One person is brain-dead and another five are critically ill after taking part in a medical drug trial that went wrong at a clinic in France, authorities said Friday. Local media reports said that the drug, developed by a European laboratory in Rennes, was a cannabis-based painkiller.

The French health ministry said the six patients had been in good health until taking the drug orally, adding that the test was carried out by a private establishment “specialized in carrying out clinical trials.”

Marisol Touraine, the country’s health minister, called the incident a "very serious accident" and said that she would visit the city in northwestern France on Friday, BBC reported. She said that authorities will launch an investigation into the incident.

However, Touraine denied media reports that the drug used was a cannabis-based painkiller.

Touraine also said that the study was a phase one clinical trial, in which healthy volunteers take the medication to “evaluate the safety of its use, tolerance and pharmacological profile of the molecule.”