How To Tell if Fish Oil Capsules are Rancid

Original Date of Publication: April 15, 2023

Original Author: Judy Blatman is OmegaQuant’s consumer health writer. Her blogs tee-up practical suggestions for health, with an emphasis on omega-3s and omega-3 testing, so readers can decide their own path to good health. Judy’s knowledge about dietary supplements developed during her 16 years spearheading communications for the Council for Responsible Nutrition, where she led an award-winning healthy lifestyle consumer education initiative focused on diet, dietary supplements and exercise. Learn more

Date of Review/Update: Jan 25, 2026

Reviewer: Stacy Cappadona MS, RD, CSSD, CSCS, TSAC-F

Stacy Cappadona is OmegaQuant’s science writer and reviewer. She is a registered dietitian and board-certified specialist in sports dietetics. Stacy has spent most of her career supporting the health and performance of United States military communities, applying evidence-based nutrition strategies to meet the demands of high-stress, high-performance environments. Learn more

Fish oil products are among some of the most well-researched dietary supplements, and their potential health benefits are well-established(1). Yet, just as fat-rich foods can go bad and turn rancid, so too can fat-rich omega-3 supplements. With apologies to Dr. Seuss, if the smell isn’t swell, well, that’s a tell, that the fish oil’s not necessarily well.

All kidding aside, fish oil rancidity is no joking matter. A mild fishy smell for fish oils (and other marine-based oils like krill and algae) is not unusual. Some companies spend more (and charge more) to make sure their fish oil supplements have no fishy smell. Others flavor their fish oil supplements to make them more palatable to the nose, which in turn makes them more palatable to swallow. 

But, when your fish oil stinks like fish gone bad, it might be rancid.

It’s not just the smell; rancid fish oil can also negatively impact the taste and texture. (You try rhyming with texture.) Still, as consumption of omega-3 products have increased, so too have concerns about their stability and potential effects related to rancidity.

VIDEO: Ask Us Anything: A-Fib, Plaques & Dementia

 

What Causes Fish Oil to Become Rancid?

Rancidity refers to the unusual or unwanted odors and flavors that result from lipid (fat) oxidation(2). Oxidation is a fact of life for unsaturated fatty acids, for all lipids, in fact. However, since unsaturated fatty acids are less stable than saturated fatty acids, foods that are rich in unsaturated fats (like fish and fish oils) are more vulnerable to rancidity(2).

Here’s how it happens: when the unsaturated fatty acids in fats and oils are exposed to oxygen (in the air around us), naturally occurring hydrogen atoms found in the fatty acid chain can be replaced by oxygen atoms(3). Once oxygen reacts with the fatty acid molecule, some of the chemical bonds can rearrange and even form new molecules, like peroxides, alcohols and aldehydes(3).

The rate of oxidation can be influenced by exposure to oxygen, light, heat, and the degree of unsaturation of the fatty acid(3). The higher the amount of the unsaturated lipids, the more likely that the fish oil will go rancid over time, especially if certain precautions aren’t taken by manufacturers and by those purchasing/consuming the products. And while it is the highly unsaturated fats found in omega-3s EPA and DHA that provide so many of the benefits of fish oil supplements, it’s those same highly unsaturated fats that can lead to rancidity.

No (legitimate) company starts out thinking “let’s make some rancid fish oil capsules.” But let’s face it: some companies take more care than others to make sure that they are producing high quality fish oil supplements. 

VIDEO: Does Omega-3 Lower Cholesterol?

 

Standards Related to the Freshness of Fish Oil Products

Responsible companies are taking initiative to help ensure the highest standards for omega-3 EPA and DHA. First, a little history to bring you up to date. Back in 2002, the Council for Responsible Nutrition (CRN), a leading dietary supplement industry trade association, worked with Omega-3 companies to develop a Voluntary Monograph (non-governmental, industry-developed standard) for Omega-3s EPA and DHA as a way to help ensure safe, high quality products for consumers worldwide.

The monograph focused on developing standards for oxidative quality, environmental contaminants, and the measurement of EPA and DHA, incorporating the strictest quality and purity standards from regulations worldwide.

