What happens if you take expired fish oil supplements?

November 17, 2025 •

3 min read

According to research, many fish oil supplements may already be rancid by the time they reach store shelves. This concerning fact raises an important question: what happens if you take expired fish oil supplements, which are even more prone to rancidity?

Quick Summary

Expired fish oil is highly susceptible to oxidation, which causes it to go rancid and lose its beneficial omega-3 potency. Consuming it may lead to unpleasant digestive side effects, increased inflammation, and potential cardiovascular risks over time.

Key Points

Rancidity is Inevitable: Fish oil's polyunsaturated omega-3s are highly prone to oxidation, a process that makes the oil rancid over time.
Health Benefits Are Lost: Expired fish oil loses its therapeutic potency, meaning you will not receive the intended anti-inflammatory and cardiovascular benefits.
Digestive Distress is Common: The most immediate effect of consuming rancid fish oil is unpleasant fishy burps, nausea, and other stomach upset.
Potential for Increased Inflammation: Longer-term consumption of oxidized fats may increase cellular damage and inflammatory markers in the body.
Trust Your Senses to Check Freshness: A strong, fishy smell, a bitter taste, or a cloudy appearance are definitive signs that your fish oil is rancid and should be thrown out.
Prioritize Proper Storage: Storing fish oil in a cool, dark place and ensuring the bottle is tightly sealed can help slow down the oxidation process.
Always Adhere to Expiration Dates: The best practice is to simply discard any fish oil supplements past their expiration date to avoid any risk.

The Science of Rancidity and Oxidation

Fish oil contains omega-3 fatty acids like EPA and DHA, which are beneficial but highly susceptible to oxidation when exposed to heat, light, and oxygen. This process breaks down the fatty acids into harmful compounds like lipid peroxides and aldehydes, resulting in rancid oil. The expiration date indicates the manufacturer's guarantee of potency and freshness, and consuming fish oil past this date increases the risk of it being rancid.

Health Consequences of Consuming Oxidized Fish Oil

Consuming rancid fish oil not only means you miss out on the health benefits but also introduces potentially harmful compounds into your body. Effects can range from immediate discomfort to possible long-term issues.

Immediate and Unpleasant Side Effects

Rancid oil is known to cause digestive distress.

Fishy Burps: A common sign of oxidation is a strong, fishy aftertaste and burping.
Nausea and Indigestion: Degraded compounds can upset the stomach, causing bloating and discomfort.
Diarrhea: In some instances, consuming rancid fish oil may lead to loose stools.

Potential Long-Term Health Risks

While immediate effects are usually temporary, regular consumption of oxidized fats is more concerning. Some studies indicate potential negative long-term health consequences.

Increased Inflammation: Oxidized fats can contribute to cellular damage and inflammation, counteracting the anti-inflammatory effects of fresh fish oil.
Cardiovascular Risks: Oxidized fish oil has been linked to potential increases in LDL cholesterol and negative effects on lipoproteins, potentially canceling out the heart benefits of fresh omega-3s.
Cellular Damage: Oxidized fats can generate free radicals, leading to cellular damage and oxidative stress over time.

How to Identify Expired or Rancid Fish Oil

It's easy to avoid consuming bad fish oil by using your senses.

The Smell Test: A reliable indicator. Fresh oil has a mild, oceanic smell, while a strong, pungent, or overly "fishy" smell signifies rancidity. Flavored products can hide this.
The Taste Test: A sour, bitter, or unpleasant taste is another sign. Discard the oil if it tastes bad.
Visual Inspection: Look for changes like cloudiness, darker color, or different consistency.
Expiration Date: Always check the date on the bottle as the simplest safety measure.

What to Do With Expired Fish Oil

Do not consume expired or rancid fish oil. Proper disposal is necessary.

Throw it in the trash: Small amounts can be put in the garbage. For liquid oil, mix with absorbent material like kitty litter before sealing in a bag and disposing.
Look for municipal waste options: Check local services for potential take-back programs for expired supplements.
Do not flush or pour down the drain: This can cause clogs and harm the environment.

Comparison of Fresh vs. Expired (Rancid) Fish Oil


Feature	Fresh Fish Oil	Expired (Rancid) Fish Oil
Appearance	Clear, amber, or golden liquid	Cloudy, darker, or discolored liquid
Smell	Mild, oceanic, or neutral	Strong, pungent, or overly "fishy" odor
Taste	Neutral or slightly fishy	Sour, bitter, or unpleasant
Potency	High concentration of EPA and DHA	Significant loss of active omega-3 content
Oxidative Stress	Low to neutral effect	Increased free radicals and oxidative stress
Digestive Impact	Generally well-tolerated, minimal side effects	Increased risk of burping, nausea, and indigestion
Health Effects	Fights inflammation, supports heart health	May increase inflammation and cardiovascular risk

Conclusion

While a single dose of expired fish oil might only cause temporary digestive issues, consistent consumption of rancid supplements is a concern due to the oxidation of omega-3s. This reduces the benefits and introduces harmful compounds that can lead to inflammation and oxidative stress. Always follow the expiration date. If unsure about freshness, use the smell and taste tests. Prioritizing freshness and proper storage ensures you get the full benefits of fish oil. For supplement disposal guidance, refer to the U.S. Food and Drug Administration.

Frequently Asked Questions

While expired fish oil is not typically considered acutely toxic in small doses, its oxidation can produce harmful compounds. Consuming it won't provide the intended health benefits and may cause digestive upset or, with long-term use, contribute to inflammation and other health risks.

The most common side effects include an unpleasant fishy aftertaste, excessively fishy burps, nausea, and indigestion. Some people may also experience loose stools or a sour taste in their mouth.

To check for rancidity, break open a capsule and smell the oil. A strong, pungent, or overly fishy odor indicates that it has gone bad. Other signs include a bitter taste, a change in color, or a cloudy appearance.

Refrigeration can significantly slow down the oxidation process by protecting the oil from heat. Storing fish oil in a cool, dark place like the refrigerator is an excellent way to prolong its freshness, but it will not prevent it from eventually expiring.

If you have accidentally taken a single expired fish oil pill, you are likely to be fine. The worst you might experience is some mild digestive upset or fishy burps. The primary risks are associated with long-term consumption of oxidized oil.

It is not a typical food poisoning risk in the way that contaminated meat might be. Instead of bacterial contamination, the issue with expired fish oil is chemical degradation through oxidation. The result is rancidity, which causes stomach upset rather than a bacterial illness.

For capsules, you can simply toss them in the household trash. For liquid oil, mix it with an unappealing absorbent material like kitty litter or coffee grounds, place it in a sealed container, and then throw it away. Do not flush it down the toilet or pour it down the drain.