Who hasn’t popped a few ibuprofens at the first sign of a headache? Well, you may want to rethink taking those pills next time.

Non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, are some of the most common over-the-counter painkillers worldwide and have long been considered relatively harmless. But they may increase the risk of a heart attack, according to a study published in the journal European Heart Journal — Cardiovascular Pharmacotherapy.

“Allowing these drugs to be purchased without a prescription, and without any advice or restrictions, sends a message to the public that they must be safe,” said author Professor Gunnar H. Gislason, professor of Cardiology at Copenhagen University Hospital Gentofte, Denmark. “Previous studies have shown that NSAIDs are related to increased cardiovascular risk which is a concern because they are widely used.”

The Danish study examined all NSAID prescriptions, which in Denmark include ibuprofen and naproxen — two drugs that are available over the counter in the United States as Advil and Aleve. The only NSAID painkiller sold over the counter in Denmark without a prescription is 200 mg tablets of ibuprofen, which is equivalent to regular strength Advil in the U.S.

The researchers compared the medical records of nearly 30,000 people who had heart attacks between 2001 and 2010, as well as all NSAID prescriptions filled at pharmacies since 1995.

The data revealed that over 3,370 people who had heart attacks had taken an NSAID in the 30 days before the cardiac arrest. In comparison to their prescriptions from previous months before the heart attack, the researchers discovered that filling a NSAID prescription raised the risk of a heart attack by 31 percent.

Specifically, diclofenac, which is also sold by prescription in the U.S., raised the risk of a heart attack by 50 percent, and prescription-strength ibuprofen raised the risk by 31 percent.

“The findings are a stark reminder that NSAIDs are not harmless,” said Professor Gislason. “Diclofenac and ibuprofen, both commonly used drugs, were associated with significantly increased risk of cardiac arrest. NSAIDs should be used with caution and for a valid indication. They should probably be avoided in patients with cardiovascular disease or many cardiovascular risk factors.”

The research was inconclusive on how long a person had to take the medication before their risk went up, but the average duration of treatment in the study was from 13 to 29 days.

“Previous studies have observed enhanced cardiovascular risk associated with less than 30 days of treatment with NSAIDs,” the researchers wrote.

The researchers noted that naproxen, celecoxib and rofecoxib were not associated with a higher risk of cardiac arrest because they were not widely prescribed in Denmark.