90 people were given the experimental drug in various doses
One man is in a coma and brain dead while five others are seriously ill after a medical trial in France went wrong.
The trial involved a drug manufactured by Portuguese company Bial.
90 people were given the experimental drug in various doses, and there is no known antidote.
"A serious accident took place during a therapeutic trial near Rennes," French Health Minister Marisol Touraine said.
She added that the trial had been halted and all volunteers taking part recalled.
The drug was initially reported to have been a cannabis-based painkiller, but BBC says those reports have been denied by the health ministry.
Testing company Biotrial said in a statement that "serious adverse events related to the test drug have occurred in some subjects.
"The trial has been conducted in full compliance with the international regulations and Biotrial’s procedures were followed at every stage throughout the trial, in particular the emergency procedures for the transfer of subjects to the hospital," the statement adds.
The study was a Phase I clinical trial, where healthy volunteers take medication to evaluate its safety for wider use.
Ms Touraine expressed her "deep determination to get to the bottom...of this tragic accident".
The first volunteers to fall ill were taken into hospital earlier this week, French media said.
A statement issued by the French health ministry confirmed that one of the volunteers is in an "état de mort cérébrale", or "brain dead".
Medical lawyer Philippe Courtois told French media: "This is the most serious case I have come across".
A formal investigation has been opened in Paris.