High-risk Heart attack patients given morphine for their Chest pain have almost a 50 percent higher risk of dying, U.S. researchers reported on Thursday.

The morphine could be masking the symptoms of the most seriously ill patients and could itself be causing harm, the team at Duke University in North Carolina said.

Cardiologists should begin treatment with enough nitroglycerin to relieve pain before resorting to morphine, they advised.

“The results of this analysis raise serious concerns about the safety of the routine use of morphine in this group of heart patients,” said Duke cardiologist Dr. Trip Meine, who led the study.

“Since randomized clinical trials evaluating the safety or effectiveness of morphine for these patients have not been conducted, official guidelines for its use are based solely on expert conjecture,” Meine added in a statement.

Writing in the American Heart Journal, the researchers noted that both drugs have been used since before the days of modern medicine. Morphine was first used to relieve the Chest pain caused by heart attacks in 1912, while the use of nitroglycerin dates back more than 130 years.

Meine and colleagues looked at the records of 57,000 heart attack patients.

They looked specifically for patients with non-ST-segment elevation Myocardial Infarction or non-STEMI, a type of Heart attack that causes chest pain but not always other typical heart attack symptoms.

This type of heart attack sends an estimated 1.3 million Americans to the hospital every year.

Nearly 30 percent of the 57,000 patients were given morphine within the first day after being admitted to the hospital.

Those given morphine had a 6.8 percent death rate, compared to 3.8 percent for those given nitroglycerin.

“Nitroglycerin has a physiological effect that may, at least temporarily, influence the underlying ischemia,” Meine said.

“Morphine, on the other hand, doesn’t do anything about what is actually causing the pain. It just masks it, and may, in fact, make the underlying disease worse,” he added.

“Morphine has the well-known and potentially harmful side effects of depressing respiration, reducing blood pressure and slowing heart rate. These side effects could explain the worse outcomes in patients whose heart function has already been compromised by disease.”