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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, has not been shown 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.

Studies regarding cancer risk

Cannabis smoke contains numerous carcinogens. Cannabis smoke (but not the plant itself) has recently been added to a "list of substances California regulators say cause cancer". A 2012 literature review by the British Lung Foundation identified cannabis smoke as a carcinogen and also found awareness of the danger was low compared with the high awareness of the dangers of smoking tobacco particularly among younger users. Other observations include increased risk from each cigarette due to drawing in large puffs of smoke and holding them; lack of research on the affect of cannabis smoke alone due to common mixing of cannabis and tobacco and frequent cigarette smoking by cannabis users; low rate of addiction compared to tobacco; and episodic nature of cannabis use compared to steady frequent smoking of tobacco.

Professor David Nutt, a UK drug expert, points out that the study cited by the British Lung Foundation has been accused of both “false reasoning” and “incorrect methodology”. Further, he notes that other studies have failed to connect cannabis with lung cancer, and accuses the BLF of "scaremongering over cannabis".

In the largest study of its kind, researchers found no cancer-cannabis connection. Donald Tashkin, a pulmonologist at University of California, Los Angeles who studied marijuana for 30 years, "hypothesized that there would be a positive association between marijuana use and lung cancer, and that the association would be more positive with heavier use". Instead, the study found "no association at all, and even a suggestion of some protective effect". 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.

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.

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.

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.

References

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2. http://www.lunguk.org/Resources/British%20Lung%20Foundation/Migrated%20Resources/Documents/A/A_Smoking_Gun.pdf
3. Tashkin DP, Simmons MS, Sherrill DL, Coulson AH (1997). "Heavy habitual marijuana smoking does not cause an accelerated decline in FEV1 with age". American Journal of Respiratory and Critical Care Medicine. 155 (1): 141–8. doi:10.1136/thx.2006.077081. PMID 9001303. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
4. Australian Government Department of Health: National Cannabis Strategy Consultation Paper, page 4. "Cannabis has been described as a 'Trojan Horse' for nicotine addiction, given the usual method of mixing cannabis with tobacco when preparing marijuana for administration."
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13. WebMD (23 May 2006). "Pot Smoking Not Linked to Lung Cancer". ScienceNOW, Abstract
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16. K.A. Rosenblatt; et al. (1 June 2004). "Marijuana Use and Risk of Oral Squamous Cell Carcinoma". Cancer Research. 64 (11): 4049–4054. doi:10.1158/0008-5472.CAN-03-3425. PMID 15173020. {{cite journal}}: Explicit use of et al. in: |author= (help)
17. ^ Dr. Steven M. Dubinett (July 2006). "Study Finds Marijuana Ingredient Promotes Tumor Growth, Impairs Anti-Tumor Defenses". {{cite news}}: Unknown parameter |org= ignored (help)
18. ^ "Study Finds No Link Between Marijuana Use And Lung Cancer". Science Daily. 26-05-2006. Retrieved 10-12-2011. {{cite news}}: Check date values in: |accessdate= and |date= (help)
19. Fred Gardner (2006-07-06). "Marijuana Smoking Does Not Cause Lung Cancer". {{cite news}}: Unknown parameter |org= ignored (help)
20. Tashkin, D. P., Simmons, M. S., Sherrill, D. L., and Coulson, A. H. 1997. Heavy habitual marijuana smoking does not cause an accelerated decline in FEV1 with age. American Journal of Respiratory and Critical Care Medicine 155(1): 141-148. Retrieved on 5 March 2007
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25. Hii, S.W., Tam, J.D.C., Thompson, B.R. & Naughton, M.T. (2008). Bullous lung disease due to marijuana. Respirology 13, 122-127
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