Six others needed emergency hospital treatment after they reacted to a new substance, BIA 10-2474, that treats pain and mood disorders.

Experts in France—where the drug trial took place—say the injuries and death were directly caused by the substance being tested. They emphasised the “astonishing and unprecedented” nature of the accident, which caused a reaction in the brain “unlike anything seen before”.

The investigators from France’s National Agency for Drug Safety want to know why extensive animal testing failed to detect any dangers. The animals suffered no ill effects despite being given doses that were 400 times stronger than those given to the human volunteers.

They are also concerned by the extent of animal testing, which suggests the manufacturer, the Portuguese drug company Bial, may have been worried about the substance’s possible toxicity.

The Biotrial research institute, which was testing the drug, recruited 108 volunteers. Of these, 90 were given the drug, and the rest had a placebo. The seven who suffered severe reactions had been given a 50mg dose.