Smokers who give up cigarettes but still vape have more than a 50 per cent higher chance of developing lung cancer than those who stop altogether, according to a study of more than 4.5 million people. But the research also emphasises that using e-cigarettes to help you quit is safer than continuing to smoke.

“The study adds to the rapidly growing body of evidence that e-cigarettes are absolutely not as low-risk as initially claimed,” says Becky Freeman at the University of Sydney, Australia, who wasn’t involved in the work. “[It’s] important that people who are trying to quit smoking try other safer [but] effective methods first, and only use e-cigarettes after exhausting other methods if they are unable to quit.”

Just over 40 per cent of smokers in the UK who quit the habit in 2024 used e-cigarettes to help the process, and 20 per cent of ex-smokers were vaping a year or more after ditching the habit. While some argue e-cigarettes are a pathway to successfully giving up smoking, they have been linked to airway irritation, reduced lung function and, in animal studies, lung cancer.

To better understand their effects on people, Yeon Wook Kim at Seoul National University in South Korea and his colleagues followed more than 4.5 million adult smokers, who participated in the Korean National Health Screening Programme from 2018 to 2023.

The participants were classified as current smokers, short-term quitters (who hadn’t smoked since at least 2018) or long-term quitters (who hadn’t smoked since at least 2014). From 2018 to 2023, there were 35,887 cases of lung cancer and 12,807 related deaths among the participants.

When the researchers broke this data down according to participant group and self-reported use of e-cigarettes, they found that the risk of lung cancer death was substantially higher among the vaping ex-smokers than the non-vaping ones. “Compared with those who completely quit cigarettes, individuals who used e-cigarettes after quitting had a 56 per cent higher risk,” says Kim.

Longer-term studies are required, but some of the chemicals in e-cigarettes have been linked to DNA damage. Vaping has also been associated with oxidative stress (an imbalance between molecules called free radicals and antioxidants in the body, which causes cell damage), epigenetic changes (when our genes are influenced by our environment) and inflammation in respiratory and oral tissue.

But Kim and his team stress that they couldn’t prove that vaping itself causes lung cancer, and that further studies are required that include people outside South Korea.

They also found that the risk of death from any cause was significantly lower among ex-smokers who used e-cigarettes than current smokers, which supports the overwhelming health benefits of quitting smoking more broadly.

Nicole Lee at Curtin University in Perth, Australia, says the message of the study seems to be that completely stopping both smoking and vaping offers greater protection against lung cancer than quitting smoking but continuing to vape. “The findings are very relevant for people who have quit smoking,” she says.

“It doesn’t change the advice to smokers that quitting completely is safest, but if you can’t quit [without the use of e-cigarettes] or don’t want to, switching to vaping is still safer,” says Lee. “Vaping isn’t harmless, but [as] a harm-reduction approach, it’s much better than continuing to smoke.”

Bernard Stewart at the University of New South Wales in Sydney adds that further studies are required before any public health initiatives, such as further restrictions on vapes, are rolled out.