Fish oil supplements can affect several common blood test results, most notably triglycerides, LDL cholesterol, and clotting markers. If you take fish oil regularly, your doctor may already expect these shifts. But if you’ve recently started supplementing or changed your dose, the changes can be significant enough to alter how your results are interpreted.
Triglycerides and Cholesterol
The biggest impact fish oil has on blood work is lowering triglycerides. A large review of 23 studies covering nearly 44,000 people found that EPA and DHA (the two main omega-3 fats in fish oil) reduce triglycerides by about 15 percent on average. In people who already have high triglycerides, the drop can be even steeper. One study of adults with elevated triglycerides found a 26 percent reduction after 12 weeks of fish oil supplementation.
Here’s the part that surprises many people: while fish oil brings triglycerides down, it can push LDL cholesterol up. In that same study, total LDL cholesterol rose by 13 percent. That increase could be enough to move your LDL number from one risk category to another on a standard lipid panel, which might prompt concern if your doctor doesn’t know you’re taking fish oil. This is one of the most important reasons to mention your supplement use before blood work.
Blood Clotting Tests
Fish oil has mild blood-thinning properties, and for most people this doesn’t show up on standard tests. But if you take a blood thinner like warfarin, fish oil can amplify its effect. In one documented case, a patient on warfarin doubled her fish oil dose from 1,000 to 2,000 mg per day. Within a month, her INR (a measure of how long it takes blood to clot) jumped from 2.8 to 4.3, well above the safe therapeutic range. After she reduced her fish oil back to the original dose, the INR dropped to 1.6 within a week.
If you’re not on blood thinners, fish oil is unlikely to cause a meaningful shift in clotting tests. But if you are, even modest changes in your fish oil dose can throw off results and, more importantly, change your actual bleeding risk.
Blood Sugar and HbA1c
Fish oil generally does not affect fasting blood glucose or HbA1c in a clinically meaningful way. A meta-analysis of studies in people with type 2 diabetes found no significant changes in fasting glucose, post-meal glucose, HbA1c, or insulin levels with omega-3 supplementation. One exception appeared in studies of Asian populations, where fasting glucose increased modestly (about 0.42 mmol/L), though the clinical significance of that small shift is unclear.
If you’re monitoring blood sugar, fish oil is unlikely to be the reason your numbers look different from one test to the next.
Inflammatory Markers Like CRP
Despite fish oil’s reputation as an anti-inflammatory supplement, it does not appear to lower C-reactive protein (CRP), the most common blood marker of inflammation. A randomized controlled trial giving healthy adults 1,400 mg per day of EPA and DHA found no effect on CRP or another inflammatory marker called IL-6. This held true regardless of sex, body weight, or baseline inflammation levels. Even participants who achieved the highest tissue concentrations of omega-3s showed no CRP reduction.
So if your doctor orders a CRP test, fish oil is not likely to move the number in either direction.
Liver Enzymes
Fish oil is unlikely to cause liver injury, and the National Institute of Diabetes and Digestive and Kidney Diseases rates it as an “unlikely cause of clinically apparent liver injury.” That said, high-dose prescription fish oil can cause small, temporary bumps in ALT (a liver enzyme measured in routine blood panels). In pooled analyses, mild ALT elevations occurred in about 13 percent of patients on one prescription formulation, but similar rates appeared in the placebo group. The elevations were always mild (less than twice the normal upper limit), caused no symptoms, and resolved on their own without dose changes.
For people taking over-the-counter fish oil at standard doses, liver enzyme changes are not a realistic concern. Prescription-strength omega-3s carry a label recommendation to monitor liver enzymes only in patients who already have liver problems.
Kidney Function
Fish oil does not harm kidney function markers like creatinine or GFR. In fact, a trial of over 2,300 heart disease patients found that 40 months of EPA and DHA supplementation slightly slowed the natural decline in kidney function compared to placebo. The difference was modest (about 2 ml/min per 1.73 m² better than placebo), but it points in a protective direction rather than a harmful one.
Vitamin A and D Levels
Standard fish oil supplements contain only omega-3 fats and will not affect your vitamin levels. Cod liver oil is different. It naturally contains vitamins A and D, and regular use can raise blood levels of both. One study in northern Norway found that people who routinely took cod liver oil had significantly higher baseline vitamin D levels than those who didn’t. If you take cod liver oil specifically, it’s worth mentioning before any test that measures fat-soluble vitamins.
Timing Around a Blood Draw
Taking fish oil shortly before a blood test can temporarily spike omega-3 levels in your plasma. In a study where healthy volunteers took 3.6 grams of omega-3s with a meal, plasma EPA and DHA concentrations jumped 47 percent within six hours. At 24 hours, plasma levels were still notably elevated. Red blood cell levels, which reflect your long-term omega-3 status, were completely unaffected by a single dose.
This matters in two situations. If your doctor is testing your omega-3 index (measured in red blood cells), timing doesn’t matter. But if you’re having a fasting lipid panel, taking fish oil that morning with water could introduce fat into your bloodstream and muddy your triglyceride reading. The simplest approach is to skip your fish oil the morning of a fasting blood draw and take it afterward.