Fish oil supplements, rich in omega-3 fatty acids EPA and DHA, have attracted attention for their potential role in cardiovascular health, including blood pressure management. Research suggests that fish oil may produce modest reductions in blood pressure, particularly in individuals with existing hypertension. Whilst the effect is typically small—around 2–4 mmHg for systolic pressure—even modest reductions can contribute to overall cardiovascular risk reduction. Fish oil should not replace prescribed antihypertensive medications but may serve as an adjunctive measure alongside lifestyle modifications. This article examines the evidence, mechanisms, dosing considerations, and safety profile of fish oil for blood pressure management.

How Fish Oil May Affect Blood Pressure

Fish oil supplements contain omega-3 polyunsaturated fatty acids, primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are thought to influence cardiovascular health through several biological mechanisms. These fatty acids may help reduce blood pressure through their effects on vascular function, inflammation, and potentially the autonomic nervous system.

Proposed mechanisms of action include the promotion of nitric oxide production in the endothelium (the inner lining of blood vessels), which causes vasodilation and improved arterial compliance. Omega-3 fatty acids may also reduce the production of vasoconstrictor substances such as thromboxane A2 and endothelin-1, whilst modulating inflammatory pathways that contribute to arterial stiffness. Some research suggests fish oil might influence the renin-angiotensin-aldosterone system, a key regulator of blood pressure, though this mechanism remains hypothetical and requires further investigation.

The blood pressure-lowering effect appears to be dose-dependent, with research suggesting an optimal range of approximately 2-3 grams of combined EPA and DHA daily rather than a linear dose-response relationship. Research indicates that the effect is more noticeable in individuals with existing hypertension or elevated blood pressure readings, rather than in those with normal blood pressure. The magnitude of reduction is typically modest—often in the region of 2–4 mmHg for systolic pressure and 1–2 mmHg for diastolic pressure—but even small reductions can contribute to cardiovascular risk reduction at a population level.

It is important to note that fish oil should not be considered a replacement for established antihypertensive medications. Rather, it may serve as an adjunctive measure within a comprehensive approach to blood pressure management that includes dietary modification, physical activity, weight management, and prescribed pharmacotherapy where indicated.

Evidence for Fish Oil in Managing High Blood Pressure

The evidence base for fish oil in blood pressure management comes primarily from systematic reviews and meta-analyses of randomised controlled trials. A substantial body of research has examined the relationship between omega-3 supplementation and blood pressure outcomes, with generally consistent findings supporting a modest antihypertensive effect.

Key findings from clinical trials indicate that fish oil supplementation can produce small but statistically significant reductions in both systolic and diastolic blood pressure. A 2022 meta-analysis published in the Journal of the American Heart Association found that omega-3 supplementation was associated with reductions of approximately 2 mmHg in systolic blood pressure and 1.5 mmHg in diastolic blood pressure across diverse populations. The effect was more pronounced in individuals with existing hypertension, those with elevated triglycerides, and older adults.

NICE guidance (NG136 on hypertension in adults and NG238 on cardiovascular disease risk assessment and reduction) acknowledges the potential cardiovascular benefits of omega-3 fatty acids but does not currently recommend fish oil supplements as a specific intervention for hypertension management. The emphasis remains on dietary sources of omega-3s—particularly oily fish. The NHS advises adults to consume at least two portions of fish per week, including at least one portion (approximately 140g) of oily fish such as salmon, mackerel, sardines, and herring.

Limitations of the evidence include variability in dosing regimens, differences in EPA and DHA ratios across studies, and heterogeneity in study populations. Whilst the blood pressure-lowering effect is reproducible, the clinical significance for individual patients remains a matter of professional judgement. Fish oil should be viewed as one component of a heart-healthy lifestyle rather than a standalone treatment for hypertension. Patients with diagnosed hypertension should continue their prescribed medications and discuss any supplementation with their GP or practice nurse.

Recommended Dosage and Types of Fish Oil

Fish oil supplements are available in various formulations, including standard fish oil capsules, concentrated omega-3 preparations, and prescription-grade products. The optimal dosage for blood pressure management has not been definitively established, but research suggests that doses in the range of 2–3 grams of combined EPA and DHA daily are associated with more consistent blood pressure reductions. Doses at or above 2 grams daily should be discussed with a healthcare professional, particularly if you have existing medical conditions.

