Researchers in the US suggest vaping may be a gateway to cannabis use.
Harvard Medical School researchers found that teenage e-cigarette users are 3.5 times more likely to use cannabis.
According to the US Department of Health and Human Services, although the prevalence of current cigarette smoking among young people declined from 28% in 1996 to 8% in 2016, electronic cigarette (e-cigarette) use is gaining popularity. Cannabis is also the most popular drug for high school students across the US.
The researchers analysed data from 21 previous studies - looking at more than 128,000 people between the ages of 10 and 24.
They found the likelihood of marijuana use was three to four times higher among young people who vaped than those who did not smoke at all. The chances of younger adolescents (aged 12 to 17) smoking cannabis was higher than for young adults (aged 18 to 24 years).
As the findings were made in the US, the data may not necessarily apply to the UK. Only four studies were from Europe, all of which found weaker associations between vaping and marijuana use.
Linda Bauld, professor of public health, University of Edinburgh, who was not involved with the research, said: "This doesn't prove that vaping is causing marijuana use but instead highlights something we have known for many years - that young people who are more prone to risky behaviours may try several different substances before they are legally permitted to do so. Although the paper didn't assess alcohol or other drug use, other research has found that young people who vape are also more likely to have tried alcohol, and that young people who use marijuana are also more likely to try other illegal drugs."
Prof John Britton, director of the UK Centre for Tobacco and Alcohol Studies, University of Nottingham, added: "The finding is unsurprising because marijuana is often smoked with tobacco, and tobacco smokers are more likely to use e-cigarettes. The study does not however provide any evidence that an association between e-cigarette and cannabis use is causal."
The study was published in JAMA Pediatrics.