Teen use of electronic cigarettes was associated with both a greater likelihood of smoking traditional cigarettes and greater use of both tobacco products over time, a RAND Corporation study found.

Among roughly 2,000 teens in California from ages 16 to 20, vaping at one time point was associated with greater risk of both e-cigarettes use and regular cigarette smoking later on, with Beta values of 0.09 for future smoking and 0.59 for continued vaping. E-cigarette use was also significantly associated with subsequent marijuana use in the study.

Likewise, smoking at the younger age was associated with a greater likelihood of vaping and smoking at age 20.

The longitudinal analysis in the journal Nicotine & Tobacco Research revealed "robust reciprocal associations" between vaping and smoking, such that more frequent e-cigarette use at the earlier age predicted more frequent cigarette use later on, and visa versa, said RAND researcher Michael S. Dunbar, PhD.

"We've seen a number of studies recently suggesting that e-cigarette use in teens increases the likelihood that they will smoke cigarettes," Dunbar told MedPage Today. "We know less about how these patterns of vaping and smoking unfold over time in terms of progressing to more habitual use. This study suggests that not only are teens who vape more likely to start smoking; they are also more likely to use both e-cigarettes and cigarettes more frequently as they get older."

The association between use of the e-cigarettes and cigarettes was stronger than associations between other youth risk behaviors, such as marijuana and alcohol use, and later vaping and smoking.

"While these other risk factors might be important predictors of which teens might start vaping and smoking, they don't tell us as much about how use of e-cigarettes and cigarettes escalates over time," Dunbar said. "There seems to be a unique relationship between these two nicotine products with regard to this."

The study drew on data from the Monitoring the Future (MTF) survey, which tracks behaviors, attitudes and values of teens and young adults in the U.S. A total of 2,039 teens who completed three MTF questionnaires over the ages of 16 to 20 were included in the analysis.

Auto-regressive cross-lagged (ARCL) modelling and auto-regressive latent growth modelling with structured residuals (ALT-SR) were used to examine both between-person and within-person associations between past-month frequency of e-cigarette use, cigarette use, and other risk factors (alcohol and marijuana use and mental health symptoms) over time.

The analysis indicated significant, reciprocal associations between electronic cigarette and cigarette use. In both types of models, more frequent electronic cigarette use at one time point predicted more frequent cigarette use at the next time point, and vice versa.

"In this study, both ARCL and ALT-SR models showed robust reciprocal associations between frequency of electronic cigarette and cigarette use across time," the researchers wrote. "With respect to the role of shared risk factors, ARCL models indicated that trajectories of both electronic cigarette use may be influenced by other substances or mental health problems -- suggesting that there are 'other reasons' why youth are increasing their electronic cigarette or cigarette use over time."

The ALT-SR model, however, did not tend to show a pattern of reciprocal escalation between these shared risk factors and higher electronic cigarette use.

Dunbar told MedPage Today that the findings have implications for federal health officials who are considering how to best regulate e-cigarettes to reflect their potential benefits for smoking cessation and their potential harms to young non-smokers.

"If adults really can use these products to switch from smoking to vaping that might be a good thing," he said. "But the harm reduction argument doesn't work for teens. We now know that teens are more likely to start smoking cigarettes in the future if they vape. Teen vapers are also more likely to use both of these products more frequently in the future and that suggests a trend toward more harm in the long run."