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.

Four of the six men hospitalized have "neurological problems"-- of which three could have "irreversible" brain damage, CHU de Rennes hospital chief neurosurgeon Gilles Edan said Friday at a press conference. Another man is being kept in the hospital, but is in a less critical condition.

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.

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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.

"INHERENT RISK"

In the initial Phase I stage of clinical testing, a drug is given to healthy volunteers to see how it is handled by the body and what is the right dose to give to patients.

"Undertaking Phase 1 studies is highly specialist work," said Daniel Hawcutt, a lecturer in clinical pharmacology at Britain’s University of Liverpool.

Medicines then go into larger Phase II and Phase III trials to assess their effectiveness and safety before they are finally approved for sale.

Europe has strict regulations governing the conduct of clinical trials, with Phase I tests subject to particular scrutiny. But Ben Whalley, a professor of neuropharmacology at the University of Reading, said these could only minimize risks, not abolish them.

"There is an inherent risk in exposing people to any new compound," he said.

The 2006 London trial led to the collapse of Germany's TeGenero, the company developing a medicine known as TGN1412. The drug has since gone back into tests for rheumatoid arthritis and is showing promise when given at a fraction of the original dose.

Reuters and the Associated Press contributed to this report.