Fish oil is one of the most popular supplements worldwide, but combining it with certain other supplements can reduce absorption, increase side effects, or amplify risks you didn’t sign up for. The interactions worth knowing about fall into a few categories: supplements that increase bleeding risk, fat-blocking supplements that prevent your body from absorbing the omega-3s, fat-soluble vitamins that can build to toxic levels, and blood-pressure-lowering supplements that may drop your pressure too far.
Blood-Thinning Supplements
Fish oil has a mild blood-thinning effect on its own. A 2024 systematic review and meta-analysis of randomized trials found that high-dose purified EPA (one of the two main omega-3 fats in fish oil) increased the relative risk of bleeding by 50%, though the absolute increase was small, about 0.6% compared with placebo. The bleeding risk scaled with the dose of EPA. For most people taking a standard fish oil capsule, this isn’t a major concern by itself. But stacking fish oil with other supplements that also thin the blood can compound the effect.
The supplements most likely to cause problems in combination include ginkgo biloba, garlic extract, high-dose vitamin E, nattokinase, and turmeric or curcumin. Each of these has its own antiplatelet or anticoagulant properties. Taken individually alongside fish oil, the added risk is modest. But if you’re combining two or three of them at once, the cumulative effect on clotting can become clinically meaningful, especially before surgery or dental procedures, or if you bruise or bleed easily.
Fat-Blocking Supplements
Fish oil is, as the name suggests, a fat. That makes it vulnerable to anything designed to block fat absorption in your gut. Chitosan, a popular weight-loss supplement derived from crustacean shells, works by binding to dietary fats in the digestive tract and forming complexes your intestines can’t absorb. Those bound fats get excreted instead. Research confirms that chitosan increases fecal cholesterol levels, demonstrating its ability to pull lipids out of circulation before your body can use them. If you take chitosan and fish oil at the same time, the chitosan can bind the omega-3 fats and carry them right out of your body.
The same logic applies to orlistat (sold over the counter as Alli), which blocks fat-digesting enzymes, and to high-dose soluble fiber supplements like psyllium or glucomannan when taken in large amounts alongside a fat-based supplement. The fix here isn’t necessarily to stop taking these supplements. It’s to separate them by at least two to four hours so the fish oil has time to be absorbed before the fat blocker reaches the same part of your gut.
Cod Liver Oil and Fat-Soluble Vitamins
This one catches people off guard. Standard fish oil (the kind labeled as omega-3 or EPA/DHA) contains negligible amounts of vitamins. But cod liver oil is a different product. It’s rich in both vitamin A and vitamin D, and if you’re taking it alongside a multivitamin or separate vitamin A and D supplements, you can push your intake into risky territory.
Vitamins A and D are fat-soluble, meaning your body stores them rather than flushing out the excess. Over time, that accumulation can cause toxicity. Vitamin D toxicity triggers dangerously high calcium levels in the blood, a condition called hypercalcemia. A report in the British Journal of Clinical Pharmacology documented seven pediatric cases of hypercalcemia linked to a fish oil supplement where the actual vitamin D dose turned out to be far higher than the label stated. In one case, an infant supposedly receiving 2,000 IU per day was actually getting 6,000 IU daily. While mislabeling was the culprit there, it highlights a broader concern: the American College of Cardiology notes that non-prescription fish oil supplements have inconsistent content and purity, meaning what’s on the label doesn’t always match what’s in the capsule.
If you take cod liver oil, check the vitamin A and D content on the label and add it to whatever you’re getting from your multivitamin and any standalone vitamin supplements. The tolerable upper limit for vitamin A in adults is 10,000 IU per day, and for vitamin D it’s 4,000 IU per day from all sources combined (though some practitioners recommend higher D levels under monitoring).
Blood Pressure Supplements
Fish oil has a modest blood-pressure-lowering effect, typically in the range of 2 to 5 mmHg for systolic pressure at doses of 2 to 4 grams per day. That’s a benefit if your pressure runs high, but it becomes a concern if you’re also taking supplements specifically designed to lower blood pressure. CoQ10, garlic extract, magnesium (at higher doses), and hawthorn berry all have documented hypotensive effects. Layering several of these with fish oil could drop your blood pressure enough to cause dizziness, lightheadedness, or fainting, especially when you stand up quickly.
This is particularly relevant if you’re already on prescription blood pressure medication. The combination of a prescription drug, fish oil, and one or two herbal blood pressure supplements can overshoot the target and leave you feeling washed out or unsteady.
Vitamin E: A Complicated Relationship
Vitamin E and fish oil have an interesting interaction that cuts both ways. On one hand, high-dose fish oil increases your body’s need for vitamin E. The omega-3 fats in fish oil are highly susceptible to oxidation, and vitamin E acts as the primary antioxidant that protects them. A clinical study in healthy young men found that taking 15 grams of fish oil per day without additional vitamin E increased markers of lipid oxidation and actually worsened blood clotting behavior. When the same dose was paired with 400 IU of vitamin E, blood aggregation decreased (the desired effect) and blood sugar stayed stable.
On the other hand, vitamin E itself has blood-thinning properties. So while a moderate amount of vitamin E may actually improve how fish oil works in your body, taking high doses of both together amplifies the bleeding risk discussed earlier. If you take high-dose fish oil regularly, a moderate vitamin E supplement (200 to 400 IU) may be beneficial, but megadoses above 1,000 IU combined with fish oil push you further into bleeding-risk territory.
Iron and Omega-3 Interactions
The relationship between fish oil and iron is still being mapped out, but research published in peer-reviewed journals has established a clear metabolic link between omega-3 fatty acids and iron metabolism. Clinical studies and animal research both show that omega-3s can affect how the body handles iron, and that various iron-related disorders are influenced by omega-3 intake. The exact mechanisms aren’t fully understood, but if you take iron supplements, it’s worth separating them from fish oil by a couple of hours to minimize any potential interference with absorption, and to reduce the stomach upset that both supplements can independently cause.
Timing and Practical Advice
Most of these interactions don’t mean you need to abandon one supplement or the other. In many cases, the solution is simple timing. Taking fat-blocking supplements two to four hours apart from fish oil protects absorption. Spacing out blood-thinning supplements throughout the day rather than taking everything at once can reduce peak anticoagulant effects. And checking vitamin A and D totals across all your supplements takes about two minutes with the bottles in front of you.
For fish oil quality specifically, the American College of Cardiology has flagged concerns about non-prescription fish oil products, noting inconsistent content, purity issues, and a lack of proven benefit in multiple randomized studies for cardiovascular risk reduction. If you do take fish oil, choosing a product that has been third-party tested for purity and accurate labeling reduces the risk of accidentally getting more (or less) than you bargained for, including hidden fat-soluble vitamins or contaminants that could interact with your other supplements.