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Many breast cancer patients are using cannabis without telling their doctors, study finds

Almost half believed cannabis could treat cancer itself.

Marisa Weiss is founder and chief medical officer of  She is also an adjunct investigator at the Lankenau Institute for Medical Research and a radiation oncologist at Lankenau Medical Center.
Marisa Weiss is founder and chief medical officer of She is also an adjunct investigator at the Lankenau Institute for Medical Research and a radiation oncologist at Lankenau Medical Center.Read moreCourtesy of Main Line Health

A wide-ranging survey of breast cancer patients found that 42% had used some form of cannabis to reduce symptoms while only 39% of cannabis users told their doctors about it, according to a new study published in Cancer, a journal of the American Cancer Society. Almost half believed that medical cannabis could treat cancer itself, although there is no scientific evidence from human trials to support that idea.

The study concluded that both patients and their doctors need more information about marijuana, because the drug could interfere with some treatments or make patients vulnerable to new side effects. Patients who are immunocompromised because of their cancer treatments should be especially wary of marijuana purchased illegally, as street drugs often contain dangerous impurities.

The study was spearheaded by, which is based in Ardmore, and Main Line Health. It was led by Marisa Weiss, founder and chief medical officer of, and a radiation oncologist at Lankenau Medical Center.

Although other studies have found widespread use of cannabis among cancer patients, Weiss said this one was important because it included a national sample, not just patents from one institution. Medical marijuana is now available in 36 states, including Pennsylvania and New Jersey, she said.

“More and more patients were using it and asking questions about it,” said Weiss, who is qualified to certify patients for Pennsylvania’s medical marijuana program. “Its use is widespread and we know relatively little about it.”

The anonymous online survey involved 612 patients recruited through and On average, they had received a cancer diagnosis 20 months earlier.

Of those who reported use of medical or recreational cannabis for symptoms, 78% said they used it for pain and 70% for insomnia. About half said they used it for anxiety, stress or nausea. Seventy-nine percent said they had used it during treatment. Three quarters said cannabis was “extremely or very helpful” for their symptoms, and more than half said they had found no other successful alternatives. Younger patients were more positive about the impact of cannabis than those over age 66.

Most were unaware that the quality of cannabis products can vary widely and saw the drug as a natural product that is better than “chemicals.” For the record, natural products also contain chemicals and many well-studied medications are based on ingredients initially discovered in plants.

The survey found that many patients were uncomfortable talking with their doctors about cannabis. Some who tried were disappointed in the resulting conversation.

In an education section meant to help doctors understand the evidence for cannabis use, the study found stronger research support for THC, one element of cannabis, than for CBD, another type of cannabinoid. THC can cause intoxication, mood elevation and sedation. CBD can cause sedation in high doses, but generally has less noticeable effects. Only a quarter of the patients in the survey preferred THC-dominant products. Most sought CBD only or a combo of CBD and THC.

The report said that studies support the use of THC for pain, nausea, anxiety and insomnia. It said research on CBD and symptoms is “scarce.” Weiss is currently conducting a trial at Lankenau of CBD — the Coala-T-CBD Study — for neuropathic pain after treatment for breast, ovarian or colon cancer. Weiss said that there is evidence that CBD is helpful for anxiety, sleep and pain, but that it is “not as rigorous as we’d like it to be.”

» READ MORE: Temple research finds that tweaking CBD may help with neuropathy from cancer treatment

Patients ingested cannabis in a variety of ways, including smoking and vaping. Weiss said smoking or vaping is a bad idea for patients undergoing active treatment for breast cancer. Some treatments can affect immune and lung function. Smoking cannabis adds another insult to stressed lungs. Street drugs are especially dangerous because they may be contaminated with bacteria and fungi, heavy metals, pesticides and solvents. She thinks it is important for cancer patients to spend the extra money on medical-grade cannabis products.

Cannabis can also impact how the body breaks down medications used to treat cancer or symptoms. For example, Weiss said, “anybody who’s on warfarin [a blood thinner] should not be using medical cannabis.”

The study said that pre-clinical work suggests that cannabis may suppress the immune system, a sign that it should be used with “great caution” in patients receiving immunotherapy.

As for cannabis as a direct treatment for cancer, the study said that very early research in animals shows that THC and CBD may reduce tumor growth and metastases in animals. Results in mice often do not hold true in humans, however, and the effect of cannabis on cancer in people is unknown.

“There is no established role in people on the safety and efficacy [of cannabis] as a cancer treatment,” Weiss said.

Nonetheless, she said, some patients are trying high-dose THC because of information they find online. “It’s not a good thing,” Weiss said. “I’ve had people that are completely stoned.”

Weiss said it has been difficult to study medical marijuana. It is still illegal on the federal level and researchers face much extra regulation. It is also hard for researchers to find high-quality cannabis and funding is a challenge. “We absolutely need more research in this arena,” she said.