Sodium valproate, which is marketed under several names including Epilim, is prescribed for epilepsy, migraine and bipolar disorder.
Babies exposed to the drug in pregnancy have a 30 to 40 per cent risk of serious developmental disorders and an 11 per cent risk of congenital malformations.
The most up-to-date advice from the UK’s Medicines and Healthcare products Regulatory Agency (MHRA) says it should not be given to women of childbearing age or during pregnancy unless alternative treatments are ineffective or not tolerated.
But documents from July 1973 show that regulators decided not to warn patients directly of the harms for fear it “could give rise to fruitless anxiety”.
Health professionals were told at the time: “This compound has been shown to be teratogenic in animals, meaning it could harm the human foetus.”
But the Committee on Safety of Medicines, which existed before the MHRA, said the warning should “not (go) on the package inserts, so that there would be no danger of patients themselves seeing it”.
The licence issued in 1974 for Epilim, a trade name for sodium valproate, said doctors and pharmacists should use it only in severe cases of epilepsy or where there was no alternative.
Karen Keely, from the Foetal Anti-Convulsant Syndrome Forum in Ireland, told the EMA hearing the “risks have been known for many years.”