Fish Oil Side Effects

Studies have consistently shown that taking fish oil supplements on a daily basis is a highly effective and efficient way of getting enough Omega 3 fatty acids in our diet to make a positive difference to our health, but what are the potential risks of any side effects?

Fortunately, side effects are rare and when taken in the correct dosage most people experience little if any side effects at all, but as with any supplement, there is always the possibility of experiencing some symptoms and fish oil is no different. So what should we be looking out for?

Fishy burps

By far the most common side effects reported are fishy burps, fishy breath or a fishy aftertaste in the mouth. This is completely harmless albeit slightly unpleasant but can largely be avoided by keeping your fish oil in the fridge and taking the fish oil along with food.

Stomach upsets

Some people have experienced other gastrointestinal side effects such as diarrhoea, indigestion, abdominal bloating, heartburn, mild stomach cramps and perhaps slight feelings of nausea but again, these side effects can be reduced or eliminated by taking the fish oil along with meals and by taking a lower dose to begin with and then gradually increasing it over a few weeks.

Increased risk of bleeding

The fatty acids in fish oil have an anticoagulant (blood thinning) effect which in most cases is a good thing, however, when fish oil is taken in particularly large doses or alongside other blood thinning agents, there is an increased risk of bleeding which can take the form of easy bruising or nosebleeds or longer bleeding times. Serious side effects are extremely rare but it is important that anyone who is already taking Aspirin, Warfarin (also known under the brand names Coumadin, Jantoven, Marevan and Waran) or any other blood thinning drugs should discuss the potential risks with their doctor before taking any fish oil supplements.

Mercury poisoning

A major worry with eating fresh fish is the amount of mercury and other toxins they can contain. Sadly, our seas are polluted and consequently so are our fish and so any toxins in the fish can also find their way into fish oil supplements unless the fish oil has had these contaminants removed. Pharmaceutical grade fish oil is an informal term that has been applied to fish oil that has been through processes such as molecular distillation and sophisticated filtering systems where the mercury and other undesirable elements in the fish oil are removed leaving purified and concentrated fish oil and so this risk is not relevant for high grade fish oils. A major advantage in buying a good quality or pharmaceutical grade fish oil is that it can be concentrated to contain very high levels of the most important fatty acids such as Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DHA). It also means you can take one or two capsules a day to get the same effect of maybe five or six capsules of lower or standard grade fish oils.

Other potential side effects

If the fish oil supplement contains vitamin A then there is an increased risk of vitamin A toxicity. Cod liver oil for example, is produced from the liver of fish where vitamin A is stored and can contain potentially high levels of Vitamin A so it is best avoided.

Fish oil can also go rancid very quickly and as Vitamin E is a powerful antioxidant it is added to some fish oils to keep them fresh. There have been some studies indicating that taking fish oil can increase the need for antioxidants so it is advisable to look for fish oil that has vitamin E added.

Other studies have suggested that fish oil might increase levels of low density lipoprotein (LDP) or bad cholesterol, but this depends on the dosage.

There have been some indications that fish oil can have a short term effect on blood glucose levels in people with Type 2 diabetes so anyone with diabetes should seek the advice of their doctor before taking fish oil supplements.


In general, the side effects from fish oil are few and far between if any at all and serious side effects are in fact extremely rare whereas the health benefits to be gained from taking fish oil supplements are enormous.

If you are on existing medication or have other health problems or are worried about the possibility of side effects then a chat with your doctor before taking fish oil will put your mind at ease.

Finally, if you take the correct dose, keep your fish oil in the fridge and take your supplements at mealtimes then the chances are the only effect you will feel is a greater sense of wellbeing and perhaps a lighter mood.