Medical Marijuana: The State of the Science

Michael E. Schatman, PhD


February 06, 2015

In This Article

Cannabis Use and Physical Health

Safety issues associated with the habitual use of smoked cannabis are discouraging. For many years, the myth of smoked cannabis being "healthier" or "no worse" than smoked tobacco was perpetuated by pro-marijuana groups. Nevertheless, clinical research has provided some answers about the comparative safety of marijuana and tobacco smoking, although certain factors may complicate their direct comparison.

Henry and colleagues[20] noted in 2003 that both smoked cannabis and tobacco contain approximately 4000 chemicals and that these chemicals are essentially identical in both plants. However, given the ever-changing potency of cannabis and the fact that very few people smoke as much cannabis as tobacco, this direct comparison is difficult to make. Furthermore, the authors note that a high percentage of cannabis smokers are also tobacco smokers, thereby further confounding their relative safety risks.

In a recent systematic review by the American Academy of Neurology examining the cardiopulmonary impact of heavy marijuana smoking,[21] the authors concluded that "smoking and possibly even use of vaporized preparations expose users to carbon monoxide and other respiratory toxins." Although the authors of another recent review[22] stated that the relationship between the long-term smoking of marijuana and lung cancer is unclear, they also concluded that marijuana smoking is associated with the inflammation of large airways, increased airway resistance, and lung hyperinflation, all of which are consistent with the development of chronic bronchitis.

In another recent review,[23] the authors concluded that "smoking of cannabis is not medically recommended due to the potential respiratory tract, dangers of noxious compounds such as polycyclic aromatic hydrocarbons, tar and carbon monoxide." Suffice it to say, recent literature reviews addressing the pulmonary implications of smoking marijuana do not endorse the safety of its use.

Physical Health Risks

An abundance of literature has recently emerged identifying the physical and mental health risks associated with the use of THC. This is concerning, given that the THC content of marijuana has increased dramatically over the past two decades.[20] Whereas Aggarwal and colleagues[24] recently noted that natural cannabis contains a maximum THC concentration of 10%-15%, dispensary websites advertise strains with concentrations as high as 33%.[25] There is also some evidence suggesting that the THC potency of "medical marijuana" is higher than that of marijuana illegally sold for recreational use.[19]

In a recent review of the cardiovascular, cerebrovascular, and peripheral vascular effects of smoked marijuana, Thomas and colleagues[26] identified an association between marijuana inhalation and higher rates of acute myocardial infarction and increased cardiovascular mortality. In addition, they described published case reports that identify a safety signal between cannabis use and stroke. Of considerable concern are data indicating diminution of the volumes of the hippocampus, amygdala, and cerebellum in adult and adolescent heavy users compared with healthy controls.[27,28,29]

Although cannabis is not necessarily hepatotoxic, a study of 272 patients with chronic hepatitis C virus infection determined that daily marijuana smoking was a risk factor for progression of liver fibrosis.[30] A relatively new diagnosis associated with heavy marijuana use is cannabinoid hyperemesis syndrome, which is characterized by cyclic nausea and vomiting and compulsive bathing; it is thought to be due to the rising levels of THC in marijuana.[31]Conjunctivitis is the most common ocular adverse event associated marijuana use and may be related to an allergic response to Cannabis sativa pollen.[32]

Long-term use of cannabis has an adverse impact on reproductive potential, as it disrupts the menstrual cycle, suppresses oogenesis, and impairs embryo implantation and development.[33] Cannabis use may also have a detrimental effect on male reproductive health, as described in a recent review of published data by Barazani and colleagues.[34]

Several problems are associated with marijuana use during pregnancy, because prenatal exposure influences brain development and can result in permanent cognitive impairment.[35] Cognitive deficits resulting from prenatal exposure include inattention; impulsivity; and impairment in learning, memory, and certain aspects of executive functioning.[36] In addition, prenatal exposure to cannabis has been associated with reduction of fetal growth.[37]

Long-term cannabis use also has oral health implications. Uvulitis and nicotinic stomatitis are probably the two most common of several potential oral adverse effects.[38]

Finally, from a population health perspective, we cannot ignore the association between cannabis use and mortality. A systematic review found that people who had used marijuana had higher rates of mortality from fatal motor vehicle accidents, AIDS, and lung cancer.[39] In its totality, the available evidence should dispel the belief that marijuana is physically innocuous.


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