Juul Labs, which is under fire in the United States for marketing its e-cigarettes to teenagers and children, wanted to cash in, too. But just days after it started business in China, its products were removed from Alibaba and JD.com, two of the biggest e-commerce platforms. Neither the government nor the company gave any explanation. Juul had paid to be at the e- cigarette fair in Shanghai starting in late October but withdrew at the last minute because of the crisis in the United States, according to Li Wangfeng, project director of the expo.

Concerns are mounting about the hazards of vaping among the young, prompting many Chinese to call for regulations. A 2018 tobacco survey commissioned by China’s Center for Disease Control found that people 15 to 24 years old were the most avid vapers, with most buying their devices online.

In recent weeks, the state news media has kept up its drumbeat of negative coverage of the industry. On Nov. 4, the most-read article on the website of People’s Daily, the Communist Party’s official newspaper, noted that most e-cigarette companies continued selling their products online despite the ban. The next day, China’s state broadcaster, China Central Television, showed Beijing officials summoning companies to comply with the ban. The day after that, e-cigarettes were no longer available on Alibaba’s Taobao and JD.com, two of China’s most popular e-commerce platforms.

For years, the Chinese government allowed the lucrative e-cigarette industry to thrive with no supervision. There was never any consensus on whether e-cigarettes should be classified as tobacco, health or electronics products and which agency should regulate them. Part of the problem, too, is that China’s top tobacco authority is both a regulator and a producer of cigarettes.