A common objection against taking fish oil supplements is a fishy taste and the "burp" effect, both of which may be signs that the oil has started to oxidise.
Fish oils tend to be unstable, and they undergo oxidation during processing and after encapsulation. Oxidation occurs when the oil comes into contact with oxygen. It's accelerated by the presence of light and heat.
Oxidation is inevitable - but oils shouldn't oxidise to the point of rancidity. Rancid oils will taste and smell horrible. There's also concern that rancid oils may have bad health effects.
When we tested omega-3 supplements in 2007 we found four products that had oxidised to unacceptable levels.
Date marking
The older the oil is, the more likely it is to be rancid.
Best-before dates are a worry, because you've got no way of knowing how old the oil is.
We'd like to see a packed-on date - as well as a best-before date - because then you'd know exactly how old the product was. We'd also like the best-before date shortened to one year after manufacture. Some products in our 2007 test had best-before dates as long as three years.
Fish oil per capsule
Don't rely on the "total fish oil per capsule" stated on the label.
A claim of "1000mg of total fish oil" is meaningless if the fish oil is from a species low in omega-3. It's more useful to know the total omega-3 content.
Some labels don't give the total omega-3 content per capsule. We'd like to see this clearly stated.
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