Splitting parents who shack up with new partners can unwittingly push their children towards cannabis use.
University of Queensland research has shown a link between cannabis use and marital changes after studying 3008 mothers and their children up to age 21, between 2001 and 2004 in Brisbane.
"Our study suggests that exposure to three or more changes in maternal marital status during childhood and early adolescence more than doubles the risk of a child beginning to use cannabis," lead researcher Dr Reza Hayatbakhsh said.
"More frequent changes in marital status predict greater risk and also the earlier onset of use of cannabis."
Dr Hayatbakhsh, from UQ's School of Population Health, said almost half of the young adults had used cannabis at some time, about a quarter before the age of 15.
Children who experienced no change in their parents' marital partners between ages five and 14 were less likely to report cannabis use.
The results are the latest from the ongoing Mater-UQ Study of Pregnancy -- one of the world's longest running health studies.
The Mater Study was started at UQ in 1981 as a health and social study of 7223 pregnant women.
Researchers continue to extract new information about social, emotional and medical issues as the families grow.
Dr Hayatbakhsh, who is now studying for his PhD in epidemiology, said mothers and children in the study were asked how often children used cannabis and when they first began to use it.