A man died in a French hospital today after taking part in a drug trial for a painkiller based on a compound similar to cannabis.

The Rennes University Hospital announced the death in a statement but did not identify the patient, who had already been in a state of brain death.

He was among six male volunteers between 28 and 49 hospitalised last week after volunteering to take the experimental drug.

French health authorities said three of the hospitalised volunteers face possible brain damage and an investigation is under way into what French health minister Marisol Touraine called "an accident of exceptional gravity".

The trial, which began on January 7, involved 90 healthy volunteers who were given the experimental drug in varying doses at different times.

The hospital said it has contacted the 84 other volunteers exposed to the new painkiller.

Ten of those volunteers underwent medical exams yesterday but the hospital found no anomalies, the statement says. It said another five will have medical exams closer to their homes.

The drug, based on a natural brain compound similar to the active ingredient in marijuana, was given orally to healthy volunteers as part of a phase one trial by Biotrial, a drug evaluation company based in Rennes, on behalf of the Portuguese pharmaceutical company Bial.

In addition to treating pain, the drug was intended to ease mood and anxiety troubles as well as motor problems linked to neurodegenerative illnesses by acting on the endocannabinoid system. In this system, natural brain compounds act on specific receptors to exert their effects.

It is rare for volunteers to fall seriously ill during phase one trials, which study safe usage, side effects and other measures on healthy volunteers, rather than drug effectiveness.