(CNN) A man has died in France after participating in a clinical drug trial, the University Hospital of Rennes said in a statement Sunday.
The conditions of five other volunteers have been improved to stable.
The French Health Ministry is investigating what it calls a "serious accident."
Four of the patients may be permanently disabled with neurological damage, doctors in the northwestern city of Rennes said. One other person doesn't have symptoms but remains under medical surveillance.
The company conducting the testing said on its website the trial was administered "in full compliance with the international regulations and Biotrial's procedures were followed at every stage throughout the trial."
Biotrial International said it was "in close and regular contact with the Health Authorities and Ministry in France."
The first patients to get sick started to show symptoms January 10 and were hospitalized in Rennes. The testing has been stopped, Health Minister Marisol Touraine said in a news conference Friday, and health authorities have been reaching out to the rest of the volunteers to let them know about these adverse effects.
The drug being tested is a painkiller meant to treat anxiety and motor disorders, and was designed to work on the body's endogenous cannabinoid system, which deals with pain. Earlier reports said the drug was related to cannabis, but Touraine insisted that the drug -- developed by the Portuguese science group Bial -- does not contain cannabis or cannabis extracts.
A total of 128 volunteers took part in the drug test. The subjects were between the ages of 18 to 55 and were considered healthy. Ninety people were given different level dosages of the drug and the rest got the placebo.
Volunteers began taking the drug on January 7 in early testing known in the industry as Phase I, in which it's determined if the drug is safe for human consumption. Scientists also measure the effectiveness of a drug at this phase and can watch to see how it reacts at different doses in the human body.
Earlier testing of the drug in question involved animals, the health minister said. While animals can react similarly to humans, their bodies can process substances differently. In order for any drug to get to market, it must go through several government-approved steps along the way that involve human subjects.
Such extreme adverse reactions to drugs in these early phases of drug trails are incredibly rare.
Bial pharmaceutical said in statement that throughout this trial, the new drug had already been administered to 108 patients "without any moderate or serious adverse reaction."
The French health ministry said it immediately sent agents to the medical facility to determine if all the rules had been followed in the testing and if the facility where the patients were staying during the trial maintained sanitary conditions. The facility was inspected in 2014 and the ministry said it had favorable inspections.
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