Medical News Today: Is marijuana use associated with a higher risk of cancer?

As the use of marijuana is increasing in the United States, researchers are asking whether the use of this substance — particularly smoking joints — is associated with an increased risk of any form of cancer, and, if so, which.

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Marijuana is one of the most widely used drugs in the United States, with more than one in seven adults reporting that they used marijuana in 2017.

Statistical reports project that sales of cannabis for recreational purposes in the U.S. will amount to $11,670 million between 2014 and 2020.

According to recent research, smoking a joint remains one of the main ways in which individuals use marijuana recreationally.

While specialists already know that smoking tobacco cigarettes is a top risk factor for many forms of cancer, it remains unclear whether smoking marijuana can increase cancer risk in a similar way.

To try to find out whether there is a link between recreational marijuana use and cancer, researchers from the Northern California Institute of Research and Education in San Francisco and other collaborating institutions recently conducted a systematic review and meta-analysis of studies assessing this potential association.

In their paper — which appears in JAMA Network Open — the team notes that marijuana joints and tobacco cigarettes share many of the same potentially carcinogenic substances.

“Marijuana smoke and tobacco smoke share carcinogens, including toxic gases, reactive oxygen species, and polycyclic aromatic hydrocarbons, such as benzo[alpha]pyrene and phenols, which are 20 times higher in unfiltered marijuana than in cigarette smoke,” write first author Dr. Mehrnaz Ghasemiesfe and colleagues.

“Given that cancer is the second leading cause of death in the United States and smoking remains the largest preventable cause of cancer death (responsible for 28.6% of all cancer deaths in 2014), similar toxic effects of marijuana smoke and tobacco smoke may have important health implications,” they go on to emphasize.

‘Misinformation — a threat to public health’

Dr. Ghasemiesfe and team identified 25 studies assessing the link between marijuana use and the risk of developing different forms of cancer. More specifically, eight of these studies focused on lung cancer, nine looked at head and neck cancers, seven examined urogenital cancers, and four covered various other forms of cancer.

The studies found associations of different strengths between long-term marijuana use and various forms of cancer.

The researchers note that the study results regarding the link between marijuana lung cancer risk were mixed — so much so that they were unable to pool the data.

For head and neck cancer, the researchers concluded that “ever use,” which they define as exposure equivalent to smoking one joint a day for 1 year, did not appear to increase the risk, although the strength of the evidence was low. However, the studies produced mixed findings for heavier users.

There was insufficient evidence to link this drug to a heightened risk of nasopharyngeal carcinoma, oral cancer, or laryngeal, pharyngeal, and esophageal cancers.

Among urogenital cancers, the investigators found that individuals who had used marijuana for more than 10 years appeared to have a higher risk of testicular cancer — more specifically, testicular germ cell tumors. Once again, however, the strength of the existing evidence was low.

There was insufficient evidence that marijuana use was associated with an increased risk of other forms of cancer, including prostate, cervical, penile, and colorectal cancers.

Dr. Ghasemiesfe and colleagues note that the studies that they had access to had many limitations, including numerous methodological problems and an insufficient number of participants who reported high levels of marijuana use.

Going forward, the team suggests that there is an urgent need for better quality studies assessing the potential relationship between marijuana and cancer. The researchers conclude:

“Misinformation [on this topic] may constitute an additional threat to public health; cannabis is being increasingly marketed as a potential cure for cancer in the absence of evidence, with enormous engagement in this misinformation on social media, particularly in states that have legalized recreational use.”

“As marijuana smoking and other forms of marijuana use increase and evolve, it will be critical to develop a better understanding of the association of these different use behaviors with the development of cancers and other chronic conditions and to ensure accurate messaging to the public,” they add.