What is the problem with fish oil?

January 5, 2026 •

4 min read

While often promoted for its heart and brain benefits, up to half of fish oil supplements tested have shown signs of rancidity by the time they reach consumers. This surprising statistic highlights a serious quality control issue and raises questions about whether these popular supplements deliver on their promises.

Quick Summary

Fish oil supplements face problems with rancidity, inconsistent quality due to minimal regulation, and potential side effects, especially with high dosages. Sustainable sourcing is also a growing concern.

Key Points

Rancidity is Common: Many over-the-counter fish oil supplements are oxidized and rancid, which can make them less effective and potentially harmful.
Limited FDA Regulation: Unlike prescription omega-3s, dietary fish oil supplements are not strictly regulated by the FDA, leading to inconsistent quality, purity, and potency.
High-Dose Side Effects: Excessive intake can increase the risk of bleeding, especially when combined with blood-thinning medications.
Common Digestive Issues: Minor side effects include a fishy aftertaste, burping, indigestion, nausea, and diarrhea.
Sustainability Concerns: The high demand for fish oil contributes to overfishing of forage fish and other environmental problems.
Whole Fish is Superior: Eating whole fish provides a broader, more bioavailable nutrient profile and synergistic benefits that supplements lack.
Look for Certifications: Consumers should prioritize products with third-party certifications (e.g., IFOS, USP) to ensure quality and freedom from contaminants.

The Pervasive Issue of Rancidity

One of the most concerning problems with fish oil supplements is their susceptibility to oxidation, which causes the oil to go rancid. Omega-3 fatty acids are highly unsaturated and fragile, making them vulnerable to degradation from exposure to heat, light, and oxygen. When the oil oxidizes, its chemical structure changes, and it loses its beneficial properties. A 2023 study found that a significant portion of fish oil products on the market were rancid, often with the fishy smell and taste masked by added flavorings. Consuming rancid oil is not only unpleasant but may also be less effective and potentially harmful, contributing to cellular inflammation rather than fighting it.

Recognizing Rancid Supplements

For consumers, detecting rancid fish oil is crucial. High-quality, fresh fish oil should not have a strong, fishy odor or leave an unpleasant fishy aftertaste. If you experience fishy burps or if the liquid from a broken capsule smells strong, the oil is likely rancid. Proper storage in a cool, dark place and checking for freshness indicators like TOTOX values are important steps.

A Lack of Regulatory Oversight

In many countries, including the United States, dietary supplements are not regulated with the same rigor as pharmaceuticals. The Food and Drug Administration (FDA) treats over-the-counter fish oil supplements as a food category, which means manufacturers are responsible for ensuring their product's safety and effectiveness. This lax oversight creates significant inconsistencies in potency, purity, and quality among different brands. Products may contain less EPA and DHA than advertised, or include undisclosed filler oils. To combat this, reputable manufacturers often seek voluntary third-party testing and certification from organizations like the International Fish Oil Standards (IFOS) or U.S. Pharmacopeia (USP). These seals help verify the product’s purity, potency, and freshness. Prescription-grade omega-3 products, which are FDA-approved, are held to much stricter standards.

Potential Health Risks and Side Effects

While generally safe for most people at recommended doses, fish oil supplements can cause a range of side effects, particularly at higher intakes.

Gastrointestinal Issues: Common, minor side effects include indigestion, heartburn, nausea, diarrhea, and bloating. These can sometimes be reduced by taking the supplement with food or by splitting the daily dose.
Bleeding Risks: Due to its blood-thinning effect, high doses of fish oil can increase the risk of bleeding. This is a serious concern for individuals on anticoagulant medications (e.g., warfarin) or those about to undergo surgery.
Cardiovascular Risks: Some studies have shown that very high doses of fish oil may slightly increase the risk of atrial fibrillation (A-fib), an irregular heartbeat, especially in people with pre-existing cardiovascular conditions.
Vitamin A Toxicity: Certain fish oils, like cod liver oil, contain high levels of vitamin A. While beneficial in moderation, excessive intake can lead to vitamin A toxicity, causing symptoms like dizziness and potentially liver damage over time.

