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The risks of pregnant women taking a common epilepsy drug were kept from patients for 40 years, a hearing was told.

An estimated 20 000 British children have been severely harmed because their mothers took sodium valproate.

Campaigners presented evidence to a hearing of the European Medicines Agency in London, revealing that regulators knew of problems as far back as 1973. Even the drug’s manufacturer Sanofi was open about the risks.

Yet documents show UK officials decided not to put the warnings on the packets for fear it ‘could give rise to fruitless anxiety’, the hearing was told. It was not until 2015 that clear warnings were published.

Sodium valproate – sold under the brand name Epilim to treat epilepsy, migraine and bipolar disorder – can cause physical deformities in children as well brain damage and problems such as autism.

Unborn babies exposed to it have a 30 to 40 per cent risk of serious developmental disorders and an 11 per cent risk of congenital malformations. But the Foetal Anti-Convulsant Support Association said documents from July 1973 show the Committee on Safety of Medicines – which was replaced in 2005 – said warnings should ‘not go on the package inserts, so there would be no danger of patients ?...?seeing it’.

Sanofi told the hearing: ‘Sanofi had always provided the most up to date scientific information, with the approval of the health authorities.’ The EMA will now consider if any further measures are needed to raise awareness of the risks.