Fish oil supplements are full of omega-3 fatty acids, which can lower risk of heart failure and coronary disease. A new study, however, has discovered the supplement might actually increase a person’s risk of developing potentially fatal health conditions.

Published in BMJ Medicine, the study analyzed data from more than 450,000 participants to determine how fish oil can affect both healthy users and those suffering from cardiovascular disease or atrial fibrillation. The researchers discovered the supplement increased certain health risks in healthy users, but could be beneficial to users with poor heart health.

“Regular use of fish oil supplements might be a risk factor for atrial fibrillation and stroke among the general population but could be beneficial for progression of cardiovascular disease from atrial fibrillation to major adverse cardiovascular events, and from atrial fibrillation to death,” according to the study.

It’s a discovery that largely mimics observations reported by the National Institutes of Health.

“Overall, research indicates that consuming fish and other types of seafood as part of a balanced diet promotes heart health, especially when the seafood is consumed in place of less healthy foods,” the institute said.

“Fish oil and other LC omega-3 supplements lower triglyceride levels and might reduce the risk of some cardiovascular endpoints, especially among people with low dietary omega-3 intakes. Evidence of a protective effect for omega-3 supplementation is stronger for people with existing coronary heart disease than for healthy individuals.”

The new research, however, determined healthy supplement users were associated with precisely a 13% higher risk of developing atrial fibrillation and a 5% increased risk of developing a stroke.

However, the study concluded, “(f)urther studies are needed to determine the precise mechanisms for the development and prognosis of cardiovascular disease events with regular use of fish oil supplements.”