Many teenagers and young adults believe cannabis is relatively harmless, especially as it becomes legal in more countries and regions.
However, scientists continue to find evidence that cannabis may affect the developing brain, particularly in people who already have a higher chance of developing mental illness.
A new study suggests the risk may become even greater when cannabis is used together with tobacco.
Researchers followed more than one thousand people taking part in the North American Prodrome Longitudinal Study. These participants were considered to be at clinical high risk for psychosis because they had mild symptoms that sometimes appear before disorders such as schizophrenia develop.
The scientists examined different patterns of substance use over two years, including tobacco alone, cannabis alone, using both together, other substances, and no substance use. They then looked at which participants later developed psychotic disorders.
The results showed that people who regularly used cannabis or tobacco experienced more anxiety, depression, and early psychotic symptoms than non-users. Surprisingly, people using both substances did not have noticeably worse symptoms in the short term than those using only one.
However, over time, the difference became much clearer. Participants who heavily used cannabis together with lighter tobacco use were almost three times more likely to develop psychosis than people who used neither substance.
Scientists think there may be several reasons for this finding. Tobacco may increase the body’s absorption of THC, making cannabis affect the brain more strongly. It is also possible that people who are naturally more likely to develop psychosis are also more likely to use both substances. More research will be needed to separate these possibilities.
The findings highlight the importance of discussing cannabis and tobacco use with doctors, especially for young people with early mental health symptoms or a family history of psychotic disorders. Early support to reduce or stop substance use may become an important part of preventing severe mental illness.
The study was published in Nature Mental Health.
Overall, this research provides important evidence that using cannabis and tobacco together may increase the risk of developing psychosis in people who are already vulnerable. Because the study followed more than 1,000 participants over time, it is stronger than studies that only look at one point in time.
However, it cannot prove that cannabis and tobacco directly cause psychosis. Other genetic, environmental, or mental health factors may also contribute.
The findings suggest that doctors should routinely ask high-risk patients about both cannabis and tobacco use and provide support to help them quit.
Future studies will need to determine whether stopping both substances can actually reduce the risk of schizophrenia and other psychotic disorders.
If you care about mental health, please read studies that vegetarian diet may increase your depression risk, and Vitamin D could help reduce depression symptoms.
For more health information, please see recent studies about why pizza is a very addictive food, and MIND diet could improve cognitive health in older people.
Source: Vanderbilt University Medical Center.
