Six volunteers were taken to hospital last week after taking part in the Phase I trial of a new medication meant to treat mood disorders such as anxiety, developed by Portuguese pharmaceutical company Bial.
Touraine said the men, aged between 28 and 49, were part of a group of around 90 people who had taken the drug, while about 30 others had received a placebo.
Pierre-Gilles Edan, head of the neurology department at the hospital in Rennes where the volunteers were taken, said that aside from the man who was clinically dead, three others were suffering a "handicap that could be irreversible" and another also had neurological problems.
The sixth volunteer had no symptoms but was being monitored.
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"This is unprecedented" in France, said the health minister, vowing to "shed light" on who was responsible.
"The shock is even greater given the fact that the people taking part in clinical trials are healthy."
The study was a Phase I clinical trial, in which a drug is tested on humans for the first time, after likely tests on animals and in the laboratory to ensure its safety.
Touraine said the drug molecule had previously been tested on chimpanzees.
France's national drug safety body (ANSM) confirmed it was the worst-ever incident to have taken place in a drug trial in the country.
In a statement, the Portuguese firm insisted it had followed "international best practice" in developing the drug and said it would cooperate with the investigation to "determine in a rigorous and exhaustive manner" what had happened.
Founded in 1924, it produces treatments for a range of ailments including problems with the nervous system and cardiovascular health, as well as antibiotics and anti-allergens.