One person has been left brain dead and five others are in serious condition after taking part in a clinical trial in France 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 specialises 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. They started taking the drug on Jan 7. One person started feeling ill on Sunday and the other five afterwards.
In total, 90 people have taken part in the trial, taking some dosage of the drug, she said, adding that others took a placebo.
All trials on the drug have been suspended and all volunteers who have taken part in the trial are being called back.
A spokeswoman for the European Medicines Agency in London said it did not have full details of the case but was monitoring the situation.
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. One was described as looking like "the elephant man" after his head ballooned. Another lost his fingertips and toes.