The US Food and Drug Administration has launched an investigation into the safety of GlaxoSmithKline's anti-nausea drug Zofran and its generics on concerns the drug may be associated with changes in the electrical activity of the heart.

The agency said yesterday that the active ingredient in Zofran - ondansetron - may prolong the QT interval on electrocardiograms, an abnormality which can lead to the potentially fatal abnormal heart rhythms such as Torsade de Pointes.

The concern is so great that the FDA is making an interim change to the labelling of all ondansetron products on the US market to strengthen warnings about their potential for QT prolongation.

"The labels are being revised to include a warning to avoid use in patients with congenital long QT syndrome because these patients are at particular risk for Torsade," it said.

Additional ECG testing requirements for are also being added to the label for certain high-risk patient groups, including those with electrolyte abnormalities, congestive heart failure, bradyarrythmias and patients receiving concomitant treatment with other drugs linked to QT prolongation.

GSK is being asked to carry out a study to assess the potential for the drug to prolong the QT interval, with the results expected in the summer of 2012. Thereafter additional labelling changes may be ordered, said the FDA.

Former blockbuster Zofran is rapidly losing its importance among GSK's product portfolio since the advent of generic competition, achieving £109 million in sales in 2009. Since last year it no longer features among GSK's breakout of drugs by sales in its financial reporting.