When Marty Weinstein decided to quit smoking, he took a friend's advice and tried electronic cigarettes rather than government-approved nicotine replacement products.

Weinstein, 58, has gone from a pack a day nine months ago to the equivalent in nicotine of four or five cigarettes. The e-cigs have a familiar look and feel, and quench his desire to hold on to a cigarette and puff.

"I fully understand I'm still addicted to nicotine," said Weinstein, a Connecticut taxi driver who had smoked for more than 20 years. "But I'm now so much healthier."

E-cigarettes, metal tubes that heat liquids typically laced with nicotine and deliver vapor when sucked, are transforming the market for smoking cessation products and slowing the $2.4 billion in global sales of long-standing aids such as nicotine patches and gums. But their impact on health remains unclear, experts say, raising difficult questions for regulators who are starting to impose limits on e-cigarette use.

E-cigarette makers in the United States are barred from explicitly marketing the products as smoking cessation devices, but have found ways to appeal legally to smokers who are thinking of quitting.

“You never say 'quit' because it’s not approved by the FDA as a smoking cessation device,” said Jose Castro, the chief executive of A1 Vapors in Miami, referring to the U.S. Food and Drug Administration.

A1 Vapors runs an ad on its website urging customers to "kiss tobacco goodbye" and give themselves the "gift of your life. literally", adding a disclaimer that e-cigs are not a smoking cessation product.

E-cigarettes, or e-cigs, have only come into widespread use in the past few years, but have already made inroads into traditional quitting therapies.

About a third of British smokers trying to quit were using e-cigarettes, according to a University College London survey in January of 1,800 people, including 450 smokers.

E-cigs are used by almost twice as many people as government-approved nicotine gums, lozenges and patches, according to the survey. That was a reversal from 2011, when only about 5 percent of people were using e-cigarettes and more than 30 percent used over-the-counter products.

Similar data is not yet publicly available for the United States.

Worldwide sales of all nicotine replacement therapies grew just 1.2 percent last year, to almost $2.4 billion, according to data from commercial researcher Euromonitor. U.S. sales, at $900 million, grew 0.2 percent, and are expected by Euromonitor to drop this year by that amount.

Big tobacco companies like Altria, Lorillard and Reynolds American have rushed into the e-cig market. The entire U.S. market for "vapor devices" such as e-cigs grew in 2014 by 40-50 percent to $2.5 billion to $3 billion, Euromonitor said. The global market is worth $5 billion.

RULES ON E-CIGS TIGHTENING

Mark Strobel, a consumer health analyst at Euromonitor, said e-cigarettes have slowed nicotine replacement therapy sales, along with relatively high prices and a shrinking population of smokers, especially in the United States.

"For some consumers it has been a direct substitution."

GlaxoSmithKline (GSK) and Johnson & Johnson don't break out the data on their smoking cessation products, which are relatively small parts of their sales, but the companies have noted the change.

"It's definitely taken a bit of our market, no question at all - but there's a lot of competition in that space," GSK chief executive Andrew Witty told Reuters in an interview this month.

GSK's nicotine replacement therapies and smoking cessation products include the brands Nicorette, NicoDermCQ and the medicine Zyban.

There is little long-term safety data on e-cigarettes, although some healthcare professionals say they may be better for consumers than tobacco cigarettes because they have no carbon monoxide and fewer cancer-causing chemicals.

A growing number of states, cities and countries - including Israel and Australia - are considering or have approved legislation to ban or limit the devices or the liquids, which come in exotic flavors from bacon to bubble gum.

California's top public health official on Wednesday slammed e-cigs as addictive, saying they were leading to nicotine poisoning among children and threatened to unravel the state's decades-long effort to reduce tobacco use.

Earlier this week, California introduced a bill that would ban the devices in public places, and New York Gov. Andrew Cuomo proposed a similar ban earlier this month.

Last year, the World Health Organization recommended that smokers should be encouraged to try already approved treatments rather than e-cigarettes. The FDA last April proposed rules for electronic cigarettes that would, among other things, ban sales to those under 18, but not restrict flavored products, online sales or advertising.

MAKING SMOKING COOL AGAIN?

Many health experts worry that e-cigarettes will become established as smoking cessation aids before enough research is done to determine their health impact. Another concern is that they may stop people from quitting tobacco completely and deter people from trying potentially more effective methods.

Dr. Albert Rizzo, senior medical advisor for the American Lung Association, said that when patients ask about the products, he tells them it's good that they are trying to quit but: "We don't know enough to recommend them."

Some healthcare professionals said that even if they are not opposed to e-cigarettes, they are concerned about their marketing, especially to young people.

The Federal Trade Commission declined to comment on specific e-cig ads but said “advertising must be truthful, non-deceptive and supported by competent and reliable scientific evidence."

E-cigs risk bringing the "cool" back to smoking, reversing the progress over decades in which smoking has become less socially acceptable, said Dr. Robert K. Jackler, a professor at Stanford University School of Medicine.

"A lot of us are very concerned about the renormalization phenomenon," he said. "These glamorize smoking behavior."

Still, some doctors point to the low efficacy of traditional ways to quit smoking.

"They have better results than placebos, but their rates of success are quite low," said Dr. Michael Siegel, a professor at the Boston University School of Public Health, who said e-cigarettes are an alternative, especially for people who have tried the conventional therapies and failed.