France's medicines watchdog said on Monday that it had been charged with manslaughter and causing unintentional injury in a major scandal over an anti-epilepsy drug that caused thousands of birth defects.

The charges relate to the drug valproate, marketed as Depakine by French pharmaceutical giant Sanofi, which studies say has caused disabilities in about 15,000 to 30,000 children whose mothers took the medicine while pregnant.

On the market since 1967, the drug is used to treat epilepsy, migraines and bipolar disorder.

But research has found that when pregnant women took the drug, their children had an elevated risk - between 10 to 40 per cent - of congenital disorders, autism and learning difficulties.

In a statement, the French National Agency for the Safety of Medicines said it took "the full measure of the victims' suffering and has been working for several years to limit exposure to valproate in women of childbearing age."

It added it would continue to cooperate with the judiciary.

A court already found the French state liable for the scandal in July, ordering it to pay thousands of euros in damages.

The court's advisory expert said health officials knew about the risks regarding birth defects from using Depakine starting in 1983, and the risks of learning disabilities and autism from 2004.

Prosecutors have also charged Sanofi, accused by victims' families of taking too long to warn about the risks of taking the drug during pregnancy, with manslaughter over the case.

Sanofi has denied any wrongdoing, saying it warned health authorities of the risks beginning in the 1980s.

Under the French legal system, charges do not automatically result in a trial as prosecutors can decide not to proceed based on a lack of evidence.