When someone uses both e-cigarettes and combustible cigarettes, it's called "dual use." And those interested in public health and tobacco policy disagree on whether dual use is harmful, potentially beneficial or neutral – because there isn't much research on what dual use actually means in day-to-day life.
"There's lots of disagreement as to whether dual use is a good thing or a bad thing," said Matthew Carpenter, Ph.D., co-leader of the Cancer Prevention and Control research program at MUSC Hollings Cancer Center and the Flora McLeod Edwards Distinguished Endowed Chair in Cancer Research at MUSC.
"You can make an argument for either, and you can find evidence either way that dual use could be a good thing or a bad thing – it really depends on what you're comparing it to," he said.
To begin to answer that question, Carpenter is partnering with Elias Klemperer, Ph.D., of the University of Vermont, on a nationwide study dubbed DUET, or Dual Use Evaluation and Tracking, to follow dual users and see how their usage changes – or doesn't – over time.
The $3.2 million study, funded by the National Institute on Drug Abuse, follows a naturalistic model. The researchers will track participants' usage but won't intervene to help or persuade people to stop smoking or vaping.
This fall, they'll begin recruiting participants for three research groups: a group of dual users, a group that exclusively smokes cigarettes and a group that exclusively uses e-cigarettes.
"Our main purpose is to look at the patterns and trajectories of use over time, within and between the groups. How much are people changing? Is it a stable behavior or is it something that fluctuates from day to day, week to week, situation to situation?" Carpenter said.
Although "dual use" is a commonly used term to describe people who both vape e-cigarettes and smoke combustible cigarettes, dual usage can look quite different from person to person.
Some may primarily smoke cigarettes and reserve vapes for places where cigarettes are banned. Others may be trying to stop smoking and are using e-cigarettes as a bridge to completely quit. Some may have almost equal usage between the two products while others may use one product heavily and the other sparingly.
Carpenter and other tobacco researchers at Hollings advocate for e-cigarettes as a less harmful option for people who have unsuccessfully tried to stop smoking combustible cigarettes. Other public health advocates, however, argue that vaping is more likely to become an "add-on" to cigarette smoking rather than a substitute, which would mean it doesn't actually help the individual to reduce negative health effects.
Identifying the pattern of dual use that's most likely to lead to quitting cigarettes altogether can help researchers to develop smoking cessation strategies for dual users. Carpenter suspects that frequency of use will be a top factor, but it could be that flavored vapes, a certain type of vaping device, the amount of use or some other variable turns out to affect whether people completely stop using combustible cigarettes.
"Is there a threshold that we can identify as a tipping point? Obviously, the more somebody uses an e-cigarette and the less they use a combustible cigarette, the more likely they are to quit smoking," Carpenter said. "But where is that tipping point? That's the essence of our second question."
The team will accept participants regardless of desire or intention to stop smoking cigarettes. They will track participants' intentions over time, however.
Because this is a nationwide study, the researchers will also look for patterns of use influenced by state tobacco policy.
"We can't look at it as a major focus, because we won't have sufficient numbers of people in every single state, but we can explore that," Carpenter said.
They'll also consider how cannabis usage influences participants' vaping and cigarette usage.
The study will be the largest and longest specifically designed to track usage patterns among dual users.
