E-cigarettes are often promoted as a better way to quit smoking than nicotine patches or gum. But Duke University School of Medicine researchers say a key piece of the story is often missing: how many people end up using both cigarettes and e-cigarettes.
In a paper published in Nicotine and Tobacco Research, researchers reviewed 71 clinical trials included in a major 2025 analysis of e-cigarettes for smoking cessation. More than 80% of the studies never reported rates of "dual use,” or smoking and vaping at the same time. Only 12 reported it directly, and just one clearly defined the term.
"We found that dual use is very infrequently reported," said senior study author Joseph McClernon, PhD, professor of psychiatry and behavioral sciences and associate director of the Duke Clinical & Translational Science Institute.
"And even when it is reported, how dual use is defined is typically unclear. This impedes having a clear picture of the efficacy of e-cigarettes as an intervention to help people quit smoking."
That matters because people who both smoke and vape tend to have a harder time quitting smoking and often remain highly dependent on nicotine. Their exposure to harmful chemicals is also much closer to that of cigarette smokers than to those who fully switched to e-cigarettes.
Among studies that measured it, dual use averaged 42.6% four weeks after participants started using e-cigarettes.
The paper's two co-first authors are Makayla Stewart, an undergraduate at North Carolina Central University who worked on the project through a joint Duke-NCCU summer research program, and Abigail Hoyt, a recent Duke graduate who completed an independent study with McClernon.
Looking beyond quit rates
Why focus on dual use rather than quit rates alone? Cessation treatments were traditionally studied only briefly or were not sold on the open market, McClernon said, so continued use was not a concern.
"But e-cigarettes are different — they are available in the market, are relatively cheap, and don't require a prescription. And, unlike other treatments, e-cigarettes can be addictive in their own right," McClernon said.
One reason dual use may persist is that vaping allows people to consume nicotine in more places where smoking is prohibited. Someone who can’t light a cigarette indoors may instead reach for an e-cigarette, increasing opportunities to use nicotine throughout the day.
That doesn’t necessarily mean e-cigarettes aren’t helping, McClernon said. Cutting back on cigarette smoking can reduce exposure to many of the toxic chemicals produced by burning tobacco. For some smokers, switching partially may be a step toward eventually quitting altogether.
Researchers say quit rates remain the most important outcome. But to fully understand the impact of e-cigarettes, they also need to understand who keeps using both products.