Contaminants contained in natural cannabis continue to pose a health threat to patients who must obtain their medicine from unscrupulous illicit markets. One popular fear is that illegal marijuana might be intentionally saturated with other illegal drugs. But that possibility is unlikely, because intentional contamination with other illegal substances would not be economically feasible for the typical drug dealer. Ignorant purchasers might be duped into buying oregano or some other green vegetable material ("GVM" is a common term used by police) resembling marijuana, but such costly deceptions are not often repeated. Several other contaminant hazards pose a much more serious threat to medical marijuana users.
Harmful pesticides may be absorbed by the marijuana plant during cultivation and then enter the human body through administration. Repeated exposure to unknown chemical poisons may build to toxic levels in a user’s system. In one example from Hayward California, a well-known marijuana patient with an unusual immune disease died from repeated exposure to the miticide "Avid". Avid is now illegal to sell in the US, though it remains available from Canadian sources. Directed for used on ornamentals only, Avid contains a powerful neurotoxin that is absorbed by all types of living tissue and builds to toxic levels cumulatively. Because marijuana patients commonly consume cannabis every day for many years, there is a substantial threat of health complications through repeated exposure to toxic pesticides. Researchers have frequently linked health hazards to a variety of common compounds long after the products have been put in use. Even well-intentioned marijuana farmers may unwittingly deliver harmful contaminants.
The US government provides another contaminant risk in its spraying programs intended to stop the use of marijuana. Herbicides used to eradicate illegal crops have many known health hazards. The United States government sprayed the herbicide Paraquat on marijuana crops in Mexico years ago, and in Hawaii more recently. Paraquat and other Drug War pesticides permeate vegetative growth and are definitely injurious to human health. Excessively yellow marijuana may be tainted with Paraquat that can cause serious lung damage. Gernment sources have assured us that this military surplus poison is no longer in use, yet there are many other noxious poisons routinely applied to outdoor marijuana plants. In 1998, the Drug Enforcement Administration released plans to spray another herbicide, Triclopyr, on wild hemp fields across America. Typically, these widespread crop eradication programs are executed without adherence to environmental impact studies required by law.. Even more alarming, crop eradication programs outside the United States operate with no concern for environmental or public health issues. For example, the industrial weed killer, Tebutheriurin, is so strong that a few granules spread on grass tufts can kill trees located several yards away. Contamination of water supplies with Tebuthiurin could cause widespread destruction of entire ecosystems. Dow Chemical Corporation, manufacturer of the toxic defoliant called Agent Orange during the Vietnam War, is one of several corporations that sided with environmentalists in resisting a US plan to use Tebuthiurin in crop eradication programs in Columbia on an estimated 150,000 acres. The intentional poisoning of rural landscapes by the government bodes ill for both the targeted outdoor marijuana gardens and for native wildlife. Drug eradication plans to unleash a new marijuana-eating fungus in America have met with harsh criticism. Bill Graves, senior biologist at the University of Florida Research Center, is concerned about the possibility of unpredictable mutations. Graves has said, “I believe that if this fungus is unleashed for this kind of problem, it’s going to create its own problems. If it isn’t executed effectively, it’s going to target and kill rare and endangered species.” The effects of mutant bioherbicides on the health of medical marijuana consumers are unknown and impossible to predict.
A third contaminant hazard is found in bacterial or fungal infections produced by careless cultivation techniques. A majority of the several hundred organisms associated with marijuana are strictly plant pathogens that cannot infect humans. A much smaller number of plant pathogens are also found in cannabis that has been improperly dried or stored. Some of these organisms may infect immune-suppressed individuals and become human pathogens. Also, a small handful of human pathogens have been isolated from samples of poor quality marijuana. These contaminants are highly infectious and potentially toxic. One bacterial contaminant, Aspergillus, sometimes found in poorly-stored marijuana, has been cited by transplant specialists as a risk that may disqualify a candidate from the transplant program. Such was the case of Tim Garon of Washington State who was refused a liver transplant on those exact grounds and subsequently died in 2008.
Suppliers of medical grades of cannabis must be particularly stringent in their cultivation techniques because HIV and AIDS patients suffering from immune suppression typically comprise 75% of medical user populations. Properly cultivated marijuana and its constituent cannabinoid compounds are among the safest drugs known to medical science. , , 
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 Grinspoon and Bakalar, Marijuana: The Forbidden Medicine. New Haven: Yale University Press, 1993
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