Heavy cannabis use may impact the human genome, according to new research. Not only that but people who use tobacco in addition to cannabis appear to be more genetically impacted than those who have never smoked cigarettes. In the journal Translational Psychiatry, the researchers write that their findings suggest a “biological response to cannabis” that results in DNA changes. 

The prolonged use of cannabis has been linked to mental health disorders and illicit drug abuse, both of which can have genetic components. Heavy use of cannabis, for example, may make a person more likely to be diagnosed with schizophrenia. To explore this further, University of Canterbury researchers in New Zealand investigated how heavy cannabis use may lead to chemical changes in the body that influence how genes work, a process known as DNA methylation.

Blood samples were taken and analyzed from 48 heavy cannabis users around the age of 28 registered in a longitudinal birth study, which has studied the health, education, and life progress of more than 1,200 New Zealand children. Some participants of the cohort included cannabis users that had never smoked cigarettes, which provided additional insight into the potential genetic differences of those who have and have never smoked tobacco.  

People who smoked cannabis were shown to have distinct DNA changes at more than 500 genetic sites, yet those who smoked both tobacco and cannabis saw the greatest differences. While tobacco was shown to have a stronger effect on DNA than cannabis, cannabis appeared to more directly impact genes involved in brain and heart function. The researchers add that their findings suggest caution should be exhibited when interpreting cannabis exposure studies that don’t exclude cigarette smokers.

"We think assessing cannabis's potential effect on DNA is timely. It's currently the most widely used illicit psychoactive substance in the world and this could be predicted to increase with decriminalization or legalization," said Osborne.

The study authors note that their findings are based on a relatively small sample size, adding that more research is needed to fully understand the effects of cannabis smoking on DNA.