Environmental and Sustainability Problems

The rising global demand for fish oil supplements has put significant pressure on marine ecosystems. The primary source of fish oil comes from forage fish like anchovies, sardines, and mackerel. Overfishing these species can disrupt the marine food web, affecting larger predators that rely on them for food. This has led to concerns about overfishing and the broader environmental footprint of the industry, which also contributes to greenhouse gas emissions through fishing and processing operations. While some companies are committed to sustainable sourcing, consumers must actively seek out products with certifications from organizations like the Marine Stewardship Council (MSC) to support ethical practices. Alternatives like omega-3s derived from microalgae offer a more sustainable option that bypasses the marine food chain entirely.

Fish Oil vs. Whole Fish: A Nutrient Breakdown


Feature	Whole Fish	Fish Oil Supplement
Nutrient Profile	Provides a broader spectrum of nutrients including high-quality protein, selenium, iodine, and vitamins A, D, and B12.	Concentrated source of only EPA and DHA; lacks the full complement of vitamins and minerals found in fish.
Bioavailability/Absorption	Omega-3s are naturally integrated into the fish tissue matrix, which improves their absorption by the body.	Absorption depends on the form (triglyceride vs. ethyl ester) and meal context. Triglyceride form is generally more bioavailable.
Contaminant Risk (Mercury)	Risk depends on the species; larger, predatory fish higher up the food chain have higher mercury levels. Smaller fish have less.	High-quality supplements are molecularly distilled and purified, removing heavy metals and contaminants.
Synergistic Benefits	Offers the combined benefits of a range of nutrients, which may work together for a greater overall effect.	Isolated compounds; does not offer the same synergistic benefits as a whole food source.

The Final Verdict: Navigating the Fish Oil Dilemma

The problems with fish oil are not insignificant, but they primarily revolve around the quality and quantity of the supplement, not the underlying benefit of omega-3s themselves. For most healthy individuals, prioritizing a balanced diet that includes two servings of fatty fish per week is the most effective and sustainable strategy. This approach offers a broader range of nutrients and superior absorption while minimizing the risks associated with supplementation.

For those who cannot or do not eat fish, a high-quality supplement can be a viable alternative to fill the nutritional gap. However, it is paramount to choose a reputable brand that provides third-party certification (IFOS, USP) to guarantee freshness, purity, and potency. You should also consult a healthcare provider before taking high doses of fish oil, especially if you have pre-existing health conditions or take other medications.

In conclusion, while the allure of an omega-3 boost in a convenient capsule is strong, a thoughtful and informed approach is necessary. Understanding the potential for rancidity, inconsistent regulation, and side effects empowers consumers to make safer, healthier choices that benefit both their well-being and the planet. Ultimately, fresh, whole food sources remain the gold standard, with supplements best reserved for specific circumstances and guided by medical advice.

Frequently Asked Questions

A strong, fishy taste or smell can be a sign that the fish oil has become oxidized and gone rancid. Fresh, high-quality fish oil should have a very mild odor, if any.

Yes, taking high doses of fish oil can have a blood-thinning effect and increase the risk of bleeding. This is especially a concern for individuals on anticoagulant medications.

In many regions, including the U.S., the FDA does not strictly regulate over-the-counter supplements for potency and purity in the same way as medications. This highlights the importance of choosing third-party certified products.

High-quality fish oil supplements are typically purified to remove heavy metals like mercury. The risk of mercury contamination is generally associated with eating large predatory fish, not refined supplements.

Common side effects include indigestion, heartburn, nausea, diarrhea, and a fishy taste. These can often be mitigated by taking supplements with food or opting for high-quality, fresh oil.

For most people, eating whole fish is superior because it provides a more complete nutritional package, including vitamins and minerals not found in supplements. Nutrients from whole foods are also often better absorbed.

Look for products with third-party certifications like IFOS or USP, which verify freshness, potency, and purity. Choose brands that source small, cold-water fish and use opaque packaging to prevent oxidation.

The high demand for fish oil contributes to overfishing of forage fish, which are vital for marine ecosystems. It also raises concerns about unsustainable harvesting and processing-related pollution.