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Cannabis may not be dangerous, but its contaminants are.

Samples from 20 unidentified Northern California marijuana dispensaries contained bacterial and fungal pathogens that may cause serious and even fatal infections if smoked or vaped by people with impaired immune systems, according to a new study by UC Davis scientists.

“Medicinal marijuana has potential benefits — and because it is allowed for medicinal purposes, that suggests it is safe,” said Dr. Joseph Tuscano, a professor of internal medicine at UC Davis and a lead author of a study in the journal Clinical Microbiology and Infection.

But for some patients, he said, “it really isn’t.”

“Inhaling marijuana in any form provides a direct portal of entry deep into the lungs where infection can easily take hold,” he said.

The tested cannabis — collected and catalogued by Berkeley’s Steep Hill Labs, with DNA extraction by MO BIO Labs in Carlsbad — contained a wide diversity of microorganisms, many of which are implicated in serious lung infections, including Cryptococcus, Mucor, and Aspergillus fungi and Escherichia coli, Klebsiella pneumoniae and Acinetobacter baumannii bacteria.

Medical marijuana is frequently used to control nausea, pain and lack of appetite in patients with a host of conditions, including those immunocompromised from AIDS, cancer treatment, or therapy following an organ transplant.
Smoking marijuana does not heat the plant long enough to make it safe, the team said. Cooked edibles or “topicals,” applied to the skin, may be better, they said.

“For most of my patients, there are many alternative medications that can be used but if insist on cannabis, the edible or topical forms should be considered,” Tuscano said.

Tuscano first became interested in the research when one of his cancer patients developed a rare and incurable fungal infection after using aerosolized marijuana, inhaled as a mist.  Such unusual lung infections are usually attributed to hospital or community exposure, he noted, because as it does not occur to most patients and doctors that the source could be marijuana.

His patients had cancer and were undergoing intensive chemotherapy and a stem cell therapy, so their immune systems were weak. In one patient, a fungal infection was lethal.

What about users whose immune systems are healthy? There are no specific recommendations, said Dr. George Thompson, associate professor of clinical medicine at UC Davis, who also contributed to the study.

But one young man had used recreational marijuana before his diagnosis with leukemia, and may be acquired the infection before he knew he was sick, said Tuscano.

“We never know what is around the corner for any of us,” he said.

The only way to completely prevent microbes is to grow plants in a sterile environment, like a “clean room,” said biologist Anthony Torres of Steep Hill Labs, who contributed to the study.  Irradiation of the plant can reduce risk, he added.

But microbial testing can detect the levels of these contaminants, he said.

“It is important for individuals who utilize cannabis to make sure it has been tested for pesticides, mold and mycotoxins and microbiological contaminants,” said Torres.

“If you’re at a dispensary and they have the test results and reports for the samples, that creates transparency…I am a patient, and I look for the test report.”

Last October, a study conducted by Steep Hill Labs found that 84 percent of marijuana samples submitted to its lab were positive for pesticide residues. The most common was myclobutanil, a key ingredient in Eagle 20, a pesticide commonly used to combat powdery mildew. It converts into dangerous hydrogen cyanide when burned.

For now, testing is only voluntary, not mandatory. Consumers can look for labels on products before purchase.

That will change starting next January 1, when microbial testing and labeling will be required under pending state regulations. The rules, created by passage of Prop. 64 and the Medical Cannabis Regulation and Safety Act, will move California away from an unregulated marijuana marketplace to a state-regulated system.

“I believe the regulations that the state will come up with ensure that safer cannabis materials are out on the market,” said Torres.

The states of New Mexico, Colorado, Oregon and Washington require testing for mold and yeast contamination. Typical standards call for 100,000 cfu/gram (colony-forming units per gram) for yeast and mold and 100,000 cfu/gram for bacteria.

What are safe levels of contaminants?

“I don’t think anyone really knows,” said Tuscano. Despite legalization within states passing cannabis legislation, the U.S. Drug Enforcement Agency does not permit possession of marijuana for research purposes within institutions receiving federal funding. For this reason, the actual testing of study samples was conducted in a private laboratory and existing departmental funds at UC Davis, not public funds, at UC Davis were used for this research.

“There needs to be more research,” he said.