(CNN) A French court has heard the start of a trial involving a weight loss pill that public health officials say may have been responsible for as many as 2,000 deaths.

The massive trial, with 25 defendants and 2,600 plaintiffs, centers on Mediator, a Type 2 diabetes drug that was found to suppress appetite and was later prescribed as a weight loss aid.

However, the pill caused pulmonary hypertension -- high blood pressure in blood vessels that supply the lungs -- and heart complications, leading to death from heart failure and cardiac arrest in some patients.

The drug also affected the aorta, the body's largest artery, and left many users in need of heart valve replacements.

Defendants in the trial, which began Monday at the Paris Criminal Court, include the drug's manufacturer, Servier; France's drug watchdog, the French National Agency for Medicines and Health Products Safety; and a senator.

Servier is accused of fraud, manslaughter and deceit by marketing a drug that was known to be harmful, while the drug watchdog faces charges of manslaughter by negligence and involuntary injury.

Other defendants are accused of illegal conflicts of interest and influence peddling.

Mediator was released in 1976 and was prescribed in France for 33 years before it was eventually banned in 2009 because of its side effects.

Servier is one of France's largest pharmaceutical laboratories, and the case has raised questions over the apparently cozy relationship between drug companies and regulators.

The company has already paid out €131.8 million ($145 million) of a total €164.4 million offered to 3,732 patients on August 30.

In July last year, Servier said in a statement that it "has made every effort to compensate all patients who have suffered as a result of Mediator."

Claude Lienhard, a lawyer representing 50 of the plaintiffs, said those affected will be able to testify at the long-awaited trial.

"The truth was hidden from the patients, a lie was created," Lienhard said on a local radio station.

"That's why there are doctors, pharmacologists and officials that are being accused of conflicts of interest."

He said some of the alleged victims accepted compensation payments because they needed the money, and he hoped the case would help to prevent similar scandals happening again.

"We expect exemplary sanctions, because what's at stake is not only what Servier did, it's also about prevention because we cannot say today that these practices do not exist."

Plaintiff Michele Jetot said she had to have heart surgery after taking Mediator for two years.

"I hated Servier. I said if I meet him I'll kill him. Just take into account all the people he's killed," Jetot said in a video interview with Le Parisien, referring to Jacques Servier, the pharmaceutical company founder who died in 2014 at age 92.

"They stole my life. I had just retired and thought I was going to begin a new life but I can't leave my house now. I can't see anyone. I get tired all the time."

On Monday, Francois de Castro, a lawyer for Servier, told CNN affiliate BFMTV that "the Servier laboratory didn't know that their drug caused heart problems and it's hurtful to everyone who worked in this laboratory to be accused of having known that and not having acted."

A 2010 report by France's national health insurance entity, CNAM, revealed that over about 30 years, as many as 500 patients may have died from complications directly related to Mediator.

However, Agnes Fournier, an epidemiologist with the French national institute of health and medical research (INSERM), wrote a 2012 report in which she estimates the drug could have been responsible for up to 2,000 deaths.

Mediator was banned in Belgium in 1978, Switzerland in 1997 and Spain in 2003, according to a report presented by France's social affairs commission to the National Assembly in 2011, and the trial will examine why the pill remained available in France for so long.