The French trial over Mediator, a diabetes drug that was also used to lose weight and is suspected in the death of 500 people, opened in Nanterre.

Servier, one of France's biggest pharmaceutical groups, is suspected of "aggravated deception" after being accused of hiding critical information about the product in a bid to make profits.

The group is accused of dissimulating the fact that the drug, known as Mediator, contained an amphetamine called benfluorex.

Benfluorex was only taken off the market in France in 2009 after it was found it caused heart valves to thicken. It had been withdrawn in Spain, Italy and the United States 10 years ago.

The health scandal has led to criticism of French authorities for allowing the drug to be sold for years after other Europeans countries had already pulled it from their markets.

The nature of benfluorex was allegedly hidden so the Servier group could obtain approval for the drug, which was first marketed in 1976.

The medicine was also used, with a state subsidy, as an appetite suppressant.

Charges have been filed against up to seven people, including 90-year-old Jacques Servier, the group's founder and president, who was present at the hearing.

The group's legal team, led by Hervé Témime, challenged the jurisdiction of the Nanterre tribunal, saying an investigation already under way in Paris could lead to the laboratory being judged twice on the same grounds.

The Nanterre trial is set to focus on whether Servier made misleading claims. The trial in Paris, which will take place later since the investigation is still underway, will focus on allegations of manslaughter and corruption.

Mediator was sold to as many as 5 million people in France between 1976 and November 2009.