One man is brain dead and three others are facing possible permanent brain damage after volunteering to take part in a drug test in western France, the French Health Ministry said Friday.

The prosecutor's office has opened an investigation into what French Health Minister Marisol Touraine called "an accident of exceptional gravity" at the private Biotrial clinical lab in Rennes.

The drug trial, which was testing a new painkiller compound, involved 90 healthy volunteers who were given the experimental drug in varying doses beginning on Jan. 7, she told reporters Friday at a news conference in Rennes.

Six male volunteers between 28 and 49 years old have since been hospitalized, including one man now classified as brain dead, she said.

"A serious accident took place," the minister said earlier in a statement.

The chief neuroscientist at the hospital in Rennes, Professor Gilles Edan, said in addition to the brain-dead volunteer, three others could have "irreversible" brain damage. A fifth man is suffering from neurological problems and a sixth volunteer is being kept in the hospital but is in a less critical condition, he said.

Edan said there's no known treatment for the experimental drug that Biotrial was testing. The drug was based on a natural brain compound similar to the active ingredient in marijuana.

Touraine said the medication was not based on cannabis. Earlier Friday, a report from Reuters cited a person familiar with the situation as saying the drug was a cannabis-based painkiller.

The brain has a system of enzymes and molecules that respond to both plant-based cannabis compounds and ones we produce naturally. 

"The aim with this synthetic approach is to mimic a cannabis-like effect, not by administering a cannabis drug, but by blocking a naturally occurring enzyme," said Dr. Mark Ware of Montreal's McGill University Health Centre, who researches the medical use of cannabis. 

"My initial reading of this means it shouldn't interfere with work on the plant cannabis or plant cannibinoids.  It does raise important questions about the role of that specific enzyme, and why blocking it has this devastating result, if this is what was responsible. The scientific community will be watching this story very closely."

Touraine urged calm, saying that no drug currently on the market was implicated in the failed trial. She said the drug was produced by the Portuguese pharmaceutical company Bial.

On a statement on its website, Bial called the drug an experimental molecule in the field of pain. The company said  the new drug had already been administered to 108 people without any moderate or serious adverse reactions.

All the other 83 volunteers are being contacted, Touraine said.

It's rare for volunteers to fall seriously ill when testing new drugs. Researchers generally start with the lowest possible dose for humans after extensive drug tests in animals. The French ministry statement said those who had fallen ill had taken an oral medication in the first phase of testing, which was studying safe usage, tolerance and other measures on healthy volunteers.

Biotrial

There was 'an accident of exceptional gravity' at the private Biotrial clinical lab in Rennes, France, the health minister says. (Damien Meyer/AFP/Getty )

Biotrial, with headquarters in Rennes and offices in London and Newark, New Jersey, says it has over 25 years of experience in clinical trials and uses "state-of-the-art facilities." In France, adults volunteering for Biotrial tests can earn between 100 euros and 4,500 euros ($110 US to $4,922 US).

There have been very few instances worldwide of trials going wrong, said pharmacy and medicine  professor Jack Uetrecht of the University of Toronto.

"I would say it's probably safer to be part of a phase 1 trial than to take a prescribed drug," Uetrecht said, because they start with a low dose and monitor so carefully. 

Uetrecht recalls only two disasters in thousands of phase 1 clinical trials. 

Decades ago, a hepatitis B drug caused liver failure, a delayed response that wasn't picked up in pre-clinical studies on typical animal models.

The other was in 2006 in Britain.

Six previously healthy men were treated for organ failure only hours after being given an experimental drug targeting the immune system. That prompted a review of procedures and resulted in the U.K. regulatory agency imposing new testing standards.

Regulations for clinical trials are largely the same in Europe and Canada, Uetrecht said.‚Äč

With files from CBC's Vik Adhopia, Amina Zafar and Reuters