Cannabis is generally assumed by patients to be safe for human use especially when purchased from one of Health Canada’s Licensed Producers (LP). Random tests conducted on illicitly sold “on the street” cannabis (i.e. not provided by a licensed producer authorized by Health Canada) suggests unsafe levels of mould, pests, pesticides, and other potentially harmful compounds can be found in the product. 
Many of these bacterial and mould species, along with their toxic products, such as aflatoxins, may cause allergic reactions and illnesses, and can even cause life threatening infections. While healthy individuals may not be affected by mild levels of contamination, chronically ill people who may have compromised immune systems, such as patients suffering from cancer or HIV/AIDS, have a significantly higher risk of infection from cannabis contamination.
Illicit market (‘on the street’) cannabis is not cultivated in a controlled environment and generally not tested for contamination. This can result in pathogens, heavy metals and toxin levels that are not only too high, but also may contain pesticides that are banned or unsafe for human consumption.
In a most recent study pesticide residues in cannabis smoke was studied by a group of researchers in California. They found that the potential of chemical and pesticide residue exposures to cannabis users is substantial and may pose a significant risk in the absence of adequate regulatory frameworks  Medical cannabis samples collected in Los Angeles have been found to contain pesticide residues at levels 1600 times the legal digestible amount.
According to a recent survey in Canada about one third of self-producers of cannabis for medical purpose reported that it was difficult or very difficult to learn to cultivate cannabis. In most cases these self-producers may not have the knowledge or access to proper technology and equipment to analyze and ensure safety of their cannabis.
In another study conducted in the Netherlands, the level of contaminants in the cannabis supplied by the coffe shops far exceeded the acceptable levels for medical cannabis. It also showed that all coffe shops samples were contaminated with bacteria and fungi .
Pharmaceutical grade medical cannabis produced under strict quality and safety measures is needed to prevent and manage contamination.
Health Canada’s Marijuana for Medical Purpose Regulation (MMPR) relies on commercial production of cannabis in order to achieve purity and quality of the finished dried cannabis product.
Health Canada provides licenses to medical cannabis producers License Producers who are inspected and found in compliance with applicable sections of the following Canadian acts and regulations:
- Food and Drug Act (FDA)
- Controlled Drugs and Substances Act (CDSA)
- Food and Drug Regulations (FDR)
- Natural Health Product Regulations (NHPR)
- Narcotic Control Regulations (NCR)
- Marihuana for Medical Purposes (MMPR) (introduced June 19, 2013)
Under MMPR, licensed producers must also abide by standards set in Good Production Practices (GPP), ensuring a controlled, procedure-driven cultivation, processing and testing systems. Read more here.
 Nicholas Sullivan, Sytze Elzinga, and Jeffrey C. Raber, “Determination of Pesticide Residues in Cannabis Smoke,” Journal of Toxicology, vol. 2013, Article ID 378168, 6 pages, 2013. doi:10.1155/2013/378168
 Hazekamp, A. (2006). An evaluation of the quality of medicinal grade cannabis in the Netherlands. Cannabinoids 1, 1–9.