Fish oil benefits and risks: what omega-3 supplements can and can't do

Short answer

Fish oil supplements provide the omega-3 fats EPA and DHA. Their best-established effects are lowering high triglycerides (at prescription-level doses), lowering blood pressure slightly (about 2.6/1.8 mmHg at 2–3 g a day) (1) and lowering resting heart rate by about 2 beats a minute (2). A meta-analysis also found reduced anxiety symptoms, mainly at 2 g a day or more (3).

What fish oil does not do is prevent heart attacks, strokes or cancer in generally healthy people: the large VITAL trial of nearly 26,000 adults found no reduction in major cardiovascular events or cancer with a standard 1 g dose (4). A prescription form of purified EPA at 4 g a day did reduce events in people with high triglycerides already on statins (5), but that is a medicine, not a typical supplement.

The main risk to know about is atrial fibrillation: in a meta-analysis of seven trials with 81,210 people, omega-3 supplements were linked to a higher risk of this irregular heart rhythm, especially at doses above 1 g a day (6). At high doses fish oil can also have antiplatelet effects, so people on blood thinners should tell their doctor (7).

If you have been taking fish oil "for your heart" for years and now feel unsure, that is not your fault. The advice has changed as large trials reported. Eating oily fish twice a week remains a well-supported habit; supplements are useful for specific goals.

Note: this article is for general education and is not medical advice. Talk to your doctor before taking high-dose fish oil, especially if you take blood thinners, have a heart rhythm problem or are preparing for surgery. Call 911 for chest pain, fainting or a rapid, irregular pulse, and for black, tarry stools or vomiting blood.

What fish oil contains

Fish oil is rich in two long-chain omega-3 fatty acids:

• EPA (eicosapentaenoic acid), linked to effects on inflammation, triglycerides and mood;

• DHA (docosahexaenoic acid), a building block of the brain and eyes, important in pregnancy and early life.

The body can make small amounts of EPA and DHA from ALA (alpha-linolenic acid), the omega-3 in flaxseed, chia and walnuts, but conversion is limited (7). That is why plant omega-3s are not a direct substitute for fish oil.

Labels can be confusing. A "1,000 mg fish oil" capsule often contains only around 300 mg of EPA and DHA combined. When comparing products or studies, look at the EPA + DHA amount, not the total oil.

Fish oil benefits: the evidence at a glance

Fish oil and the heart: what changed

For years, fish oil was widely recommended for heart health. Two large trials shifted that picture:

• VITAL gave 25,871 healthy US adults 1 g of omega-3 a day or placebo for about five years. There was no significant reduction in major cardiovascular events or cancer overall (4).

• REDUCE-IT gave 8,179 people with raised triglycerides who were already on statins 4 g a day of a purified EPA medicine (icosapent ethyl) or placebo. Major cardiovascular events fell by about a quarter (5). Some experts have questioned whether the mineral oil placebo exaggerated the difference, and the result does not apply to ordinary fish oil capsules.

The practical message: fish oil is not a general heart-protection pill for healthy people. For people with high triglycerides and existing heart risk, prescription omega-3 may be an option your doctor discusses.

Fish oil and atrial fibrillation

Atrial fibrillation (AF) is an irregular heart rhythm that raises stroke risk. A 2021 meta-analysis of seven randomised trials with 81,210 people found that omega-3 supplements were associated with a higher risk of AF. The risk appeared greater in trials using more than 1 g a day (6).

The absolute increase in risk is small for most people, but it matters if you already have AF, have had it before, or have other risk factors. Signs of AF include an irregular, often fast pulse, palpitations, breathlessness and tiredness. If you notice an irregular pulse or palpitations while taking fish oil, contact your doctor (9); call 911 for chest pain, fainting or a rapid, irregular pulse (10).

Fish oil, heart rate and HRV

Omega-3s have measurable effects on the heart's electrical behaviour. In a meta-analysis of 51 randomised trials with about 3,000 people, omega-3 supplements lowered resting heart rate by about 2.2 beats a minute compared with placebo, with the effect coming mainly from DHA (2). Some studies also report small increases in heart rate variability (HRV), a marker of the balance in the nervous system that controls the heart.

These are small changes, within the range of normal day-to-day swings in resting heart rate, so a handful of readings cannot show whether fish oil changed yours.

Fish oil for anxiety and mood

A 2018 meta-analysis of 19 trials found that omega-3 supplements reduced anxiety symptoms, with the clearest effects at doses of at least 2 g a day and in people with clinical conditions rather than healthy volunteers (3). For depression, some trials show modest benefits, mostly with products higher in EPA than DHA, often as an addition to standard treatment. Fish oil is not a replacement for therapy or antidepressants.

