She said the six 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 serious accident took place," the minister said, adding that the study had been halted and all volunteers taking part recalled. The incident occurred yesterday.
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.
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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 getting 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 Paris prosecutor's office said an investigation had been opened.
Touraine said she was determined to "shed light on" what happened. She has also called for an inspection of the research site.
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 among two French children treated for "bubble baby" syndrome, a chronic lack of immune defences.