Dronedarone, marketed as Multaq, increases the risk of heart failure, and was directly responsible for 10 deaths in a group of 310 people who were using the drug.
The drug, manufactured by Sanofi-Aventis, is in its last trial phases before it is granted a license, and sold to the public.
But, say researchers, the drug increases the chance of dying among patients with severe heart failure as it worsens their condition.
The patients in the trial were given 400 mg of dronedarone twice a day. Over a two-month period, 25 patients died, and 10 of those directly as a result of their heart condition worsening after taking the drug.
(Source: New England Journal of Medicine, 2008; 358: 2678-87).