The companies who worked to produce the monograph later formed the Global Organization for EPA and DHA Omega-3s (GOED), a not-for-profit trade group, which today represents more than 170 companies from fisheries and crude oil suppliers to refiners, concentrators and brands exclusively dedicated to increasing omega-3 EPA and DHA consumption with quality products that consumers can trust.

It made sense for GOED to rebrand the monograph under its banner, as the GOED Voluntary Monograph, and to review and update it as needed to help ensure that omega-3 oils used by consumers are both safe and of high quality(4). The monograph was most recently updated in January 2022 and today still serves as the benchmark for quality in the omega-3 industry worldwide.

Presently, the GOED Monograph standards set oxidation limits for consumer omega-3 products — including generally acceptable limits for analytical measures related to oxidation like the peroxide value (PV), the para-anisidine value (p-AV), and total oxidation value (TOTOX) — to measure rancidity(5). A companion piece, known as the GOED Guidance Documents, serves as a technical blueprint for manufacturers to comply with the GOED Monograph, including oxidation recommendations(6). Still, these guidelines are voluntary, and reviews have reported varying degrees of compliance.

Moreover, scientists suggest that the analytic values set forth by the monograph should also be interpreted with caution, as numbers like the PV and p-AV can be falsely elevated for several reasons.

For example, researchers report that the inclusion of natural or synthetic flavorings or colors can interfere with accurate measurements. One study reported that lemon flavorings, which are commonly included in fish oil products to improve smell and taste, can increase p-AV levels more than 12-fold(3).

Moreover, oils from different types of fish, like salmon or krill, can have different properties that can lead to false elevations on p-AV testing(7). The GOED Monograph does take some of these nuances into consideration, stating that the p-AV assay is not applicable to flavored oils for these reasons.

Responsible fish oil supplement manufacturers have also developed effective strategies to help slow down oxidation in the production process. Some of these steps include limiting exposure to ambient air during production, refining oils in a vacuum, blanketing storage containers with inert gases like nitrogen that displace oxygen and using antioxidants to slow the rate of oxidation(3).

VIDEO: Omega-3 VS Fish Oil: What’s Better?

 

Why is Freshness so Important with Fish Oil?

Research on the effects of oxidized omega-3 oils is still emerging. Yet there is fear among some health experts and consumers that consuming oxidized products will either be ineffective, or worse, lead to adverse health effects.

Studies examining the effectiveness of oxidized fish oil warrant further attention. One study involving healthy subjects exposed to oxidized fish oil, high-quality fish oil, or sunflower oil demonstrated that high-quality fish oil and oxidized fish oil affected participant blood lipid levels differently, with more favorable effects seen in the high-quality fish oil group(8).

Similarly, a trial that assigned women to an oxidized versus less-oxidized fish oil supplement group for 30 days found that ingestion of less-oxidized omega-3 supplements reduced circulating triglyceride and cholesterol levels compared to the oxidized supplement(9). Together, these studies suggest that oxidation levels may affect omega-3 supplement effectiveness and desired health benefits. But what about health risks?

Studies examining the safety of oxidized fish oil have yet to report alarming findings. For example, one study randomized 83 subjects to an oxidized fish oil (TOTOX = 45), unoxidized fish oil (TOTOX = 11), and sunflower oil (TOTOX = 11) supplementation for 3-7 weeks and found no significant changes in markers of lipid peroxidation (oxidative degradation of lipids), inflammation, or oxidative stress(10).

The authors of this study also reported that those in the oxidized fish oil group had no significant changes to other measures of health, like markers of atherosclerotic risk(10). Some investigators have even found beneficial effects associated with oxidized oil consumption, noting resolution of inflammation in the body(11).

While rancid fish oil supplements may not be good for you—in part because a bad experience with rancid fish oil might deter you from trying it again, thereby depriving you of all the potential benefits— GOED says the science does not support the claims that suggest rancid fish oil is toxic or harmful to health.

Even recent reports align with this, concluding that the oxidative state of consumer omega-3 products is not, at this time, a demonstrated hazard(7). Although, when looking at the entire body of evidence, even though oxidized fish oil may not cause harm, the effectiveness of oxidized products may vary based on the health outcome being measured.  

