Fish oil supplements may increase, not reduce, the risk of first-time stroke and atrial fibrillation among people in good cardiovascular health, a new study reveals

For people without heart issues, regular use of fish oil supplements was associated with a 13% higher risk of developing atrial fibrillation and a 5% heightened risk of having a stroke, according to the study, published Tuesday in the journal BMJ Medicine.

However, the new study found that people with existing heart disease at the beginning of the research had a 15% lower risk of progressing from atrial fibrillation to a heart attack and a 9% lower risk of progressing from heart failure to death when they regularly used fish oil, CNN reported.

Over-the-counter fish oil suffers from a lack of purity and consistency, as well as the potential contaminants and heavy metals such as mercury that come with fish, Dr. Andrew Freeman, director of cardiovascular prevention and wellness at National Jewish Health in Denver has been quoted as saying by the CNN.

“If you are healthy and you seek to use fish oil capsules for preventing heart disease, it might be better to think over your decision,” Dr. Sarah Bonza, a board certified family physician and founder of Bonza Health in Columbus, Ohio, has been quoted as saying by Healthline, an American website which provides information on Health.