The Stanford University team found that rats dosed with it for 16 hours sustained twice the muscle damage when they had heart attacks, compared to those spared nitroglycerin.
Nitroglycerin is often used to help treat angina, while it is also used immediately after a heart attack.
Daria Mochly-Rosen, a professor of translational medicine, said they carried out the study because they were concerned that nitroglycerin use in angina patients could be increasing the severity of heart attacks.
The team has found that giving an enzyme killed by nitroglycerin at the same time protected rats' hearts from the harmful side-effect.
The study is published today (THUR) in the journal Science Translational Medicine.