Krill oil vs fish oil

Krill oil benefits are marketed heavily. Krill oil contains EPA and DHA bound to phospholipids, which some studies suggest are absorbed slightly better, and it contains the pigment astaxanthin. However:

• krill oil capsules usually contain less EPA and DHA than fish oil capsules;

• there is little evidence that krill oil produces better health outcomes than fish oil at the same EPA and DHA dose;

• krill oil is more expensive.

For most people, fish oil gives more omega-3 for the money. Krill oil may suit people who get fishy burps from fish oil, though enteric-coated fish oil is another option.

Cod liver oil benefits and cautions

Cod liver oil contains EPA and DHA plus vitamin A and vitamin D. That makes it different from regular fish oil:

• It can be a useful source of vitamin D in winter.

• But it also contains preformed vitamin A, which can build up to harmful levels if you take cod liver oil alongside a multivitamin or other vitamin A sources. Intakes of preformed vitamin A above the adult upper limit of 3,000 mcg a day can cause birth defects, so experts advise against high-dose vitamin A supplements in pregnancy (12).

If you take cod liver oil, add up the vitamin A and D from all your supplements, and avoid it in pregnancy unless your doctor or midwife agrees.

Flaxseed oil and other plant omega-3s

Flaxseed oil benefits are often presented as equivalent to fish oil. Flaxseed, chia seeds, hemp seeds and walnuts provide ALA, a plant omega-3. ALA is healthy in its own right, but the body converts only a small fraction of it into EPA, and even less into DHA (7). Flaxseed oil does not replace fish oil for the effects that depend on EPA and DHA.

For vegetarians and vegans, algae oil supplements provide EPA and DHA directly, from the same source fish get them from.

What about omega-6?

Omega-6 fats, found in vegetable oils such as sunflower, corn and soybean oil, nuts and seeds, are also essential. Most Western diets contain plenty, so omega-6 supplements are rarely needed. The idea that omega-6 is "inflammatory" and must be strictly limited is not well supported; replacing saturated fat with vegetable oils containing omega-6 is generally linked to better heart health. What most people lack is not omega-6, but EPA and DHA from fish.

How much fish oil per day

• General health: eating oily fish such as salmon, mackerel, sardines or herring twice a week provides roughly 250–500 mg of EPA and DHA a day on average, which is what most guidelines aim for.

• Blood pressure: studies used about 2–3 g of EPA and DHA a day (1).

• Anxiety: effects were clearer at 2 g a day or more (3).

• High triglycerides: around 4 g a day, as a prescription medicine under medical supervision.

Doses above about 1 g a day are linked to a higher AF risk (6), so higher doses should be agreed with a doctor, especially if you take a blood thinner. The FDA has concluded that supplements providing no more than 5 g a day of EPA and DHA are safe when used as recommended (7); that is a ceiling, not a target.

Best time to take fish oil: with a meal containing fat, which improves absorption and reduces fishy burps. Splitting a higher dose between two meals helps some people.

Fish oil side effects and safety

• Common: fishy aftertaste and burps, heartburn, nausea, loose stools.

• Bleeding: fish oil can have antiplatelet effects at high doses. Most research finds that 3–6 g a day does not significantly change how warfarin works, but prescription omega-3 labels advise INR monitoring with anticoagulants, so tell your doctor and ask about stopping before surgery (7).

• Atrial fibrillation: higher risk, especially above 1 g a day (6).

• Blood sugar: a European safety review found that doses up to about 5 g a day did not affect blood sugar control (7).

• Mercury: omega-3 supplements have not been found to contain mercury (7).

• Freshness: fish oil can oxidise; a strong rancid smell is a reason to discard it.

• Allergy: people with fish or shellfish allergy should check with a doctor before taking fish or krill oil.

Living with fish oil: the questions people ask next

Is fish oil good for your heart?

It depends on who you are. In generally healthy people, a standard 1 g dose did not prevent heart attacks or strokes in the large VITAL trial. In people with high triglycerides already taking statins, a prescription purified EPA medicine at 4 g a day reduced cardiovascular events. Fish oil also slightly lowers blood pressure and resting heart rate. Eating oily fish twice a week remains a well-supported heart-healthy habit.

Does fish oil cause atrial fibrillation?

A meta-analysis of seven trials with over 81,000 people found that omega-3 supplements were linked to a higher risk of atrial fibrillation, especially at doses above 1 g a day. The absolute risk increase is small for most people, but it matters if you have had AF or have other risk factors. If you notice an irregular pulse or palpitations, contact your doctor; call 911 for chest pain, fainting or a rapid, irregular pulse.

Should I take fish oil or eat fish?

Eating oily fish twice a week is the first choice for most people: it provides EPA and DHA along with protein, vitamin D, selenium and other nutrients, and it is linked to better heart health in observational studies. Supplements make sense if you do not eat fish, or for specific goals such as high triglycerides, where your doctor may recommend a prescription product.

How long does fish oil take to work?

