Omega-3 fish oil is a dietary supplement extracted from fatty fish like salmon, mackerel, and anchovies. It delivers two key fatty acids your body needs but can’t produce on its own: EPA and DHA. These two compounds play different roles in your body, and understanding what fish oil actually does (and doesn’t do) can help you decide whether it belongs in your routine.
What EPA and DHA Do in Your Body
EPA and DHA are both polyunsaturated fats, but they serve distinct functions. EPA is primarily involved in reducing inflammation throughout the body, which is why it’s linked to cardiovascular and joint health. DHA is a structural component of your brain and retinas, making up a significant portion of the fat in brain tissue. Together, they influence everything from blood vessel function to cell membrane flexibility.
Your body can technically convert a plant-based omega-3 called ALA (found in flaxseed and walnuts) into EPA and DHA, but the conversion rate is extremely low. That’s why fish oil supplements and fatty fish remain the most reliable dietary sources of these two fats.
Effects on Heart Health and Triglycerides
The strongest evidence for fish oil involves triglycerides, a type of fat in your blood that raises cardiovascular risk when elevated. At prescription-strength doses of 4 grams per day, EPA and DHA reduce triglycerides by roughly 20% to 30% in people with elevated levels. For those with very high triglycerides, reductions of 30% or more are typical, and one study in people with very low baseline omega-3 levels reported a 48% drop. Even when added on top of statin therapy, prescription omega-3s still lowered triglycerides by about 21%.
That said, the American Heart Association does not recommend omega-3 supplements for people who don’t already have a high cardiovascular risk. For those with existing coronary heart disease, the AHA suggests about 1 gram per day of combined EPA and DHA, preferably from eating oily fish rather than capsules. The triglyceride-lowering doses of 4 grams daily are prescription products, not the same as picking up a bottle at the drugstore.
Effects on Mood and Depression
A large meta-analysis covering 67 randomized trials found that each 1 gram per day of omega-3 supplementation significantly improved depressive symptoms in both people with and without depression. The benefit was larger for people who already had a depression diagnosis. In that group, dose-response analysis revealed a U-shaped curve: the greatest improvement occurred at around 1.5 grams per day, meaning more wasn’t necessarily better.
The same analysis estimated that omega-3 supplementation increased depression remission by 19 additional people per 100 patients treated, though researchers rated this as low-certainty evidence. The effect sizes were generally small to moderate. Fish oil isn’t a replacement for established depression treatments, but the data suggests it can be a meaningful add-on for some people.
Fish Oil vs. Algal Oil
If you’re vegetarian, vegan, or concerned about ocean sustainability, algal oil is the main alternative. It’s derived from the same microalgae that fish eat, cutting out the middleman. Algal oil typically contains about 50% more DHA per serving than standard fish oil, but it contains little to no EPA depending on the formulation. A typical fish oil serving provides around 180 mg of EPA and 120 mg of DHA, while algal oil delivers 100 to 300 mg of DHA with variable EPA content.
If you’re supplementing specifically for heart health or inflammation, where EPA plays a central role, standard fish oil or an algal product that includes EPA is the better fit. For general brain health and DHA intake, algal oil works well.
How the Form Affects Absorption
Fish oil supplements come in two main chemical forms: triglyceride form (closer to how fat exists naturally in fish) and ethyl ester form (a cheaper, more concentrated synthetic version). The difference in absorption can be significant.
In one human study, EPA absorbed at 68% and DHA at 57% from the triglyceride form, compared to just 20% and 21% from ethyl esters. Pancreatic enzymes break down the ethyl ester form 10 to 50 times more slowly than the triglyceride form. Taking ethyl ester fish oil with a high-fat meal (around 40 grams of fat) improved absorption to about 60%, which was still below what the triglyceride form achieved without a fatty meal.
Some studies using very large doses found no meaningful difference between the two forms, and a few researchers have reported similar plasma levels over time. But the weight of evidence favors triglyceride-form fish oil for better absorption at typical supplement doses. If your bottle doesn’t specify, it’s likely ethyl ester, since that’s the cheaper default. Look for “triglyceride form” or “rTG” on the label if absorption matters to you.
How Much to Take
There’s no official recommended daily intake for EPA and DHA from major regulatory bodies. The FDA caps supplement labels at 2 grams per day of combined EPA and DHA. The AHA’s specific recommendations depend on your situation: about 1 gram daily for people with existing heart disease, and 4 grams daily (prescription only) for managing high triglycerides. For the general population without elevated cardiovascular risk, the AHA’s position is simply to eat fish twice a week rather than supplement.
For mood support, the research points to 1 to 1.5 grams per day of total omega-3s as the sweet spot based on the dose-response data from clinical trials.
Quality and Freshness Matter
Fish oil is prone to oxidation, which produces rancid byproducts that can reduce effectiveness and cause unpleasant fishy burps. The industry uses three main markers to measure freshness. Peroxide value (PV) captures early-stage oxidation, with quality standards recommending no more than 5 mEq/kg. Para-anisidine value (p-AV) measures secondary oxidation products, with a recommended ceiling of 20. TOTOX combines both into an overall oxidation score, capped at 26 by the Global Organization for EPA and DHA Omega-3s. Many premium brands aim well below these limits.
Third-party testing programs like IFOS (International Fish Oil Standards) independently verify oxidation levels, potency, and contaminant purity. If you’re comparing brands, an IFOS-certified product or one that publishes its TOTOX score gives you more confidence that what’s in the capsule is fresh and accurately labeled. Storing fish oil in the refrigerator and choosing dark or opaque bottles also slows oxidation after purchase.
Common Side Effects
At typical supplement doses (under 2 grams of EPA and DHA daily), fish oil is well tolerated by most people. The most common complaints are fishy aftertaste, burping, and mild digestive discomfort. Taking capsules with food, choosing enteric-coated versions, or freezing capsules before swallowing can reduce these issues.
At higher doses, fish oil has a mild blood-thinning effect. While clinical evidence hasn’t established a firm threshold where bleeding risk becomes dangerous, people taking blood-thinning medications should be aware of the potential interaction, especially at doses above 2 grams per day. The FDA’s 2-gram label cap for supplements exists partly for this reason.