A landmark trial over one of France’s biggest healthcare scandals has begun after a weight-loss pill was believed to have killed up to 2,000 people and left many more injured for life.

The trial for manslaughter and deceit will attempt to lift the lid on France’s massive pharmaceuticals industry.

Servier, one of France’s biggest and most powerful privately-owned laboratories, is accused of covering up the killer side-effects of a widely prescribed drug called Mediator. The French state drug regulator is accused of lenience and not acting to prevent patient deaths and injuries.

The Mediator pill was an amphetamine derivative marketed to overweight diabetics but it was often prescribed to healthy women as an appetite suppressant if they wanted to lose a few pounds. Even healthy, slim and sporty women were prescribed it by their doctors who advised they should take it in order to avoid weight gain.

As many as 5 million people were given the drug between 1976 and 2009, despite the fact that it was suspected of causing heart and pulmonary failure. The health ministry found at least 500 people died of heart valve trouble in France because of exposure to Mediator’s active ingredient, but other estimates by doctors put the figure closer to 2,000. Thousands more live with debilitating health problems.

Some women, who began taking the drug while in good health, found themselves unable to climb a flight of stairs and were left with permanent cardiovascular problems that limited their daily lives. Servier has paid out almost €132m (£116m) in compensation.

The trial will seek to establish why the drug was on the market for so long in France. Lawyers argue that Servier laboratory deliberately misled patients for decades, helped by lenient authorities. The drugmaker has been accused of making at least €1bn from the drug, while knowing of its dangers.

The French drug regulator, the Agence National de Sécurité du Médicament, is on trial accused of not taking sufficient steps to check and control the drug. It has been accused of being too slow to act and being too close to pharmaceutical companies. The watchdog has said it would cooperate with the trial and was now abiding by stricter ethics rules.

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The alarm was raised in 2007 when Irène Frachon, a lung specialist from a Brittany hospital, assessed patients’ records and warned of a link between Mediator and serious heart and pulmonary damage.

“The trial comes as huge relief. Finally, we are to see the end of an intolerable scandal,” Frachon said this week. “This so-called medicine is in reality a poison.

The drug was not withdrawn from the market in France until 2009, two years after Frachon raised the alarm and many more years after it had been pulled in Spain and Italy. It was never authorised in the UK or US.

In the 677-page French indictment, magistrates wrote that Servier “knowingly concealed the medication’s true characteristics” from the 1970s and hid medical studies unfavourable to the product, perpetrating a long-term fraud.

The scandal has raged for more than a decade, sparking a political row about drugs regulation and the lobbying power of pharmaceutical companies in France, which has one of Europe’s highest levels of consumption of prescription drugs.

The vast trial, with 21 defendants and more than 2,600 plaintiffs, will last six months and is set to be one of the longest court cases in Paris for decades. It has been likened in its time frame to the 1997 trial of the former police chief, Maurice Papon, convicted for his role in sending 1,700 Jews to Nazi death camps between 1942 and 1944.

It has taken more than 10 years for the case to come to court. “The fact that a trial is ultimately taking place is, in itself, a victory for the victims,” said Charles Joseph-Oudin, a lawyer for 250 plaintiffs.

Servier has said it did not lie about the effects of the treatment and hoped to demonstrate it did not act against patients’ interests.