One person has been left brain dead and five others are in a serious condition after taking part in a clinical trial in France of an experimental medicine from an unnamed drug company, the French Health Ministry said on Friday.
The ministry did not say what the medicine was intended to be used for. A person familiar with the situation said the drug was a cannabis-based painkiller in the initial stage of human testing.
The ministry said the six volunteers in Rennes, in western France, had been in good health until taking the oral medication, developed by "a European laboratory."
Britain's GW Pharma, which markets an approved cannabis-derived treatment for spasticity caused by multiple sclerosis, and France's largest drugmaker Sanofi both said they were not involved in the trial.
"This test was carried out at a private facility specialised in carrying out clinical trials," a ministry statement said.
The brain dead individual was admitted to hospital in Rennes on Monday. Other patients went in on Wednesday and Thursday.
All trials on the drug have been suspended and all volunteers who have taken part in the trial are being called back.
A spokeswoman for the European Medicines Agency in London said it did not have full details of the case but was monitoring the situation.
Cases of early-stage clinical trials going badly wrong are rare but not unheard of. In 2006, six healthy volunteers given an experimental drug in London ended up in intensive care. One was described as looking like "the elephant man" after his head ballooned. Another lost his fingertips and toes.
In the initial so-called Phase I stage of clinical testing, a drug is given to healthy volunteers to see how it is handled by the body and what is the right dose to give to patients.
"Undertaking Phase 1 studies is highly specialist work," said Daniel Hawcutt, a lecturer in clinical pharmacology at Britain's University of Liverpool.
Medicines then go into larger Phase II and Phase III trials to assess their effectiveness and safety before they are finally approved for sale.
Europe has strict regulations governing the conduct of clinical trials, with Phase I tests subject to particular scrutiny. But Ben Whalley, professor of neuropharmacology at the University of Reading, said these could only minimise risks rather than abolishing them.
"There is an inherent risk in exposing people to any new compound," he said.
The 2006 London trial led to the collapse of Germany's TeGenero, the firm developing the medicine known as TGN1412. The drug has since gone back into tests for rheumatoid arthritis and is showing promise when given at a fraction of the original dose.