One man is brain-dead and another five people are in hospital after an experimental drug was administered to 90 people in a French clinical trial.
There is no known antidote to the drug, the chief neuroscientist at the hospital in Rennes said.
Of the five men in hospital, three could have permanent brain damage, Gilles Edan added.
Reports that the drug is a cannabis-based painkiller have been denied by the health ministry.
The trial, which involved taking the drug orally and has now been suspended, was conducted by a private laboratory in Rennes.
The experimental drug was manufactured by the Portuguese company Bial.
All those who volunteered for the trial have been recalled and the Paris prosecutor’s office has opened an investigation.
Health Minister Marisol Touraine pledged to “get to the bottom… of this tragic accident”.
“I was overwhelmed by their distress” she told reporters. “Their lives have been brutally turned upside down”.