Types of fish oil supplements include:

• Standard fish oil capsules: Typically contain 30% omega-3 fatty acids (approximately 300mg EPA and DHA per 1g capsule)

• Concentrated omega-3 preparations: Contain higher percentages of EPA and DHA, often 60–90%, allowing for smaller capsule sizes

• Prescription omega-3 products: Highly purified preparations licensed in the UK as medicines for specific indications such as hypertriglyceridaemia, not specifically for blood pressure control

• Krill oil: Contains omega-3s in phospholipid form, though evidence for superior efficacy in blood pressure reduction is limited

• Algal oil: A vegetarian source primarily of DHA, suitable for those avoiding fish-derived products, though evidence for blood pressure effects may differ from combined EPA/DHA products

When selecting a fish oil supplement, look for products that clearly state the EPA and DHA content per serving, rather than just total fish oil content. Quality assurance is important—reputable manufacturers should provide evidence of purity testing for contaminants such as heavy metals, polychlorinated biphenyls (PCBs), and dioxins.

Practical considerations include taking fish oil with meals to improve absorption and reduce gastrointestinal side effects. Some individuals find that refrigerating capsules or choosing enteric-coated formulations helps minimise fishy aftertaste or reflux. For those unable to tolerate capsules, liquid fish oil preparations are available, though these may have a more pronounced taste.

It is advisable to discuss supplementation with a healthcare professional before starting fish oil, particularly for individuals taking anticoagulant or antiplatelet medications, those with bleeding disorders, or anyone scheduled for surgery. Pregnant women should avoid fish liver oil supplements due to their vitamin A content. Your GP can help determine an appropriate dose based on your individual circumstances and existing medication regimen.

Safety Considerations and Potential Side Effects

Fish oil supplements are generally well tolerated, but like all supplements and medications, they can cause side effects and may interact with other treatments. Understanding these potential issues is essential for safe and effective use.

Common side effects include:

• Gastrointestinal symptoms such as nausea, diarrhoea, or indigestion

• Fishy aftertaste or reflux

• Mild headache

• Unpleasant body odour or breath

These effects are usually mild and often resolve with continued use or by taking the supplement with food. Enteric-coated preparations may reduce gastrointestinal symptoms.

Important safety considerations relate primarily to fish oil's effects on blood clotting. Omega-3 fatty acids have mild antiplatelet properties, which theoretically could increase bleeding risk, particularly at higher doses (above 3 grams daily). Whilst clinically significant bleeding is rare, caution is warranted in individuals taking anticoagulants (such as warfarin, apixaban, or rivaroxaban) or antiplatelet agents (such as aspirin, clopidogrel, or ticagrelor). If you are taking these medications, consult your GP or anticoagulation clinic before starting fish oil supplementation.

Higher doses of omega-3 supplements have been associated with a small increased risk of atrial fibrillation (irregular heartbeat) in clinical trials. Seek medical advice if you experience palpitations or irregular heartbeat while taking fish oil supplements. Some omega-3 preparations may also slightly increase LDL-cholesterol levels in some individuals.

If you are scheduled for surgery, inform your surgical team about all supplements you are taking. They may advise you to discontinue fish oil supplements before the procedure based on your individual circumstances.

Allergic reactions are possible in individuals with fish or shellfish allergies, though reactions to highly purified fish oil are uncommon. Those with known allergies should consider algal oil as an alternative source of omega-3s.

Pregnancy and breastfeeding: Pregnant women should avoid fish liver oil supplements due to their vitamin A content, which could be harmful during pregnancy. Follow NHS guidance on oily fish consumption during pregnancy and discuss any supplementation with your midwife or GP.

When to seek medical advice: Contact your GP if you experience unusual bruising, prolonged bleeding from minor cuts, blood in urine or stools, palpitations, or any signs of allergic reaction such as rash, swelling, or difficulty breathing. Seek urgent medical attention if your blood pressure is very high (≥180/120 mmHg) or if you experience symptoms such as severe headache, chest pain, difficulty breathing, vision changes, or neurological symptoms.

Suspected adverse reactions to fish oil supplements can be reported through the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

Frequently Asked Questions