China has launched an investigation into search giant Baidu after the death of a student who tried an experimental cancer therapy he found online.
Wei Zexi, who died last month from a rare form of cancer, had sought the treatment from a hospital that came top of the list on his Baidu web search.
Baidu has come under fire for selling top search positions to the highest bidder without checking their claims.
In a statement Baidu said it was investigating the matter.
The company told the BBC: "We deeply regret the death of Wei Zexi and our condolences go out to his family.
"Baidu strives to provide a safe and trustworthy search experience for our users, and have launched an immediate investigation of the matter."
Baidu owns search engine and social media services, and is often compared to Google. Unlike Google, its search results page does not identify which listings have paid for a prominent placement.
Shares slumped in the US on news of an investigation by China's internet regulator, with Baidu's Nasdaq-listed shares falling 7.92%.
According to state news agency Xinhua, Wei was diagnosed with synovial sarcoma in 2014.
He and his family said he found out about a controversial treatment at the Second Hospital of the Beijing Armed Police Corps through an advertisement on Baidu.
But the treatment was unsuccessful and the 21-year-old student died on 12 April.
Before his death, Wei publically accused Baidu of promoting false medical information and the hospital of misleading advertising.
Baidu has said on its Weibo account that it had filed a request for the hospital to be investigated. The hospital has yet to comment and efforts to contact hospital officials have been unsuccessful.
In addition to the Cyberspace Administration of China, several other government agencies including the State Administration of Industry and Commerce, and the National Health and Family Planning Commission are looking into the matter.
The outcry over Mr Wei's case follows a similar scandal in January involving ethical practices regarding healthcare advertising.
The story has also reignited public concern over Baidu's advertising ethics, following an earlier scandal where the hospital admitted it allowed healthcare companies to moderate online health forums.
On popular microblogging network Sina Weibo, the hashtag #Wei Zexi Baidu Advertising Incident# has been trending for days as netizens have called for a boycott of Baidu.