ConsumerLab.com's tests of fish oil supplements have found none to contain mercury and most, although not all, to have only trace levels of PCBs (which can't be fully avoided since PCBs are found in water everywhere). A serving of fish meat is likely to contain far more contamination than a fish oil supplement. However, we have found some supplements to contain less fish oil than listed and some to be spoiled. In addition, the enteric coatings on some supplements have not worked properly.
Clinical studies have shown fish oil to be safe, but it is recommended that you get no more than 2 grams (2,000 mg) of omega-3 fatty acids (EPA and DHA) from fish oil supplements per day.
For details, including our test results and comparisons of fish oil supplements and dosage recommendations, see our Fish Oil Supplements Review