A French man in his 20s his dead after a clinical drug trial left him with irreversible drug damage.
Five others are still in hospital with three thought to have serious brain damage.
The drug was being tested on humans for the first time at a clinic near Rennes in north-west France.
The trial of the drug, which is supposed to act on the body’s neurological system and particularly pain and mood receptors, was being carried out by a private research company on behalf of a Portuguese pharmaceutical.
Three separate inquiries have been opened into what is the worst drug trial tragedy to have taken place in France.
The six men who suffered adverses effects were aged between 28 and 49 and all in good health before they started taking the drug on January 7.
It took three days before the first person fell unwell and was admitted to hospital, the five others were admitted in subsequent day.
Marisol Touraine, the French health minister, said 108 people had taken part in the trial – 90 had been given varying doses of the drug and the rest a placebo. The six men who fell ill were among a group that had taken the highest dose.
All trials of the drug have been suspended. No other volunteers have shown ill effects.