An epilepsy drug that can damage unborn babies must no longer be prescribed to girls and women of childbearing age in the UK unless they sign a form to say that they understand the risks.
Drug regulator the MHRA says the new measures it's introducing will keep future generations of children safe.
Those already on valproate medication should see their GP to have their treatment reviewed.
No woman or girl should stop taking it without medical advice though.
It is thought about 20,000 children in the UK have been left with disabilities caused by valproate since the drug was introduced in the 1970s.
Affected families have called for a public inquiry and compensation.
Epilepsy charities say one in five women on sodium valproate are unaware that taking it during pregnancy can harm the development and physical health of an unborn baby.
And more than one in four have not been given information about risks for their unborn child.
The MHRA has changed the licence for valproate, which means any doctor prescribing it will have to ensure female patients are put on a Pregnancy Protection Programme, which means:
- The patient can see her doctor every year to discuss the risks of this drug to an unborn baby
- She signs an acknowledgement form at least every year
- She is told about the importance of using contraception throughout treatment and having a pregnancy test if she thinks she could be pregnant
If valproate is taken during pregnancy, up to four in 10 babies are at risk of developmental disorders, and approximately one in 10 are at risk of birth defects.
Dr June Raine, from the MHRA, said: "Patient safety is our highest priority. We are committed to making sure women and girls are aware of the very real risks of taking valproate during pregnancy. However, we also know it is vitally important women don't stop taking valproate without first discussing it with their doctor.
"I would like to particularly thank the families involved in the Valproate Stakeholder Network who have shared their experiences and expertise with us. Their support will help keep future generations of children safe."
Simon Wigglesworth, deputy chief executive of Epilepsy Action, said: "We know there are still far too many women who haven't been made aware of the potential risks of taking sodium valproate in pregnancy.
"It is vitally important that healthcare professionals ensure that all women with epilepsy taking sodium valproate are reviewed in line with the new guidelines."