The media recently published research from the University of Auckland which evaluated the quality and content of fish oil supplements sold in Australia and New Zealand. The study looked at the amount of omega 3 EPA and DHA in various products and also measured their oxidation (rancidity).
The study concluded that many over-the-counter fish oil products do not contain the amounts of EPA and DHA shown on the label due to high rancidity levels. This means that you are not only not getting what you are paying for but are also risking these oils being pro-inflammatory, or causing inflammation rather than being anti-inflammatory which is a primary reason fish oil supplements are prescribed.
The practitioner-only brands I use and recommend in clinic undergo strict third-party testing to ensure that the oils are not oxidised or rancid and the omega-3 content meets or exceeds the amount shown on the label. I certainly believe that with fish oils, you get what you pay for and it is worth investing in the practitioner-only products for best results.