While cannabis is associated with certain health risks, it is nearly impossible to overdose from the consumption of cannabis.A recent study by the Canadian government found cannabis contained more toxic substances than tobacco smoke. The study determined that marijuana smoke contained 20 times more ammonia, and five times more hydrogen cyanide and nitrous oxides than tobacco smoke. In spite of this, a recent large-scale study found no correlation between heavy marijuana use and lung cancer, despite noting that cannabis contains the same carcinogens as tobacco. The same study found a 20-fold increase in lung-cancer rates of smokers who consumed two or more packs of cigarettes per day. These researchers postulated that the THC present may have a “protective effect” by causing aging cells to die before they become cancerous. Other recent research suggest the cannabinoid CBD may stop certain cancers from spreading, although not in concentrations consumed during smoking.
In contrast, a study published in the January 2008 edition of the journal Respirology found that “regular” cannabis smokers who developed bullous lung disease did so on average 24 years sooner than tobacco smoking counterparts. Researchers attributed this to the inhalation of a larger volume of smoke, and typically holding it for four times longer than tobacco smokers. Bullous lung disease is considered an uncommon cause of respiratory distress. In general, habitual inhalation of any kind of smoke is detrimental to lung health.
Cannabis use has been linked to exacerbating the effects of depression, psychosis, schizophrenia, bronchitis, and emphysema by several peer-reviewed studies for those who are vulnerable to such illnesses based on personal or family history. More recently, the Dunedin Multidisciplinary Health and Development Study published research showing an increased risk of psychosis for cannabis users with a certain genetic predisposition, held by 25% of the population.
In July 2007, British medical journal The Lancet published a study that indicates that cannabis users have, on average, a 41% greater risk of developing psychosis than non-users. The risk was most pronounced in cases with an existing risk of psychotic disorder, and was said to grow up to 200% for the most-frequent users.
A 2008 study by the National Institutes of Health suggested a link between chronic marijuana smoking and increased risk of heart attack or stroke.