How to Tell if your Fish Oil is Rancid

Rancid fish oil obviously isn’t good. In a nut shell, if your fish oil is rancid then you’re not going to get the beneficial omega-3 fatty acids, and you...

Rancid fish oil obviously isn’t good. In a nut shell, if your fish oil is rancid then you’re not going to get the beneficial omega-3 fatty acids, and you will be consuming pills of toxic sludge. Not what you want from something which is meant to be a health supplement.  In some respects, knowing if your fish oil is rancid is even more important than knowing how your fish oil is processed (which, really you should know too). So, here I’ll explain how to tell if your fish oil is rancid, and how to stop your fish oil going rancid once you’ve bought it.


Telling if your oil is rancid

Visual test

I have read that you can tell if a fish oil is rancid by looking at it, because fish oils should be a clear golden colour, whereas rancid oils are cloudy. Whilst this may be true for some brands, its isn’t true for all. Anyone who has encountered Green Pastures cod liver oil will know that they are a deep and dark golden brown colour, and PurePharma’s fish oil is almost clear, and these are some of the best quality fish oils out there. So sadly, the visual inspection is not the most reliable test, but if you are familiar with the particular brand and regularly use their capsules, you could see if the oil looks different to previous purchases.

Capsule inspection

Having a look and squeeze of the capsules can help you determine if the oil has gone rancid, but again, it is’t a fool proof method. Gel caps should be firm, if they are squishy then it indicates that they lost their ability to protect the oil, and moisture and air can pass through the gel cap easier. Regular capsules should be clear and firm, if the capsules are soft or cloudy, again, it indicates that air/ moisture can transverse the capsule into the oil. As the oxygen in the air is one of the primary causes for oil to spoil, this isn’t good news! Obviously things like capsule squishiness are subjective, and the types of capsules used will vary from brand to brand, so this test isn’t fool proof by any means. Just because your capsule is squishier than you thought doesn’t mean the oil is rancid.

Smell & taste test

The best and quickest way to tell if your oil is rancid, is simply open up a capsule and smell it (or taste it if you are feeling brave). If it smells/ tastes mildly fishy, then don’t worry, many fish oils and cod liver oils will smell a little fishy, and why wouldn’t they? They are from fish, and fish smell of fish – so an oil from a fish which smells like fish should smell like fish itself – right? You can get some fish oils will be chemically cleansed to remove the smell but generally, they will have a bit of a fishy odour to them, and its nothing to worry about.

If the oil is rancid, you will know, it will smell and taste of rancid fish, and it can be overpowering. Its like when milk goes off, when its gone off, you really know.

Essentially if the smell or taste of the oil makes you want to gag, then the oil is rancid. If the oil is overly fishy in smell, it indicates that the oil is going off, and you might want to avoid it. If you don’t like the taste or smell of fish, then this could be a difficult test for you to make, but you fish-o-phobes can look at the other signs of a rancid oil.

The ultimate test

If you don’t know what rancid fish smell like, and are really worried about rancid fish oil you could always make your own rancid fish oil to act as a control for all your other fish oils. I’ve not tried this myself, but this should work to make your own rancid fish oil control:

  • Open up a couple of capsules on a small plate. Just opening the capsules should be fine, but you can pour the contents out if you like (you can also do the above tests at this point).
  • Place somewhere warm and bright – a sunny window sill is perfect.
  • Try and cover it to avoid flies etc, but still allow air to get to the oil. Something like a nylon food cover for cakes would be perfect.
  • Leave this way for at least 24 hours…
  • Have a good smell and look at the now rancid oil and compare it to the fish oil in the bottle.

You will now have a good memory of what a rancid fish oil smells like (and one you probably won’t forget any time soon). If the fish oil in your capsules is anything like this, then throw it away!


How to avoid buying a rancid oil

  1. Don’t buy in bulk – By ‘bulk’ I don’t mean getting a few bottles at a time, I mean don’t buy 1 bottle which contains enough for many months. Once the bottle is opened, and the air tight seal is broken, air is going in and out of the bottle, which will increase the speed at which the oil goes rancid.
  2. Buy fish oils sold in dark bottles – Light exposure increases the rate at which oils spoil. Dark bottles prevent any light penetrating into the oil, and prevent it going rancid.
  3. Look for 3rd party testing – This is something which does hike the price up, but if you can afford it, its worth it. The supplement market is so poorly regulated that rancid fish oils are sold all the time, and the only way to avoid this for sure is to have the products tested at a third party (Carlson Labs and Green Pasutres are places to look). If you can’t afford these products, you should go with a reputable brand, but even this isn’t without its risks. Independent tests of GNC products, (a massive brand in the US with connections to Holland & Barrett in the UK), found that 4 out of 5 products were fake. These were herbal supplements, not fish oils, but it goes to show even the big guys aren’t reliable for one reason or the other.


How to stop your fish oil going rancid

Even if you buy the best fish oil, there is still a chance that it can go rancid in your care. Just buying a good quality brand doesn’t get you out of the rancid oil forest just yet, and there are some important things you can do. First of all, use it, don’t put it in the cupboard and wait for that day when you are going to become the healthiest person on the planet. That day is unlikely to come (even with the best will in the world), so if you have bought some fish oil, make sure you use it. The longer it goes unused, the more time it has to go rancid. Secondly, if you use a liquid, once the bottle is open put it in the fridge. The low temperature slows down the spoiling process. This makes sure that you can finish the bottle before it goes off.



The omega-3 fats in fish oils are susceptible to oxidative damage, which will make the fats go rancid. This is something you want to avoid at all costs, because rancid fats are pretty bad for you. The best way to check if your fish oil is rancid is simply to taste it. If it tastes disgusting or overly fishy, then it is probably bad. A slight fishy taste is nothing to worry about though. To reduce your risk of buying rancid fish oils, try and find dark bottles which have 1 months supply in from a reputable brand, preferably one which has 3rd party tests. When you have your fish oil, don’t store it – use it, and keep it in the fridge.

Image courtesy of oter.

I hope you enjoy the site, and like what we have worked hard to create, any feedback is very much welcome, after all this site is for you! Graduate of Nutrition & Food Science (Bsc) at Reading Uni.

    The Health Cloud was created in December 2011 by Craig and Morg who have been friends since high school. Our focus is to educate our readers with unbiased health articles and on the side we run our own online health shop. This website is for you, so drop us a comment or send us a tweet, we always take the time to reply!