Side Effects

• The Creighton University Medical Center indicates that omega-3 fatty acid sometimes causes fishy-smelling skin and breath, as well as stomach upset, burping and flatulence. People who are sensitive to aspirin risk developing blood clots. The Mayo Clinic adds that excessive amounts of fish oil cause hemorrhagic stroke, blood in the urine, and slow blood clotting.
  
  

Infrequent Adverse Reactions

• The Mayo Clinic indicates that some patients who suffer from bipolar disorder or debilitating depression experience mania, restlessness and the pervasive feeling that something is crawling on their skin. Other patients have experienced increased levels in their bad cholesterol rather than decreased levels after taking fish oil capsules.
  
  

Toxic Contaminants

• Some species of fish accumulate harmful cancer-causing toxins called PCBs, or polychlorinated biphenyls. Many fish are also contaminated with methylmercury, a toxic heavy metal compound that causes brain and kidney damage. Mayo Clinic suggests that fish oil is safe. However, a 2010 lawsuit is challenging the safety of fish oil in a lawsuit, noting that PCBs were found in 10 different fish oil supplements.