Fish oil lowers blood fats called triglycerides, reduces inflammation throughout the body, and supports brain, eye, and joint health. Its active ingredients are two omega-3 fatty acids, EPA and DHA, which your body can’t make efficiently on its own. These fatty acids work by embedding themselves into cell membranes, where they change how cells communicate and respond to stress.
How Fish Oil Works at the Cellular Level
When you consume EPA and DHA, they’re rapidly absorbed and incorporated into the outer membranes of your cells. Once there, they increase membrane fluidity, which is the flexibility of the cell’s outer layer. This might sound abstract, but it has real consequences: a more fluid membrane changes which signals get in and out of the cell, how immune cells activate, and how efficiently receptors on the cell surface do their jobs. DHA in particular increases ion permeability and alters how proteins embedded in the membrane function.
These membrane changes also influence inflammation. EPA and DHA suppress the activation of certain immune cells, including T cells involved in chronic inflammatory responses. This is the foundational mechanism behind most of what fish oil does. Nearly every benefit traces back to these two fatty acids reshaping how cells behave from the outside in.
Triglycerides and Heart Health
The most well-documented effect of fish oil is lowering triglycerides, a type of fat in the blood linked to heart disease. At doses of 3 to 4 grams per day of EPA plus DHA, triglycerides drop by roughly 30%, with a range of 16% to 45% depending on the person. At prescription-strength doses (around 3.4 grams per day), reductions of 25% to 50% have been observed within a month of treatment.
The mechanism starts in the liver. Your liver packages fats into particles called VLDL and releases them into the bloodstream. Fish oil reduces the liver’s production of these fat-carrying particles. It does this partly by lowering the supply of raw materials: EPA and DHA suppress inflammation in fat tissue, which slows the release of fatty acids from fat cells into the blood. Fewer fatty acids reaching the liver means fewer triglyceride-rich particles leaving it. Fish oil also speeds up the clearance of these particles from the bloodstream, creating a two-pronged effect.
The American Heart Association recommends 4 grams per day of prescription omega-3s specifically for people who need to lower high triglycerides. For people with existing coronary heart disease, the recommendation is about 1 gram per day of EPA plus DHA, preferably from oily fish. The AHA does not recommend omega-3 supplements for people without elevated cardiovascular risk.
Reducing Inflammation
Fish oil lowers key markers of systemic inflammation. In studies of postmenopausal women, supplementation with fish oil significantly decreased levels of C-reactive protein (a general marker of inflammation in the body) and interleukin-6 (a signaling molecule that drives inflammatory responses). Both of these markers are independently linked to cardiovascular disease risk, which is one reason fish oil’s heart benefits extend beyond just lowering triglycerides.
This anti-inflammatory effect is relevant to a range of conditions, from joint pain to metabolic disease. It’s not a dramatic, drug-like suppression of inflammation. Instead, it’s a gradual shift in the body’s inflammatory baseline, driven by the membrane changes described above. The more EPA and DHA that accumulate in your cell membranes over weeks and months, the more pronounced the effect becomes.
Joint Pain and Stiffness
People with rheumatoid arthritis have been studied extensively for their response to fish oil. Randomized trials have reported reductions in the duration of morning stiffness, longer time before fatigue sets in, and decreased use of anti-inflammatory painkillers. That said, the overall evidence remains mixed, and researchers have not reached a firm consensus on the size of the benefit. Fish oil is not a replacement for standard arthritis treatment, but some people find it helps manage day-to-day symptoms when used alongside their existing regimen.
Brain Development During Pregnancy
DHA is a structural component of brain tissue, and a developing fetus needs a substantial supply of it. Higher DHA levels in maternal blood and breast milk correlate with better brain growth and sharper visual acuity in newborns. In one study, mothers who took 600 milligrams of DHA daily from before 20 weeks of pregnancy showed improved visual acuity in their newborns, with a particularly strong effect in boys.
The benefits appear to last well beyond infancy. When mothers supplemented with 500 milligrams of DHA per day during pregnancy, their children showed improved cognitive development at age five and a half. Perinatal DHA supplementation has also been linked to reduced risk of lower IQ scores in children from very low-income families, suggesting that adequate DHA may be especially important when other developmental resources are limited.
Current guidelines for pregnant women generally recommend at least 250 milligrams per day of combined DHA and EPA, with an additional 100 to 200 milligrams of DHA on top of that. Pregnant women with low DHA intake (under 150 milligrams per day) are advised to aim for 600 to 1,000 milligrams per day starting by the second trimester.
Dry Eye Relief
A meta-analysis of 19 randomized controlled trials found that omega-3 supplementation consistently improved dry eye symptoms. Compared to placebo, people taking omega-3s showed better tear film stability (how long the tear layer holds together between blinks), increased tear production, and reduced corneal surface damage. In 18 of the 19 trials, the symptom improvement was statistically significant.
The effect was strongest at higher doses and with longer supplementation periods, and higher EPA content appeared to drive better results. Omega-3s improve tear quality by stabilizing the oily outer layer of the tear film, which reduces evaporation. Based on this evidence, researchers now suggest omega-3 supplementation as a clinical tool for managing dry eye disease.
Dosage and Safety
For general health, there’s no single agreed-upon dose. Most of the meaningful clinical effects show up at 1 gram per day of combined EPA and DHA or higher. The FDA specifies that dietary supplement labels should not recommend more than 2 grams per day of EPA and DHA. Prescription formulations used for triglyceride management go up to 4 grams per day under medical supervision.
The most common side effects are mild: fishy aftertaste, digestive discomfort, and loose stools. The more serious concern people raise is bleeding risk. A systematic review and meta-analysis of randomized trials found that omega-3s in general were not associated with increased bleeding. However, high-dose purified EPA specifically did show a 50% relative increase in bleeding risk, though the absolute increase was small (0.6%). This risk was linked to the dose of EPA itself, not to whether people were also taking blood thinners.
Freshness Matters
Fish oil is highly susceptible to oxidation, which degrades EPA and DHA and produces compounds that may counteract the supplement’s benefits. The industry standard for freshness is set by the Global Organization for EPA and DHA Omega-3s (GOED), which caps the peroxide value (a measure of early-stage oxidation) at 5 meq/kg and the total oxidation value at 26. These limits are stricter than those for most other edible oils. Independent testing has found that many retail fish oil products exceed these thresholds, so storing supplements in the refrigerator, checking expiration dates, and choosing brands that publish third-party testing results all help ensure you’re getting intact omega-3s rather than rancid oil.