French authorities are investigating a Phase I clinical trial in Rennes that led to the death of one volunteer and left five others (all male) in serious or critical condition. This was the second incident of similar dimension in a clinical trial since 2006, when six volunteers taking an autoimmune drug in a London trial with a German-manufactured drug were left seriously impaired.

All further trials with the drug manufactured by Portugal’s Bial have been suspended, and the company said it is cooperating with the investigation. The study conducted by Biotrial, a private company based at Rennes, was intended to test the tolerability of a candidate drug to treat Alzheimer’s and Parkinson’s diseases.

Reports said three separate investigations have been launched to determine whether the fault was an error in the trial’s procedure or in the substance tested. The medication was identified as a compound acting on the body's endocannibanoid system, a group of receptor proteins that responds to compounds naturally made in the body as well as being components in cannabis.

Similar tests are said to have been performed with drugs made by Pfizer, Sanofi and several others without producing such severe results. The incident was called the worst of its kind to have ever taken place in France.

All trials with the Portuguese drug have now been suspended and volunteers who had taken part in the trial previously called back.

Altogether 108 volunteers are reported to have taken part in the trial, with 90 receiving the drug at varying doses and the remainder being given placebos. The six men who had severe reactions are believed to have been among those given the highest dose.