A 2021 longitudinal study conducted among populations of HIV-positive and HIV-negative adults found that smoke-related carcinogenic toxicants and biomarkers detected in tobacco smokers were also detected in exclusive marijuana smokers, including carbon monoxide (CO), polycyclic aromatic hydrocarbons (PAHs), aldehydes (such as acrolein), acrylonitrile and acrylamide metabolites, but exposures are lower compared with tobacco or dual smokers. Increased levels of acrolein exposure by tobacco smoking but not exclusive marijuana smoking were detected both in HIV-positive and HIV-negative adults, and contribute to increased diagnoses of cardiovascular diseases and respiratory diseases among tobacco smokers.Cannabis smoke contains thousands of organic and inorganic chemical compounds. This tar is chemically similar to that found in tobacco smoke or cigars. Over fifty known carcinogens have been identified in cannabis smoke. These include nitrosamines, reactive aldehydes, and polycylic hydrocarbons, including benz[a]pyrene. Marijuana smoke was listed as a cancer agent in California in 2009. A study by the British Lung Foundation published in 2012 identifies cannabis smoke as a carcinogen and also finds awareness of the danger is low compared with the high awareness of the dangers of smoking tobacco particularly among younger users. Other observations include possible increased risk from each cigarette; lack of research on the effect of cannabis smoke alone; 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".
Oral ingestion use eliminates the need to inhale toxic combustion products created by smoking and therefore negates the risk of respiratory harm associated with cannabis smoking. Effects on driving
While several studies have shown increased risk associated with cannabis use by drivers, other studies have not found increased risk. Cannabis usage has been shown in some studies to have a negative effect on driving ability. The British Medical Journal indicated that "drivers who consume cannabis within three hours of driving are nearly twice as likely to cause a vehicle collision as those who are not under the influence of drugs or alcohol".In Cannabis and driving: a review of the literature and commentary, the United Kingdoms Department for Transport reviewed data on cannabis and driving, finding although impaired, "subjects under cannabis treatment appear to perceive that they are indeed impaired. Where they can compensate, they do". In a review of driving simulator studies, researchers note that "even in those who learn to compensate for a drugs impairing effects, substantial impairment in performance can still be observed under conditions of general task performance (i.e. when no contingencies are present to maintain compensated performance). "A 2012 meta-analysis found that acute cannabis use increased the risk of an automobile crash.