how to adjust their smoking to attain a certain high. It may be that smoking joints with higher THC content results in less tar intake. Water pipes, filters, and vaporizers may also reduce the tar before it reaches the lungs.


One alternative to smoking marijuana is eating it. With the explosion of medical marijuana collectives, oral forms of THC abound. Collectives sell lollipops, drinks, and pastries. Home bakers like to make marijuana butter and use it to bake brownies and other goodies.

This THC delivery method is less reliable than smoking. Smoking produces a rapid and more controllable high. Smoked marijuana seems to go directly to the cannabinoid receptors in the brain within seconds.

Unlike smoking, eating pot takes a lot longer to get high. The food and pot have to go through the digestive system and liver before reaching general circulation which takes longer. It can take 1 to 4 hours after ingestion to feel high, and because smokers don't feel high, they eat more and more. Ingesting marijuana is further complicated if the person has recently eaten a big meal and especially one with high-fat content.

Regulating the high through ingestion of homemade goodies is hard. And, eating collective-bought goodies introduces new problems. Because cannabis is unregulated, it is impossible to know the source, THC content, and even if pesticides were used. Finding a product with a consistent, reliable dose may be challenging.

Is there a correlation between THC and cancer?

To date, no large-scale, epidemiological studies exist demonstrating a correlation between THC and cancer. During the early 1980s, Kaiser Permanente studied over 65,000 patients with a 10-year follow-up. About half of these patients admitted to having tried marijuana. One hundred eighty-two tobacco-related cancers were found but no effects of marijuana use on the risk of cancer. This study has some important limitations. First, the marijuana smokers were young (15 to 39), their usage was relatively light, and the follow-up period was short.

However, Tashkin's (2005) research at UCLA-MC showed increased redness, swelling, and white blood cell count in regular marijuana users. Furthermore, because marijuana smoke is very similar to cigarette smoke, it seems quite likely that long-term effects may be the same. Doll et al. (2005) conducted a 50-year follow-up study of 40,000 volunteers and showed that half of all regular cigarette smokers eventually die of lung, mouth, throat, larynx, pancreatic or bladder cancer, or asthma or emphysema (Doll, 2005). It may be that cannabis, too, will prove lethal and it is just a matter of time.

In sum, just because there is not a large-scale, longitudinal study proving marijuana smoke causes cancer does not mean it can be ruled out. Marijuana smoke, like cigarette smoke, contains known carcinogens. There is considerable evidence that cigarette smoking is related to lung cancer and many other health hazards. It took decades to demonstrate this. The widespread use of marijuana for recreational uses is a relatively new phenomenon. With time and more research, marijuana smoke may very well prove to be as lethal as cigarette smoke.


Doll R, Peto R, Boreham J, Sutherland I. Mortality from cancer in relation to smoking: 50 years observation on British doctors. Br J Cancer, 2005;92:426-429.

Matthias P, Tashkin DP, Marques-Magallanes JA, et al. Effects of varying marijuana potency on deposition of tar and delta-9-THC in the lung during smoking. Pharmacol Biochem Behav. 1997;58:1145-1150.

Iversen L L. (2008). The Science of Marijuana. Oxford: Oxford University Press.
Tashkin DP. Smoked marijuana as a cause of lung injury. Monalid Arch Chest Dis. 2005; 63:93-10

Wu TC, Tashkin DP, Djaheb B, Rose JE. Pulmonary hazards of smoking marijuana as compared with tobacco. New Engl J Med. 1988; 347-351.

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