One person has been left brain dead and five others are in serious condition after taking part in the clinical trial of an experimental painkiller made by Portuguese drug company Bial, the French Health Ministry said on Friday.

The medicine involved works by targeting the body’s pain-controlling endocannabinoid system, which is also responsible for the human response to cannabis.

The ministry said the six volunteers in Rennes, in western France, had been in good health until taking the oral medication at a private facility that specializes in carrying out clinical trials. The brain-dead volunteer was admitted to hospital in Rennes on Monday. Other patients went in on Wednesday and Thursday.

The volunteers are all men aged 28 to 49, French Health Minister Marisol Touraine told a news conference. Touraine also told reporters that all those hospitalized were healthy men when the trial began on Jan. 7. She said one man who has now been classified as brain dead was admitted to the hospital on Sunday.