The QT interval is the time on an electrocardiogram between the start of the heart's electrical activation and the end of its recovery. Certain medicines, electrolyte disturbances, and inherited conditions can lengthen it.
A prolonged QT interval is associated with an increased risk of a specific abnormal heart rhythm. Regulators including the European Medicines Agency have formally reviewed heart rhythm considerations for hydroxyzine-containing medicines. Anyone with a personal or family history of heart rhythm problems, or who takes other medicines affecting the QT interval, should make sure their prescriber and pharmacist know.