In reality, supplements probably contain fewer toxins than whole fish, Ismail says. Most fish oil purveyors remove heavy metals like mercury and lead during the manufacturing process. And supplements made from virgin, unprocessed oils such as krill or salmon tend to come from pristine waters with few contaminants. “Within the past year we tested 60 products from shelves around the world, and we never found one that had detectable levels of lead, cadmium, or arsenic,” Ismail says. Another recent study found that although every product tested contained mercury, it was only in trace amounts.

That said, you’re better off choosing a medium- to high-grade product, says Vannice. “The least expensive fish oil usually has the lowest concentration per pill, and it may not have the attention to purity or freshness that other products do,” she says.