In uneasy news for medical marijuana users, UC Davis researchers have identified potentially lethal bacteria and mold on samples from 20 Northern California pot dispensaries, leading them to warn patients with weakened immune systems to avoid smoking, vaping or inhaling aerosolized cannabis. 

“For the vast majority of cannabis users, this is not of great concern,” said Dr. George Thompson, professor in the UC Davis Department of Medical Microbiology and Immunology. But those with weakened immune systems – such as from leukemia, lymphoma, AIDS or cancer treatments – could unwittingly be exposing themselves to serious lung infections when they smoke or vape medical marijuana. “We strongly advise them to avoid it,” Thompson said.

The study’s findings were published online in a research letter in the journal Clinical Microbiology and Infection.

Cancer patients and others with lowered immune systems are typically told to avoid cut flowers and unwashed fruits or vegetables because they may harbor potentially harmful bacteria and mold, or fungi. Marijuana also belongs in that group, according to Thompson.

“Cannabis is not on that list and it’s a big oversight in our opinion,” Thompson said. “It’s basically, dead vegetative material and always covered in fungi.” 

The study began after Dr. Joseph Tuscano, a UC Davis cancer specialist, began seeing patients who were developing rare, severe lung infections while being treated for leukemia and lymphoma. 

At least two UC Davis patients died from lung infections believed to be caused from contaminated cannabis, although that assumption cannot be confirmed without samples of the medical pot they were using before becoming sickened.

Suspecting there might be a link between the infections and his patients’ use of medical marijuana, Tuscano teamed up with Thompson to study whether soil-born pathogens might be hiding in medical marijuana samples. 

The cannabis testing was done by Steep Hill Laboratories in Berkeley, which sampled medical marijuana from 20 Northern California dispensaries. All of the samples tested positive for multiple kinds of bacteria and fungi, Thompson said, many of which are linked to serious lung infections. 

The study comes as California and seven other states have legalized recreational marijuana and a majority of U.S. doctors support the use of medical marijuana to relieve patients’ symptoms, including pain, nausea and loss of appetite during chemotherapy and other treatments.

To avoid the risk of exposure to fungal infections, Thompson advises cancer patients and others with hampered immune systems to avoid smoking, vaping or inhaling aerosolized cannabis altogether. Cannabis edibles, such as baked cookies or brownies, could be an alternative.

Theoretically, Thompson said, the consumption of cooked edibles seems safer than smoking or vaping marijuana, but it’s not scientifically proven. “I give that advice with a caveat: We don’t know it’s safer, we think it probably is.”