US regulators on Wednesday imposed a tougher warning on GlaxoSmithKline's diabetes drug Avandia, but said the data on whether it resulted in a higher risk of heart attack remained “inconclusive”.

In a ruling less severe than some analysts had expected, the Food & Drug Administration required the UK-based pharmaceutical company to add a reference of possible side effects to its “black box”, the most prominent warning of risks in the drug information leaflet that accompanies the medicine.