One person has been left brain-dead and five others hospitalised after a "serious accident" during a drugs trial in France, Health Minister Marisol Touraine said Friday.
She said the six volunteers had been taking part in a "trial of an oral medication being developed by a European laboratory" in the northwestern city of Rennes.
According to a source close to the case, the drug was a painkiller containing cannabinoids, an active ingredient found in cannabis plants.
A separate source said the research company Biotrial had been carrying out the drugs trial for Portuguese pharmaceutical company Bial.
"A serious accident took place," the minister said, adding that the study had been halted and all volunteers taking part recalled. The incident occurred on Thursday.
The study was a Phase I clinical trial, in which healthy volunteers take a prototype medication to "evaluate the safety of its use, tolerance and pharmacological profile of the molecule", the minister added in a statement.
It was not clear how many people were taking part in the study.
Clinical trials typically have three phases to assess a new drug or medical innovation for safety and effectiveness. Human participation in such trials and scrutiny by outside watchdogs are essential for obtaining market authorisation.
Phase I entails a small group of volunteers, and focuses only on safety.
Phase II and Phase III are progressively larger trials, typically involving hundreds or thousands of volunteers, to assess the drug's effectiveness, although safety remains paramount.
Touraine was set to hold a press conference later Friday alongside a manager from the Biotrial company which has its French headquarters in Rennes.
The company conducts its Phase I trials at a 150-bed facility in Rennes and also in Newark, New Jersey, from where it carries out "a large variety of early clinical studies," according to its website.
Biotrial says it is able to fast-track early patient studies by "combining the favourable regulatory environment in Western Europe with fast and efficient patient recruitment in Eastern Europe."
The Paris prosecutor's office said an investigation had been opened.
Touraine vowed to "shed light on" what happened and has called for an inspection of the research site.
Every year thousands of volunteers, often students looking to make extra money, take part in such clinical trials which are seen as safe.
Mishaps are relatively rare, but in 2006 six men were hospitalised in London after taking part in a clinical trial into a drug developed to fight auto-immune disease and leukaemia.
In gene therapy, setbacks have included the death of an 18-year-old US volunteer, Jesse Gelsinger, in 1999, and the development of cancer in two French children treated for "bubble baby" syndrome, a chronic lack of immune defences.