"The number of e-cigarette brands sold on the Internet is large and the variety of flavors staggering," Dr. Shu-Hong Zhu, of the University of California, San Diego, department of family and preventive medicine, and colleagues said.
About 10 new brands and more than 240 new flavors appeared online each month during their study, the researchers found.
The investigators analyzed online English language websites marketing e-cigarettes between May to August 2012 and again between December 2013 to January 2014. Of the 288 new e-cigarette brands identified during the first search, 37 had disappeared by the second search. The second search did, however, find another 215 new brands.
In January 2014, there were over 460 different brands of e-cigarettes. Each brand had its own website. There were almost 7,800 different flavors, according to the study in the June 16 online edition of the journal Tobacco Control.
Nearly all brands of e-cigarettes offered menthol and tobacco flavors, the study authors noted. Other popular flavors included fruit, dessert/candy, alcohol/drinks and snacks/meals.
About 10 percent of older and newer e-cigarette brands made direct claims that the products can help smokers quit, Zhu and colleagues said in a journal news release.
It's unclear what impact the rapidly growing e-cigarette market will have on rates of new smokers and attempts to quit. This uncertainty poses "significant challenges to regulatory policy making," the study authors concluded, according to the news release.
In the United States, sales of e-cigarettes are expected to reach $2 billion this year. The U.S. Food and Drug Administration recently categorized e-cigarettes as a tobacco product, so sales to minors will be banned. However, the ban does not cover online sales, which account for up to half of e-cigarette purchases, the researchers noted.
The U.S. National Institute on Drug Abuse has more about e-cigarettes.