A clinical trial of a new drug in France has left one person brain-dead and another five people in hospital, in a serious condition.
The oral trial was conducted by a private laboratory in the western city of Rennes.
The trial has been suspended and the firm is recalling the volunteers. It is unclear how many people are involved.
Media reports that the drug is a cannabis-based painkiller have been denied by the health ministry.
The Paris prosecutor’s office said an investigation had been opened.
Health Minister Marisol Touraine, who was heading for Rennes on Friday, pledged to “get to the bottom… of this tragic accident”.
The trial was conducted by Biotrial, a French-based company with an international reputation which has carried out thousands of trials since it was set up in 1989.
According to the health ministry, the accident occurred on Thursday.
The worst of those affected is brain dead, it said, while the other five are described as being in a critical neurological condition.
The study was a Phase I clinical trial, in which healthy volunteers take the medication to evaluate the safety of its use, the ministry said.
Before any new medicine can be given to patients, detailed information about how it works and how safe it is must be collected.
Clinical trials are the key to getting that data – and without volunteers to take part in the trials, there would be no new treatments for serious diseases such as cancer, multiple sclerosis and arthritis.
Every year around the world thousands of people take part in clinical trials but incidents like this are very rare, the BBC’s Hugh Schofield reports from Paris.
New EU regulations to speed up clinical drug trials and streamline testing procedures across the 28-nation bloc are due to take effect in May.