Proton pump inhibitors are such effective stomach acid blockers they are now the third most popular class of medication in the United States.

But a new study says that they may also increase heart attack risk.

Researchers at Houston Methodist and Stanford University analyzed data that included 1.8 million Stanford hospital and clinic patients to identify those who had no history of heart attack, but were taking PPIs or H2 blockers for heartburn.

The study was published in the online journal PLOS ONE.

The researchers found that the PPIs increased the risk of heart attack 16 to 21 percent. But in a separate analysis, they found no increase for H2 blockers.

Although the study did not offer proof that PPIs were to blame for the increased risk, these medications are known to decrease nitric oxide, which is essential for maintaining the structure and the elasticity of the heart’s blood vessels.

PPIs are the strongest acid inhibitors available, and many can be purchased over the counter.

If you are taking a PPI, talk to your doctor to find out if an H2 blocker might be a better choice.

Cimetidine (Tagamet) and ranitidine (Zantac) are examples of H2 blockers.