Blood levels of EPA and DHA rise within weeks. In studies, effects on triglycerides were seen within weeks, blood pressure effects after about 10 weeks, and anxiety effects after several weeks to months. If you are taking fish oil for a specific goal, measure that goal over two to three months.

Is fish oil safe with blood thinners?

Most research finds that 3–6 g a day of fish oil does not significantly change how warfarin works, but fish oil can have antiplatelet effects at high doses, and prescription omega-3 labels advise checking INR in people on anticoagulants. Tell your doctor if you take fish oil with warfarin, a direct oral anticoagulant or an antiplatelet medicine, ask whether to stop it before surgery or dental procedures, and do not change your blood thinner on your own.

Is fish oil safe in pregnancy?

DHA is important for a baby's brain and eye development, and eating low-mercury fish such as salmon and sardines is encouraged in pregnancy. DHA supplements have been shown to reduce the risk of preterm birth, and many prenatal vitamins contain DHA; ask your doctor or midwife before adding a separate supplement. Avoid cod liver oil in pregnancy unless advised, because of its vitamin A content.

Before you blame yourself

If you have taken fish oil for years and now hear it may not prevent heart attacks, it is easy to feel misled or foolish. You followed advice that seemed sensible at the time, before large trials reported. That is not a mistake on your part.

The useful question now is simple: what do you want fish oil to do for you, and does the evidence support that goal at the dose you take?

When to see a doctor

Talk to your doctor before taking more than 1 g of EPA and DHA a day, if you take blood thinners, have atrial fibrillation or another heart rhythm problem, have high triglycerides, or are planning surgery.

Call 911 for chest pain, fainting or a rapid, irregular pulse, and for black, tarry stools or vomiting blood. These are MedlinePlus emergency signs for fast heart rhythms and gastrointestinal bleeding (10, 11).

How to bring this up with your doctor

What to ask for, by name. "Given my triglycerides and heart risk, would prescription omega-3 make sense for me?" If you take blood thinners: "Is my fish oil dose safe with my medicine?"

What to bring. Your recent cholesterol and triglyceride results, the exact product and its EPA and DHA content per capsule, and any heart rhythm symptoms.

What to say, out loud. "I take [N] mg of EPA and DHA a day for [reason]. Is that useful for me, and is it safe with my heart and medicines?"

If you're told fish oil is useless. Ask: "For my situation, is there any goal where it would help, or should I focus on diet instead?"

How long until you notice a difference? Most supplements take weeks, not days. See how long supplements take to work, and how to tell if one is working.

Start understanding your body

Learn what affects your energy, stress, sleep, and daily state. Get the app.

Explore your data

How Welltory helps — and what it cannot do

The limits first. Welltory is a general wellness product, not a medical device. It cannot measure omega-3 levels, triglycerides or detect atrial fibrillation, and it does not diagnose any condition. An irregular pulse needs a medical assessment.

What it can do is help you keep a consistent record of resting heart rate and HRV before and after a change, to look at with your doctor. It cannot tell you whether fish oil caused a change: a shift of a couple of beats can come from sleep, stress, training or illness.

Resting heart rate. On iOS, Battery starts each morning from your resting heart rate on waking and your sleep (Today screen). A stable morning routine for several weeks before and after starting makes that record easier to read.

HRV, measured consistently. If you take morning HRV measurements, take them at the same time after waking, before coffee, in the same position (accurate HRV measurements). Compare monthly averages, not single days.

Tag the change. On iOS, when Welltory marks a stress or rest episode and asks "What happened?", tap, type or talk: "started fish oil 2 g", "palpitations", "fishy burps". Over weeks, My Patterns shows which tags tend to come with stress and recovery; some sections need a paid plan.

A record for your doctor. From the web app you can export a CSV (Dashboard → choose a chart → Export) (export guide). Patterns are associations to discuss with your doctor, not proof that fish oil caused a change.

How we made it

We based this article on the NIH Office of Dietary Supplements omega-3 fact sheet, the VITAL and REDUCE-IT trials, the DREAM trial of omega-3 for dry eye, meta-analyses of omega-3 and blood pressure (2022), resting heart rate (2018), anxiety (2018) and atrial fibrillation (2021), and MedlinePlus pages on atrial fibrillation, fast heart rhythms and gastrointestinal bleeding for the warning signs. Search questions came from what people in the United States most often ask about fish oil and other omega-3 supplements. For the product section, we checked every function against the Welltory Help Center on October 21, 2026.

Made with AI tools, then edited and fact-checked by the Welltory team. See our Editorial & AI policy.

Jane Smorodnikova, co-founder of Welltory, built the methodology behind how we read physiological data.

Written by Tatsiana Yashyna.

Discounts for blog readers: up to 36% off

See what affects your energy, stress, sleep, and daily state with Welltory

Explore your data