And as for concerns by some that fish oil rancidity is widespread, GOED disagrees. A 2015 study found that 92% of products in New Zealand exceeded one of the recommended oxidation limits(12). In 2017, GOED replicated that study and found that, of the tested products, 72%, 86%, and 77% complied with the voluntary industry-set limits on PV, p-AV, and TOTOX, respectively(13).

Gerard Bannenberg, GOED’s Director of Technical Compliance at GOED and primary investigator on the replicated New Zealand study, was quoted in The Guardian, a British newspaper, as saying “I don’t think people need to be concerned about oxidation. The level we’re talking about is very low. Even if the product is slightly oxidized, it’s very unlikely that it will harm our health.”

Ultimately, whether or not consuming oxidized omega-3 oils poses any health risks should continue to be investigated, and efficacy may vary based on desired outcome. Still, GOED notes that “even a very fishy-smelling product is not unsafe, just unpleasant. If the product smells bad and is beyond its expiration date, then it should be thrown away.”(14)

 

8 Tips for Fish Oil Freshness

Let’s be clear: no one should be taking rancid fish oil. Here are some things you can do to avoid rancid fish oil supplements:

1. Expiration date. Here in the U.S., dietary supplements are regulated by the Food & Drug Administration (FDA), among other agencies. FDA does not mandate that companies list expiration dates on dietary supplements. However, many companies choose to do so in an effort to be transparent. In the case of fish oil supplements, an expiration date is one way to protect you from consuming fish oil that could be rancid. Other companies may stamp the packaging with “best by” date or the manufacture date (call to find out what that means in terms of end use date). Fish oil will go rancid—it’s just a matter of time. Throw out the product when it reaches the expiration date.
2. Don’t stockpile. It’s hard to resist a sale; however, with fish oil products, you don’t want them hanging around for too long. And if you buy more than one large bottle at a time, certainly don’t open them both. Stocking up on a fish oil sale may mean you’ll end up with expired (or possibly rancid) product that you’ll need to throw away. So much for the sale. To avoid a stockpile, use your fish oil (or per label instructions) consistently and on a daily basis.
3. Sniff away. It’s okay to break open a fish oil soft gel or capsule to do a sniff test, just don’t consume that one. Throw that capsule away. And if it smells super fishy or you recoil from the smell, throw the whole bottle away. (If it’s still within the expiration date, call the company to see if they’ll offer you a refund, or at the very least, an explanation.) And by all means, if you’re using fish oil supplements in the liquid form (generally cod oil), it’s fair game to also take a whiff.
4. Rely on your other senses. If your fish oil supplement tastes bad, makes you gag when you swallow or gives you excessive burps, don’t use it. Just as there are plenty of fish in the sea, there are plenty of companies selling fish oil supplements. Excessive burping or bad taste may be a sign of rancidity or just a hint that this is not the brand for you. Try another brand. There are flavored fish oils that have been developed to cover unpleasant sensory reactions, but some say that these added flavors may also unwittingly mask the smell or taste of a rancid product.
5. Proper storage. You want to keep your fish oil supplements away from heat, light, and air (for the latter, make sure your bottle is tightly closed)(15). Store your package in a cool, dry place—not the bathroom cabinet and not next to the stove. Some people choose to keep their fish oil supplements in the refrigerator, especially if you’re consuming fish oil liquid. You know how your olive oil or oil-based salad dressing can get cloudy when you take it out from the refrigerator? The same may be true with your fish oil. Put it out on the countertop for about 5 or 10 minutes and let it return to its normal state. But if it doesn’t, call the company’s customer service and ask about it.
6. Packaging considerations. Look for products sold in bottles with opaque plastic, dark or UV-blocking glass, or in blister packs. These options help to limit the amount of light reaching the fish oil capsules or fish oil and help preserve the product for longer.
7. Company counts. Do your research because who you buy from matters. Check for company reputation and test out the customer service before you buy that brand by asking some questions of the company or of the retailer if you purchase from a trusted retailer. What do they have to say about how the product should smell? How will you know if the product is rancid? How should their product be stored? Do they comply with the GOED Voluntary Monograph? Do they use an independent testing company or a third-party certifier?
8. Keep doc in the loop. Be sure your doctor is involved in your decision to use (or not use) fish oil supplements in case you’re contraindicated (e.g., are you allergic to fish) or are taking medications that could interact with fish oil supplements. Your pharmacist can also advise about those potential interactions.

