Gilead Sciences (GILD) has alerted health care providers, once called physicians, that sofosbuvir has probably been implicated in exacerbating the effects of the type III anti-arrhythmic drug amiodarone and causing excessively slow heart rates. There was one death and there were three pacemaker insertions. The interactions were seen both with Sovaldi (sofosbuvir) and Harvoni (sofosbuvir plus ledipasvir).
Oddly, Gilead itself is in Phase 2 clinical trials using a next-generation analogue of amiodarone called dronedarone (Multaq) in combination with Gilead's ranolazine (Ranexa) to treat paroxysmal atrial fibrillation, in which the heart generally beats too fast and irregularly.
I have not seen the warning; I would suppose that it would apply both to amiodarone and dronedarone, but that's not a certainty.
In any case, these two anti-arrhythmics are not especially common drugs. I do not think this news will affect Gilead's profits in any material way. If the stock happens to sell off tomorrow out of apparent nervousness about this news item, I would expect it to reverse shortly.
The bigger issue with GILD is, as I have sadly said some time ago, that EPS estimates for 2016 are steadily eroding. That's not what the Street wants from a stock that is up 5X since late 2011.