Who Should Not Eat Oily Fish?

October 24, 2025 •

4 min read

According to the National Health Service (NHS), women who are pregnant or breastfeeding should limit their oily fish intake to no more than two portions a week. While often hailed for its omega-3 benefits, oily fish is not suitable for everyone and can pose serious risks for certain groups due to factors like mercury levels, blood-thinning properties, and allergies.

Quick Summary

Oily fish consumption is restricted for pregnant or breastfeeding women, young children, individuals on blood-thinning medication, and those with fish allergies due to specific health risks. High levels of mercury and other pollutants are primary concerns, along with the omega-3s' effect on blood clotting. This guide highlights who should limit or avoid it.

Key Points

Allergy Warning: Anyone with a fish allergy should avoid all oily fish and fish oil supplements, and be aware of cross-contamination risks.
Pregnancy and Breastfeeding: Pregnant and breastfeeding women, as well as young children, should limit oily fish intake to prevent overexposure to mercury and other pollutants.
Blood Thinners: People on anticoagulant or antiplatelet medication should consult a doctor before consuming oily fish or supplements, as omega-3s can increase bleeding risk.
Pre-Surgery Precautions: It is often advised to temporarily stop consuming oily fish and supplements before any surgery to avoid excessive bleeding.
High-Mercury Fish: Certain large, predatory fish like shark, swordfish, and marlin should be avoided by vulnerable groups due to high mercury levels.
Dosage Matters: While moderate intake is healthy for most, extremely high, chronic consumption of omega-3s might pose other health risks like immune suppression.

Oily fish and specific health conditions

For most healthy adults, eating a moderate amount of oily fish is part of a balanced diet, offering essential omega-3 fatty acids that support brain and heart health. However, for certain individuals and demographics, the risks associated with oily fish consumption can outweigh the benefits. Understanding these risks is crucial for making informed dietary choices. These risks range from allergic reactions to drug interactions and the accumulation of environmental contaminants.

Mercury contamination and vulnerable populations

Larger, predatory fish that live longer tend to have higher concentrations of methylmercury, a neurotoxin that accumulates in their bodies over time. This poses a particular threat to developing nervous systems, making it a major concern for certain groups.

Pregnant and breastfeeding women: Regulatory bodies advise a restricted intake of oily fish for this group to limit mercury exposure to the developing fetus or baby. Excessive mercury can impair a baby's developing nervous system. Some high-mercury fish, such as shark, swordfish, and marlin, should be avoided entirely during pregnancy and while trying to conceive. It's recommended to stick to smaller oily fish like sardines and mackerel, limiting intake to two portions a week.
Young children and infants: Similarly, due to their smaller size and rapid development, children are more susceptible to mercury's harmful effects. Guidelines recommend limiting oily fish intake for children under 16, and some high-mercury fish are best avoided altogether. Parents should ensure children consume a variety of fish to balance nutrients while minimizing pollutant exposure.

Bleeding risk and anticoagulant medication

Omega-3 fatty acids, the primary beneficial component of oily fish, have anti-inflammatory and anti-clotting properties that make blood platelets less sticky. While this is a benefit for many, it can be hazardous for people with bleeding disorders or those on specific medications.

Individuals on blood thinners: Patients taking anticoagulant and antiplatelet drugs like warfarin, heparin, or clopidogrel should be cautious. Combining high doses of omega-3s from supplements or excessive oily fish can amplify the blood-thinning effect, potentially increasing the risk of bruising and bleeding. Consultation with a healthcare provider is essential for anyone taking these medications to safely manage their intake.
Before surgery: Due to the blood-thinning effects, many doctors advise patients to temporarily discontinue oily fish and fish oil supplements before a surgical procedure to prevent complications from excessive bleeding.

Fish allergies

One of the most immediate and serious reasons to avoid oily fish is a confirmed fish allergy. Reactions can range from mild hives and digestive issues to a severe, life-threatening anaphylactic response.

