A man left brain-dead after an experimental drug trial in France has died, local media report.

He was one of six people being treated in hospital in the city of Rennes.

The hospital said the other five remained in a stable condition – four had “neurological problems” and the fifth had no symptoms.

Reports that the drug was a cannabis-based painkiller have been denied by the French health ministry. The Paris prosecutor has opened an investigation

The trial, which involved taking the drug orally and has now been suspended, was conducted by a private laboratory in Rennes.

Ninety volunteers took the drug, manufactured by the Portuguese company Bial.

Ten of the other 84 have been tested, but did not display any of the “anomalies” of those admitted, the Rennes hospital said in its statement (in French).

On Friday, the chief neuroscientist at the hospital, Gilles Edan, said there was no known antidote to the drug.