According to a new warning issued last week by the U.S. Food and Drug Administration, GlaxoSmithKline’s popular drug, Zofran, which is used to treat nausea caused by chemotherapy, has been reported to trigger serious and potentially life-threatening changes in heart rhythms.

In its warning, the FDA explained that Zofran, otherwise known as ondansetron, can increase the risk for patients to develop “prolongation of the QT interval of the electrocardiogram, which can lead to an abnormal and potentially fatal heart rhythm.” The FDA also issued a letter to physicians, in which the agency defined the group of patients at greatest risk for developing heart arrhythmia. These patients included those with underlying heart conditions and those or are already predisposed to low levels of potassium and magnesium in the blood, as well as those patients currently taking additional medications that are associated with QT prolongation.

GSK, the pharmaceutical company who markets and sells Zofran, was ordered by the FDA to conduct an in depth review of its product, in order to better understand its impact on QT interval prolongation. The FDA has also ordered that the drug’s label be changed to reflect the new risks associated with its use.