Herceptin is a targeted therapy that's proven very successful in advanced cancer cases but a known complication is that it can cause heart-muscle damage.
A study in the "Journal of the American College of Cardiology" suggests the drug could be more dangerous than previously thought.
While earlier studies had excluded older patients, this one focused on them.
Researchers found an increase in heart problems of 10 percent.
That's significantly higher than previous estimates ranging between one and seven percent.