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Taking a widely prescribed blood thinner after a heart attack in combination with common medications to reduce stomach acid could significantly increase a person's chances of suffering another heart attack, a new study by Canadian researchers has found.

A five-year study of more than 13,000 patients found that those who took Plavix along with certain proton pump inhibitors in the first three months after a heart attack have about a 40-per-cent increased risk of suffering a second attack. The study was published online yesterday by the Canadian Medical Association.

The finding has sweeping public health implications because both drugs are widely prescribed and the potential risks are only now being uncovered, said David Juurlink, the study's lead author and head of the division of clinical pharmacology and toxicology at Toronto's Sunnybrook Health Sciences Centre.

"There are easily tens of thousands of Canadians, probably more, and millions of people on the planet who are taking these drugs together," Dr. Juurlink said.

Proton pump inhibitors, a class of drugs used to help prevent acid reflux, seem to decrease the effectiveness of Plavix, a blood-thinning medication often prescribed to people after a heart attack, the study found.

Researchers estimate the drug interaction is responsible for between 5 and 15 per cent of recurring heart attacks among patients taking Plavix.

The publication of the study comes after health officials in Canada and the United States announced they were looking into reports of the potentially dangerous drug interaction.

The U.S. Food and Drug Administration announced earlier this week that it's investigating whether Plavix is less effective in some patients, a decision made after reports that some acid-reducing medications seemed to reduce the drug's effectiveness.

Health Canada also said it's in the midst of reviewing reports and studies about the effectiveness of Plavix and possible drug interactions to determine what, if any, action is needed.

"New information will be made public to Canadians and Canadian health-care professionals as soon as it is available," department spokesman Stéphane Shank wrote in an e-mail.

Although the study's findings highlight potentially serious risks, the drug interaction seems to be completely avoidable, Dr. Juurlink said.

Study participants who took Plavix in conjunction with the proton pump inhibitors omeprazole, lansoprazole and rabeprazole had an elevated risk of recurring heart attacks. But those who took pantoprazole, another proton pump inhibitor, showed no increased risk.

Dr. Juurlink said patients currently taking Plavix and proton pump inhibitors should not stop taking their medication, and advised them to consult a doctor to determine what action to take. He said some patients may be switched to pantoprazole as a way of minimizing their chances for a potentially dangerous drug interaction.

"If they actually have to be on a proton pump inhibitor, the family doctor might elect to use the safer one," Dr. Juurlink said.

Although previous research has found that Plavix seems to be less effective in some people, including those taking certain proton pump inhibitors, the Canadian study is the first of its kind to document and quantify the potential risks from this drug interaction.

In the study, researchers found that the risk of drug interaction appeared highest in the first three months following the initial heart attack, but seemed to decline afterward.

More than 2.8 million prescriptions for Plavix, worth about $267-million, were distributed from Canadian retail pharmacies in 2008, according to IMS Health Canada. About 15.9 million prescriptions, worth about $1-billion, were issued for the four proton pump inhibitors cited in the research.