For the second time in three months, the US Food and Drug Administration has warned that GlaxoSmithKline’s antidepressant Paxil (paroxetine) should not be used in pregnant women because it increases the risk of birth defects.

The agency said a second study has now shown that the drug is associated with heart defects and other problems when women take it during the first three months of pregnancy. It said it is currently gathering additional data and waiting for the final results of the recent studies to evaluate the risks further.

GSK has been asked to change the pregnancy category from C to D, and it is the only antidepressant in this category. The company updated the drug's labeling in September to add the data from one study, and will now include the results of the second.

Crucially, the FDA has not banned the use of the drug in pregnancy or women of child-bearing age outright, but said it is advising health care professionals ‘to discuss the potential risk of birth defects with patients taking Paxil who plan to become pregnant or are in their first three months of pregnancy’. It did say, however, that doctors should consider discontinuing Paxil in these patients. Around 25% of Paxil patients are women of child-bearing age, according to GSK.

The two studies have shown that women who took Paxil during the first three months of pregnancy are about one and a half to two times as likely to have a baby with a heart defect as women who received other antidepressants or women in the general population. Most of the heart defects reported in these studies were holes in the walls of the chambers of the heart.

The new label change adds to the pressure on Paxil, which is sold as Seroxat outside the USA. It has been linked to an increased risk of suicidal behaviour in children and adolescents and latterly the debate has been widened to ask whether adults are also affected.