It’s been called a bigger scandal than Thalidomide. The drug sodium valproate is estimated to have harmed 20,000 children in the UK.

It’s mainly used to treat epilepsy and other conditions such as bipolar disorder.

But taking the drug when pregnant can cause serious harm to unborn babies.

Even when it was licenced fifty years ago, it was known to cause harm to foetuses in animals. But it wasn’t until 2005 that the patient information leaflet, which should come with a prescription- gave clear warnings on the risks of taking valproate in pregnancy to unborn children, beyond a small chance of spina bifida.

File on 4 asks if the health regulators in the UK and the company who make it, Sanofi, did enough to inform patients of the severity of the drug's risks soon enough.

Meanwhile, new risks of the drug are still emerging. A new study shows the drug may affect the neurological development of children fathered by men taking valproate. The evidence is still inconclusive, but neurologists are uncertain what to advise their male patients on valproate.

And why are women still getting pregnant on it? We ask if the system set up to protect women taking it is working as it should be?

A major review of the drug made a number of recommendations, including the setting up of specialist clinics and a compensation scheme for those affected. The programme asks what progress has been made in the UK to implement those changes, and are we lagging behind other countries?

File on 4 speaks to the families whose children have been left with lifelong neurological and physical disabilities as a result of taking the medication.

Reporter: Rachel Stonehouse