A man left brain dead after a drug trial in northwest France died on Sunday, said the hospital where he was being treated.
The Rennes hospital said in a statement that five other volunteers were in stable condition after they were admitted last week.
In total, 90 people took part in the trial of the drug made by Portuguese pharmaceutical company Bial, which is intended to treat mood and anxiety issues, as well as movement coordination disorders linked to neurological issues.
It was not yet known what caused the tragedy, said Francois Peaucelle, the head of Biotrial, the company that conducted the trials for Bial.
Cases of early-stage clinical trials going badly wrong are rare but not unheard of. In 2006, six healthy volunteers given an experimental drug in London ended up in intensive care.