A new study published in Nature Mental Health has found evidence that using cannabis and tobacco together appears to substantially increase the risk of developing psychosis, particularly among young people already considered at clinical high risk.
Cannabis use has long been linked to psychosis, and tobacco use is also more common among people who experience psychotic symptoms. What has been less clear is how these substances interact when they are used together. This large study helps to fill that gap by examining patterns of combined use and the likelihood of transitioning to psychosis over time.
The researchers, From Vanderbilt University Medical Centre in the USA,  followed a large group of people identified as being at clinical high risk of psychosis and assessed their cannabis and tobacco use. They found that people who used both substances were significantly more likely to develop psychosis than those who used neither or only one. Strikingly, the elevated risk was not limited to people who used both substances heavily. Heavy cannabis use combined with even light tobacco use was associated with around a three‑fold increase in the likelihood of transition to psychosis.
Cannabis and tobacco are often used together, particularly by young people. Tobacco is frequently overlooked in mental health prevention strategies, while cannabis tends to receive most of the attention. This study suggests that separating the two may miss an important part of the picture.
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What impact might this new evidence have?
For youth mental health services, the results strengthen the case for integrated approaches to substance‑use prevention. Addressing cannabis use without also tackling tobacco use may limit the impact of early intervention efforts. The authors argue that prevention and support strategies should reflect the reality of co‑use, rather than treating each substance in isolation.
At a time when cannabis laws and social attitudes are changing in many countries, evidence like this is crucial. It underlines the need for clear, evidence‑based public health messaging and for services that can support young people to reduce risk before serious mental illness develops.