Keep your doctor in the loop about all of your supplement use. And remember, when it comes to concerns about fish oil supplements and rancidity, common sense and smart choices go a long way.

 

References:

1. Swanson D, Block R, Mousa SA. Omega-3 fatty acids EPA and DHA: health benefits throughout life. Adv Nutr. 2012;3(1):1-7. doi:10.3945/an.111.000893
2. Marcus JB. Lipids basics: fats and oils in foods and health—healthy lipid choices, roles and applications in nutrition, food science and the culinary arts. In: Culinary Nutrition: The Science and Practice of Healthy Cooking. 2013:231-277.
3. Ismail A, Bannenberg G, Rice HB, Schutt E, MacKay D. Oxidation in EPA- and DHA-rich oils: an overview. Lipid Technology.2016;28(3-4):55-59. doi.org/10.1002/lite.201600013
4. Global Organization for EPA and DHA Omega-3s (GOED). GOED Voluntary Monograph. Global Organization for EPA and DHA Omega-3s; 2023. https://goedomega3.com. Accessed January 15, 2026.
5. Hands JM, Anderson ML, Cooperman T, Frame LA. A Multi-Year Rancidity Analysis of 72 Marine and Microalgal Oil Omega-3 Supplements. J Diet Suppl. 2024;21(2):195-206. doi:10.1080/19390211.2023.2252064
6. Global Organization for EPA and DHA Omega-3s (GOED). GOED Guidance Documents. Global Organization for EPA and DHA Omega-3s; 2023. https://goedomega3.com. Accessed January 15, 2026.
7. Hands JM, Anderson ML, Cooperman T, Frame LA. A Multi-Year Rancidity Analysis of 72 Marine and Microalgal Oil Omega-3 Supplements. J Diet Suppl. 2024;21(2):195-206. doi:10.1080/19390211.2023.2252064
8. Rundblad A, Holven KB, Ottestad I, Myhrstad MC, Ulven SM. High-quality fish oil has a more favourable effect than oxidised fish oil on intermediate-density lipoprotein and LDL subclasses: a randomised controlled trial. Br J Nutr. 2017;117(9):1291-1298. doi:10.1017/S0007114517001167
9. García-Hernández VM, Gallar M, Sánchez-Soriano J, Micol V, Roche E, García-García E. Effect of omega-3 dietary supplements with different oxidation levels in the lipidic profile of women: a randomized controlled trial. Int J Food Sci Nutr. 2013;64(8):993-1000. doi:10.3109/09637486.2013.812619
10. Ottestad I, Vogt G, Retterstøl K, et al. Oxidised fish oil does not influence established markers of oxidative stress in healthy human subjects: a randomised controlled trial. Br J Nutr. 2012;108(2):315-326. doi:10.1017/S0007114511005484
11. Mishra A, Chaudhary A, Sethi S. Oxidized omega-3 fatty acids inhibit NF-kappaB activation via a PPARalpha-dependent pathway. Arterioscler Thromb Vasc Biol. 2004;24(9):1621-1627. doi:10.1161/01.ATV.0000137191.02577.86
12. Albert BB, Derraik JG, Cameron-Smith D, et al. Fish oil supplements in New Zealand are highly oxidised and do not meet label content of n-3 PUFA. Sci Rep. 2015;5:7928. Published 2015 Jan 21. doi:10.1038/srep07928
13. Bannenberg G, Mallon C, Edwards H, et al. Omega-3 Long-Chain Polyunsaturated Fatty Acid Content and Oxidation State of Fish Oil Supplements in New Zealand. Sci Rep. 2017;7(1):1488. Published 2017 May 3. doi:10.1038/s41598-017-01470-4
14. Global Organization for EPA and DHA Omega-3s (GOED). FAQs. AlwaysOmega3s.com. Published 2026. Accessed February 12, 2026. https://alwaysomega3s.com/more/faqs
15. Global Organization for EPA and DHA Omega-3s (GOED). Purchasing and taking care of omega-3 supplements. AlwaysOmega3s.com. Published 2026. Accessed January 15, 2026. https://alwaysomega3s.com/what/explore-good-sources/supplements/purchasing-and-taking-care-omega-3-supplements