A man has died and five others remain hospitalised after taking an experimental medicine as part of an early stage clinical study in France.

Of the five in hospital, three are now facing permanent brain damage as a result of participating in the Phase I trial, which was evaluating a drug made by Portugese firm Bial in healthy volunteers. One other is said to be suffering neurological problems while the fifth man has no symptoms and is just being monitored, according to media reports.

The firm said that a total of 108 people have received the medicine - which acts on the endocannabinoid system to target pain - as part of the study, “without any moderate of serious adverse reaction”. Six men, aged 28 and 49, were selected to begin a higher dose of the therapy, which experts believe caused the potentially catastrophic and irreversible neurological reactions observed.

The first volunteer was taken to the neurology department at a hospital in Rennes three days later beginning treatment with the higher dose. According to department head Pierre-Gilles Edan, the man was admitted with acute symptoms similar to stroke and was later declared brain dead.

French clinical research group Biotrial insists that it carried out the trial, which is now suspended, “in full compliance with international regulations”, but The National Agency of Drug Safety has now launched its own investigation into the incident.