Heart patients who are taking Plavix and other similar drugs may be at risk of a heart attack if they also take antacid medications such as Nexium or Prevacid.

That's the conclusion of a U.S. study that repeats findings made in Canada earlier this year.

This latest study, published in JAMA, the journal of the American Medical Association, found that heart patients who take the blood-thinner clopidogrel (Plavix) have a greater risk of suffering a heart attack if they are also taking a proton-pump inhibitor (PPI), such as Nexium, Prevacid and Prilosec.

The study of 8,205 patients found that those taking clopidogrel plus a PPI were 86 per cent more likely to be rehospitalized for acute coronary syndrome -- a kind of heart attack -- than those not taking the two drugs together.

"When patients were not taking clopidogrel after hospital discharge, a prescription for PPI was not associated with death or rehospitalization for ACS, supporting the hypothesis that the interaction of PPI and clopidogrel, rather than PPI itself, was associated with increased adverse outcomes," the authors write.

The FDA warned about this possibility at the start of the year, just ahead of a study in the Canadian Medical Association Journal that found that heart attack patients who take clopidogrel have a 40 per cent greater risk of suffering another heart attack if they are also taking a PPI.

PPIs block the production of stomach acid and are used to treat conditions such as acid reflux disease. But they have also been prescribed to reduce the risk of gastrointestinal bleeding that may be associated with the use of clopidogrel.

PPI's appear to interfere with clopidogrel in a way that makes it less effective, which could increase the odds of heart problems in heart patients. Previous studies have suggested that PPIs, other than pantoprazole, prevent the liver from converting clopidogrel to its active form. The drug must be converted to its active form in order to be fully effective.

The researchers suggest that doctors could reduce the risk of heart attack among their patients by prescribing other PPI's, such as pantoprazole, to their patients who are also taking Plavix.