Patients taking both a popular blood-thinning drug and anti-acid medications may actually boost their risk of having a heart attack, a new study suggests.

Canadian researchers have found that heart attack patients who take the blood-thinning drug clopidogrel (Plavix) have a 40 per cent greater risk of suffering another heart attack if they are also taking one of a number of proton-pump inhibitors (PPIs).

PPIs block the production of stomach acid and are used to treat conditions such as ulcers and acid reflux disease.

The PPIs linked to the increased heart attack risk are: omeprazole (Losec), lansoprazole (Prevacid) and rabeprazole (Pariet).

"This is really an important finding," Dr. Dennis Ko, one of the report's authors, told CTV Newsnet.

"What we didn't know before is they have an interaction."

Ko noted the group of people at risk is large given the sheer numbers of people taking both heart attack medication and anti-acid pills.

But since acid problems aren't life threatening, patients on heart attack medication should consult with their physician and explore different options, said Ko.

"If you're on one of those medications, you should talk to your doctors about switching you in the meantime to something that's safe."

The researchers, from the Institute for Clinical Evaluative Sciences (ICES) in Toronto, say that they did not find an increased heart attack risk among patients taking the PPI pantoprazole (Pantoloc). They also did not find an increased risk among patients taking anti-acid medications known as H2 receptor antagonists, such as Pepcid or Zantac.

The researchers rushed to publish their findings after the U.S. Federal Drug Administration announced earlier this week that it would be conducting a review of Plavix's interaction with PPIs.

The findings are published in the online edition of the Canadian Medical Association Journal.

According to the researchers, previous studies have suggested that PPIs, other than pantoprazole, prevent the liver from converting Plavix to its active form. The drug must be converted to its active form in order to be fully effective.

However, this study followed thousands of patients over a six-year period, and the findings have far-reaching implications, the researchers said.

Most patients who take Plavix are also prescribed acetylsalicylic acid (ASA, or Aspirin) for heart-attack prevention, but Aspirin can also cause bleeding in the stomach.

Therefore, experts recommend that patients over 60 who are taking Aspirin also take an anti-acid medication to reduce the risk of bleeding.

"Because clopidogrel and ASA are often prescribed together following a heart attack, it is probable that millions of patients worldwide will be told to take a proton pump inhibitor in addition to clopidogrel," lead study author Dr. David Juurlink, of Sunnybrook Health Sciences Centre in Toronto, said in a statement.

"Depending on which PPI is prescribed, the effectiveness of clopidogrel in preventing recurrent (heart attack) may be reduced or eliminated altogether."

The researchers estimate that between five and 15 per cent of recurrent heart attacks among patients taking Plavix may be the result of the drug's interaction with a PPI.

According to Juurlink, doctors could reduce the risk of heart attack among their patients by prescribing Protonix to their patients who are also taking Plavix.