A clinical trial for an experimental drug France has left one man brain dead and five others hospitalized, three of whom could suffer from permanent brain damage, the Associated Press reported.

The drug was manufactured by Bial, a Portuguese pharmaceutical company, and the trial was being conducted by drug evaluation company Biotrial.

"This is unprecedented," French Health Minister Marisol Touraine said, according to Reuters. "We'll do everything to understand what happened."  

Touraine added that the trial was being conducted for a painkiller drug that was based on a brain compound with similar properties as the active ingredient in cannabis. However, she clarified that it was not based on cannabis, which others had previously reported.

The research recruited 90 volunteers, who began taking the drug on Jan. 7. Four days later, the man who became brain dead was admitted to the hospital. The three men feared to suffer from permanent brain damage were hospitalized a few days later.

Two other men were admitted to the hospital; one suffered from neurological problems while the other was in a less serious condition but was told to stay for observation. The six male volunteers' ages ranged from 28 to 29 years old.  

Dr. Gilles Edan, the hospital's chief neuroscientist, said there is no way that the drug's damage could be reversed.

The trial has now been suspended and all the volunteers recalled, Touraine said.

"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," Biotrial said in a statement.