One person is clinically dead and five others were hospitalised after a "serious accident" during a drugs trial in France
Health Minister Marisol Touraine said the six volunteers had been taking part in the trial of an oral medication in the northwestern city of Rennes.
She said the drug was manufactured by Portuguese pharmaceutical company Bial and 90 people in the trial were given varying doses of the experimental drug.
Pierre-Gilles Edan, head of the neurology department at the hospital in Rennes where the six people were admitted, confirmed that one man is clinically dead and three others were left with possibly life-altering injuries.
The volunteers are all men aged 28 to 49, Ms Touraine told a news conference.
They started taking the drug on 7 January. One person started feeling ill on Sunday and the other five afterwards.
Ms Touraine said the drug, which was being tested in a Phase I trial in France by Biotrial for Bial, contained neither cannabis nor any substance derived from cannabis.
A person familiar with the situation had said it was a cannabis-based painkiller.
The minister said the drug was meant to act on the body's endocannabinoid system, which deals with pain.
The study had been halted and all volunteers taking part recalled.
A spokeswoman for the European Medicines Agency in London said it did not have full details of the case but was monitoring the situation.
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.
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.
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.
Ms 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.