Kevin Burns, chief executive of vaping firm Juul, has stepped down, amid growing concerns around vaping health risks and criticism of its marketing.
The firm has also announced it will withdraw all US advertising.
Mr Burns will be replaced immediately by KC Crosthwaite, former chief growth officer at tobacco giant Altria, Juul said.
Juul is 35%-owned by Altria, and in the past has been accused of targeting vaping devices at children.
Mr Burns said: "Since joining Juul Labs, I have worked non-stop, helping turn a small firm into a worldwide business, so a few weeks ago I decided that now was the right time for me to step down."
At the same time, Altria said its merger talks with fellow cigarette-maker Phillip Morris would not move forward.
'Future at risk'
The changes come as Juul faces serious threats to its once explosive growth.
The Trump administration this month said it was preparing a nationwide ban on flavoured e-cigarettes. Juul is also facing multiple investigations, including into its marketing practices.
Juul has for years promoted its e-cigarettes, which contain addictive nicotine, as a safer alternative to traditional tobacco products.
However, the Food and Drug Administration recently warned Juul against making health claims without presenting scientific evidence to authorities for approval.
Juul said it would not lobby against the proposed ban on flavoured e-cigarettes.
However, Mr Crosthwaite said he remains committed to making Juul's products available to adult smokers.
"Unfortunately, today that future is at risk due to unacceptable levels of youth usage and eroding public confidence in our industry," he said.
The crackdown on Juul, which dominates the US e-cigarette market, follows a spate of serious lung injuries in the US linked to vaping.
Health authorities have not blamed the outbreak, in which nine people have died and more than 530 people been taken ill, on any one product.
Most of the patients had a history of using vaping products that contain THC, the chemical in marijuana, they said.
However, the injuries have raised alarm, especially in conjunction with surging rates of teen vaping.
Two US states, New York and Michigan, have already imposed bans on flavoured e-cigarettes, while Massachusetts has announced a four-month ban on all vaping products.
Walmart last week announced it would stop e-cigarette sales, citing the regulatory uncertainty.