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Cannabis-associated respiratory disease can refer to neoplastic processes or to structural damage to the lung.

It is often compared to the damage done by tobacco, but it is the subject of much less study. Cannabis is sometimes considered more dangerous because filters are usually not used, and sometimes considered less dangerous because different molecules are involved.

The process most popularly used to ingest cannabis is smoking, and for this reason most research has evaluated health effects from this method of ingestion. Other methods of ingestion may have lower or higher health risks. Tobacco smoking has well-established risks such as bronchitis, coughing, overproduction of mucus, wheezing, and addiction. Similar risks for smoking cannabis related to airway inflammation have been suggested in a study of healthy cannabis users who exhibited similar early characteristics to tobacco smoking.

The effects of tobacco and cannabis smoking differ, however, as they affect different parts of the respiratory tract: whereas tobacco tends to penetrate to the smaller, peripheral passageways of the lungs, cannabis tends to concentrate on the larger, central passageways. One consequence of this is that cannabis, unlike tobacco, does not appear to cause emphysema, though this claim is disputed. A 2002 report by the British Lung Foundation estimated that three to four cannabis cigarettes a day were associated with the same amount of damage to the lungs as 20 or more tobacco cigarettes a day. Unlike tobacco, regular cannabis use does not appear to cause chronic obstructive pulmonary disease.

In some cases, cannabis users mix commercial tobacco in joints, called "Spliff" (popular in Europe), tobacco mixed with hash in a chillum (India), or cannabis rolled in tobacco leaves (a blunt), which would expose the user to the additional risks of tobacco, such as rapid physical addiction to nicotine.

Cancer risk

Cannabis smoke contains numerous carcinogens. Surprisingly, an extensive study published in 2006 by Donald Tashkin of the University of California, Los Angeles found that there is no significant link between smoking cannabis and lung cancer. The study, which involved a large population sample (1,200 people with lung, neck, or head cancer, and a matching group of 1,040 without cancer) found no correlation between marijuana smoking and increased lung cancer risk, with the same being true for head and neck cancers as well. The results indicated no correlation between long and short-term cannabis use and cancer, indicating a possible therapeutic effect. Extensive cellular studies and some studies in animal models suggest that THC or cannabidiol has antitumor properties, either by encouraging programmed cell death of genetically damaged cells that can become cancerous, or by restricting the development of the blood supply that feeds tumors, or both. Unlike most other studies, this one had a very large sample size and was controlled for tobacco, alcohol, and several socio-demographic factors, which likely confounded the other studies.

In 2007 a significantly smaller study was released by the Medical Research Institute of New Zealand suggested that smoking cannabis increased the risk of lung cancer by 5.7 times over non-smokers. The small 79 person study noted that "In the near future we may see an 'epidemic' of lung cancers connected with this new carcinogen. And the future risk probably applies to many other countries, where increasing use of cannabis among young adults and adolescents is becoming a major public health problem."

Prior, a 1997 study examining the records of 64,855 Kaiser patients (14,033 of whom identified themselves as current smokers), also found no positive correlation between cannabis use and cancer.

Conversely, the 2008 case-control study from New Zealand mentioned earlier by the Cannabis and Respiratory Disease Research Group examining adults under age 55 with lung cancer concluded that smoking cannabis was significantly associated with risk of developing lung cancer, after controlling for smoking tobacco; the highest tertile of marijuana smokers were estimated to have a 5.7-fold higher risk of lung cancer compared to nonusers.

A Research Triangle Institute study concluded that THC, a dilative agent (bronchodilator), may help cleanse the lungs by dilating the bronchi, and could actively reduce the instance of tumors. Additionally, a study by Rosenblatt et al. found no association between marijuana use and the development of head and neck squamous cell carcinoma. However, a contrasting study conducted in 2000 linked the smoking of cannabis to the growth of cancerous tumors through the impairment of anti-tumor defenses in mice.

A preliminary 2009 study found that cannabis use may increase the risk of testicular cancer. In particular, the risk of developing nonseminoma testicular cancer, a more aggressive form of the disease, was increased in current cannabis users and even greater in long-term chronic users. This however is overshadowed by the fact that researchers are unable to determine what about marijuana causes the increased risk, or for that matter if a positive correlation can be established. A statement was released by the researchers that performed the study as seen below: "This is the first study to look at this question, and by itself is not definitive. And there's a lot more research that would have to be done in order to prove that marijuana use really increases a man's risk of developing testicular cancer", - Stephen Schwartz.

Cannabis smoke (but not the plant itself) has recently been added to a "list of substances California regulators say cause cancer". California's Office of Environmental Health Hazard assessment has added cannabis smoke to the list after it found that it "contains 33 of the same harmful chemicals as tobacco smoke."

UCLA study

On 23 May 2006, Donald Tashkin, M.D., Professor of Medicine at the David Geffen School of Medicine at UCLA in Los Angeles announced that the use of cannabis does not appear to increase the risk of developing lung cancer, or increase the risk of head and neck cancers, such as cancer of the tongue, mouth, throat, or esophagus. The study involved 2252 participants, with some of the most chronic marijuana smokers having smoked over 22,000 marijuana cigarettes. The finding of Donald Tashkin, M.D., and his team of researchers in 2006 refined their earlier studies published in a Dec. 17th 2000 edition of the peer-reviewed journal Cancer Epidemiology Biomarker and Prevention. Many opponents of marijuana incorrectly cite the original finding of UCLA Medical Center from 2000 as "proof" that marijuana leaves the users at higher risk for cancer of the lung, and cancerous tumors, even though the researchers at the UCLA Medical Center have revised their finding with a more in-depth study on the effects of the use of marijuana. This seemed to contradict assumptions made after some studies, like those from Dale Gieringer et al., which found that 118 carcinogens were produced when marijuana underwent combustion, and two carcinogens {2-Methyl-2, 4(2H-1-benzopyran-5-ol) & 5-azulene-3,8-dione} formed when marijuana underwent vaporization with the Volcano Vaporizer. To help explain this seemingly chemical proof of carcinogenicity inherent in the process of combustion, Tashkin noted that "one possible explanation for the new findings, he said, is that THC, a chemical in marijuana smoke, may encourage aging cells to die earlier and therefore be less likely to undergo cancerous transformation."

In a study of ten smokers with mild respiratory issues Hii et al. found evidence of lung disease in the form of severe bullae (fluid-filled, thin-walled blisters) of different shapes and sizes. Despite such lung disease, the patients' chest x-rays were normal and lung function was only mildly reduced in nearly half of the patients. The cannabis-smoking patient group was, on average, 41 years old—considerably younger than previously research tobacco-smoking patients with lung disease, who had an average age of between 62–67 years. The researchers conclude that the younger age of lung disease and poorer lung function may be due to different smoking patterns demonstrated in cannabis smokers, who have been found to inhale larger amounts of smoke, which is held in the lungs for longer periods of time.

Reference

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