A man was left brain dead and five people in a serious condition after a drug trial in France went wrong, officials said Friday.

Health Minister Marisol Touraine revealed that 90 people were involved in the trial conducted by a private clinic in the northwestern city of Rennes.

“A serious accident took place,” she said in a statement, adding that the study had been halted and all the participants recalled.

“This accident resulted in the hospitalisation of six of the volunteers at the University Hospital of Rennes. One of them is in intensive care – he is brain dead.”

The Paris prosecutor’s office said an investigation had been launched.

The national drug safety agency described the incident the most serious case in French pharmacological history.

Touraine denied media reports the drug being tested was a cannabis-based painkiller.

She told a news conference in Rennes that all those hospitalised were healthy men aged between 28 and 49. She said the man classed as brain dead was admitted to hospital on Sunday.

The oral trial was initiated by Rennes-based Biotrial on Jan. 7 and was in the first phase of testing, in which the drug is administered to healthy volunteers to evaluate its safety and patients’ tolerance.

In a message on its website, Biotrial, which also has offices in London and the U.S., said “serious adverse effects” had occurred during the study but that it had complied with all safety regulations.

Volunteers who agree to participate in trials in France can earn between 100 and 4,500 euros ($110 and $4,900), Biotrial claimed on its website. (Anadolu Agency)