Cross-contamination risk: Individuals with fish allergies must be vigilant, not only avoiding the fish itself but also being aware of potential cross-contamination in restaurants and kitchens. Even pure fish oil can trigger a reaction, and many supplements carry warnings for fish allergy sufferers.
Other considerations: While some fish allergy sufferers may tolerate specific types of fish, it is best to avoid all finned fish unless specifically advised by an allergist after testing. Shellfish allergies are separate, but cross-contamination can also occur in environments where both are prepared.

Other high-risk groups

Some research has indicated potential risks with very high, consistent consumption of oily fish.

Risk of diabetes: Some studies suggest that very high, frequent consumption of omega-3 fatty acids might be associated with a higher risk of developing type 2 diabetes, though this finding requires more research and isn't typically relevant for individuals consuming fish in moderation.
Immune suppression: High doses of fish oil supplements (far exceeding typical dietary intake) have shown a potential to suppress immune function by reducing the body's inflammatory responses. This is not a concern for normal dietary habits but should be noted for those considering large doses of supplements.

Comparison of risks for different population groups


Population Group	Primary Concern	Specific Risks	Recommended Action
Pregnant/Breastfeeding Women	Methylmercury and Pollutants	Potential harm to infant nervous system from mercury; accumulation of pollutants (e.g., PCBs).	Limit to 2 portions of low-mercury oily fish per week; avoid high-mercury fish.
Young Children	Methylmercury and Pollutants	Interference with nervous system development; higher susceptibility due to smaller body size.	Limit intake; avoid high-mercury species like shark, marlin, swordfish.
Individuals on Blood Thinners	Increased Bleeding Risk	Omega-3s can enhance anticoagulant effects, leading to excessive bruising or bleeding.	Consult a doctor before consuming oily fish or supplements; moderate intake.
Pre-Surgery Patients	Increased Bleeding Risk	Similar to those on blood thinners, risk of excessive bleeding during and after the procedure.	Temporarily discontinue oily fish and supplements as advised by a physician.
People with Fish Allergy	Allergic Reaction	Risk of mild to severe allergic reactions, including anaphylaxis.	Avoid all oily fish and be cautious of cross-contamination.
Individuals with Diabetes Risk	High Omega-3 Intake	Some studies link very high omega-3 consumption to a higher risk of type 2 diabetes.	This is not typically a risk at recommended intake levels; focus on moderation.

Conclusion

While oily fish is an excellent source of essential nutrients for most people, certain populations must approach its consumption with caution. Individuals with fish allergies should avoid it entirely, while pregnant or breastfeeding women and young children must adhere to consumption limits to avoid mercury and pollutant accumulation. Patients on blood-thinning medications should consult a doctor, as the omega-3s in oily fish can increase bleeding risk. Always consider your personal health profile and circumstances before adding or increasing oily fish in your diet. When in doubt, consulting a healthcare professional is the safest course of action.

Disclaimer: The information in this article is for informational purposes only and should not be taken as medical advice. Always consult with a qualified healthcare professional before making any dietary changes, especially if you have pre-existing medical conditions or are taking medication.

This article is based on information from various public health authorities and medical reports.

Frequently Asked Questions

You should not consume high amounts of oily fish or fish oil supplements without consulting a doctor if you are on blood-thinning medication. Omega-3s can increase the medication's effect, raising the risk of bruising and bleeding.

Pregnant women must limit oily fish intake because some species contain mercury and pollutants that can accumulate in the body and potentially harm the baby's developing nervous system.

Larger, predatory fish like shark, swordfish, and marlin have the highest mercury levels and should be avoided entirely by pregnant women and young children. Tuna should also be limited.

No, a fish allergy is an immune system response that can cause severe, life-threatening reactions. An intolerance is a less severe digestive issue. Anyone with a confirmed fish allergy should avoid all fish unless advised otherwise by a medical professional.

While generally safe, high doses of fish oil can slightly lower blood pressure. If you are already taking medication for high blood pressure, consult your doctor to ensure your blood pressure does not drop too low.

No, mercury is integrated into the fish's muscle tissue, so cooking will not remove or reduce its content. Focusing on low-mercury fish species is the best way to reduce exposure.

Women who are planning a pregnancy should follow the same guidelines as pregnant women, limiting their intake to no more than two portions of oily fish per week to prevent the accumulation of pollutants and mercury.