One person has been left “brain-dead” and five more have been hospitalized after a clinical trial on a cannabis-based drug went wrong in France, according to French Health Ministry.

The statement said the “serious accident” happened on Thursday, stopping the tests and forcing all volunteers to be recalled, while confirming that one of the test subjects “brain dead.”

French Health Ministry, though, did not confirm the name of the drug being tested, but local media claim it was a new cannabis-based painkiller.

According to the French newspaper Ouest France the trial was being carried out by Biotrial, a major clinical trial company with 200 of its 300 staff based in the country.

The Paris prosecutors office told AFP news agency it has launched an investigation.