But then their views diverge.

Pessimists like Dr. Glantz say that while e-cigarettes might be good in theory, they are bad in practice. The vast majority of people who smoke them now also smoke conventional cigarettes, he said, and there is little evidence that much switching is happening. E-cigarettes may even prolong the habit, he said, by offering a dose of nicotine at times when getting one from a traditional cigarette is inconvenient or illegal.

What is more, critics say, they make smoking look alluring again, with images on billboards and television ads for the first time in decades. Dr. Glantz says that only about half the people alive today have ever seen a broadcast ad for cigarettes. “I feel like I’ve gotten into a time machine and gone back to the 1980s,” he said.

Researchers also worry that e-cigarettes could be a gateway to traditional cigarettes for young people. The devices are sold on the Internet. The liquids that make their vapor come in flavors like mango and watermelon. Celebrities smoke them: Julia Louis-Dreyfus and Leonardo DiCaprio puffed on them at the Golden Globe Awards.

A survey from the Centers for Disease Control and Prevention found that in 2012, about 10 percent of high school students said they had tried an e-cigarette, up from 5 percent in 2011. But 7 percent of those who had tried e-cigarettes said they had never smoked a traditional cigarette, prompting concern that e-cigarettes were, in fact, becoming a gateway.

“I think the precautionary principle — better safe than sorry — rules here,” said Dr. Thomas Frieden, director of the C.D.C.

E-cigarette skeptics have also raised concerns about nicotine addiction. But many researchers say that the nicotine by itself is not a serious health hazard. Nicotine-replacement therapies like lozenges and patches have been used for years. Some even argue that nicotine is a lot like caffeine: an addictive substance that stimulates the mind.

“Nicotine may have some adverse health effects, but they are relatively minor,” said Dr. Neal L. Benowitz, a professor of medicine at the University of California, San Francisco, who has spent his career studying the pharmacology of nicotine.