A clinical trial taking place in the French city of Rennes has left one patient dead and five critically ill, according to media reports.

The study, being conducted by a CRO, was a Phase 1 trial involving healthy volunteers. It is believed the accident occurred on Thursday.

Initially, it was reported the drug on trial was a cannabis-based painkiller. However, the French health ministry has denied that was the case.

As it stands, the trial has been suspended, with the firm conducting the trial recalling all 90 volunteers that took part.

According to the BBC, the Paris prosecutor’s office has opened up an investigation. Additionally, France’s Health Minister Marisol Touraine has vowed to "get to the bottom of this tragic accident."