Smokers are turning back to traditional cigarettes amid health scares over vaping, US experts have warned.
Vaping has been linked to six deaths across the United States, and 380 people have been hospitalised with lung illnesses in what the US Centers for Disease Control and Prevention has called an "outbreak".
Three-quarters of the market is controlled by the e-cigarette company Juul.
Some smokers who started using the nicotine devices as a way of quitting said they were moving back to regular cigarettes.
Lucas McClain, 21, from Arlington, Virginia, said he had "bought a Juul to quit smoking cigarettes, now I'm smoking cigarettes to quit the Juul".
He added: "Juul made my nicotine addiction a lot worse. When I didn't have it for more than two hours I'd get very anxious."
Ryan Hasson, 25, from New York, said he stopped using e-cigarettes after getting chest pains. He told California Healthline: "I think a lot of people are quitting completely, or going back to cigarettes. "They're waking up to the reality that maybe this [vaping] isn't as safe as we once thought."
Dr Sally Satel, of the American Enterprise Institute, said Mr Trump's proposed ban on flavours could also lead to more people smoking traditional cigarettes.
She said: "Removing flavours is bad for adult smokers. There is little question that some adults will switch back to their Marlboros and Kools if their preferred flavours are no longer available.
"If vaping disappears many smokers would go back to cigarettes."
Adam Hergenreder, 18, one of the hundreds hospitalised by vaping, said he had been told by doctors his "lungs were now that of a 70-year-old’s".
He said: "I was a varsity wrestler before this and I might not ever be able to wrestle because that’s a very physical sport and my lungs might not be able to hold that exertion. It's sad. "It was scary to think about what that little device to my lungs."
Juul has said it will "fully comply" with the final policy on flavoured e-cigarettes introduced by the US Food and Drug Administration.
But there has been some suggestion that the outbreak has not been caused by regular e-cigarettes but illicit marijuana vape products.
A study published last week in the New England Journal of Medicine found that more than half of patients with the lung illness - 24 of 41 - who were extensively interviewed in Wisconsin and Illinois reported having used black market marijuana vape cartridges sold under the name 'Dank Vapes'.