Can You Take Flaxseed and Fish Oil Together?

Taking flaxseed oil and fish oil at the same time is safe for most people, and there is no known interaction between the two supplements that would make combining them dangerous. They deliver different forms of omega-3 fatty acids, so some people take both with the idea of covering more bases. The more interesting question is whether doing so actually provides a meaningful benefit over taking just one, and the answer depends on what your body does with each type of omega-3 once it arrives.

Two Different Omega-3s, Two Different Starting Points

Flaxseed oil and fish oil are both marketed as omega-3 supplements, but the omega-3s they contain are chemically distinct. Flaxseed oil is rich in alpha-linolenic acid (ALA), a plant-based omega-3. Fish oil delivers EPA and DHA, which are the longer-chain omega-3s found in fatty fish like salmon and mackerel. Your body uses EPA and DHA directly for things like managing inflammation and supporting cell membrane function. ALA, on the other hand, needs to be converted into EPA and DHA before it can do much of the same work.

This conversion step is where the practical difference between the two supplements becomes important. Your body can turn ALA into EPA reasonably well, but the conversion to DHA is less efficient and more variable from person to person. A 12-week study in people taking flaxseed oil found that plasma EPA levels rose by about 60%, and a related intermediate called DPA increased by about 25%, but DHA levels did not change at all.¹ A separate short-term study in young women confirmed this pattern: two weeks of flaxseed oil supplementation raised ALA, EPA, and DPA in the blood but left DHA unchanged.²

Fish oil, by contrast, delivers EPA and DHA in ready-to-use form. The same study in young women showed that fish oil raised EPA, DPA, and DHA levels in a dose-dependent way after just 14 days.² So if your goal is specifically to increase DHA, fish oil accomplishes that directly while flaxseed oil does not, at least not in most studies. This is the strongest reason someone might take both: flaxseed oil contributes ALA and boosts EPA effectively, while fish oil fills the DHA gap that flaxseed oil alone leaves open.

How Much ALA Actually Becomes DHA

The conversion of ALA to DHA has been one of the more debated topics in nutrition research. Older estimates painted a bleak picture, sometimes suggesting that less than 1% of ALA makes it all the way to DHA. More recent work using isotope-labeled tracking has produced higher numbers. One study using carbon-13-labeled ALA calculated a fractional conversion to DHA of roughly 30% as reflected in the LDL compartment after high-dose ALA treatment.³ That is a considerably higher figure than the traditional estimates, though it reflects conversion in a specific measurement compartment rather than whole-body efficiency.

The reality is probably somewhere in the middle for most people, and it varies by sex, genetics, and what else you eat. Women of reproductive age tend to convert ALA more efficiently than men, likely because DHA is critical for fetal brain development. People whose diets are already high in omega-6 fatty acids (from vegetable oils, processed foods, and the like) may convert ALA less efficiently because the same enzymes handle both omega-6 and omega-3 conversion, and they compete for access. If you eat a typical Western diet heavy in omega-6s, flaxseed oil’s ALA faces stiff competition for the enzymes it needs to become EPA and DHA.

Cardiovascular and Metabolic Effects Compared

When researchers have directly compared flaxseed oil and fish oil for heart health markers, the results are more similar than you might expect given how different the supplements are at the biochemical level. In a retrospective study of patients with coronary heart disease, flaxseed oil and fish oil had comparable effects on body weight, BMI, fasting blood sugar, and insulin resistance over a median follow-up of about 10 weeks. Flaxseed oil actually performed better on two markers: it reduced serum insulin levels and high-sensitivity C-reactive protein (a marker of inflammation) more than fish oil did.⁴

Animal research tells a broadly similar story. In a study of rats fed a high-fat diet, both flaxseed oil and fish oil supplementation lowered cholesterol, triglycerides, and LDL while raising HDL, with no significant difference between the two.⁵ And in diabetic rats, both oils reduced kidney oxidative stress and limited the formation of harmful compounds associated with uncontrolled blood sugar. Flaxseed oil had a slight edge in boosting certain antioxidant enzymes in the kidneys, but both diets lowered markers of inflammation and tissue damage.⁶

One thing worth noting: while omega-3 fatty acids are broadly beneficial for cardiovascular health, some research has raised questions about their effect on blood sugar regulation. There are findings suggesting that omega-3 supplementation, particularly with long-chain forms like EPA and DHA, can increase fasting glucose in some contexts.⁷ The effect is generally small and its clinical significance for people without diabetes is unclear, but if you have diabetes or prediabetes, it is worth discussing your omega-3 regimen with your doctor. Interestingly, the coronary heart disease study mentioned above found that flaxseed oil reduced insulin levels compared to fish oil, so the metabolic picture may differ depending on which omega-3 source you use.

What About Bleeding Risk

The most common safety concern people have about stacking omega-3 supplements is whether doing so increases bleeding risk. Omega-3 fatty acids have mild blood-thinning properties, so the worry is that doubling up could push this effect into dangerous territory, especially if you also take blood thinners like warfarin or aspirin.

The evidence here is more reassuring than many people assume. A systematic review and meta-analysis of randomized trials looked specifically at bleeding risk from omega-3 supplementation. At standard supplementation doses, there was no meaningful increase in bleeding. High-dose purified EPA (the kind prescribed for very high triglycerides, at doses around 4 grams per day) did show a 50% relative increase in bleeding risk, but the absolute increase was small, just 0.6 percentage points compared to placebo. The bleeding risk tracked with EPA dose, not with whether someone was also on antiplatelet therapy.⁸

What about people already on anticoagulants? A retrospective study of patients on warfarin for atrial fibrillation or deep vein thrombosis found that adding fish oil did not significantly affect how well-controlled their anticoagulation was. Both the supplement group and the control group maintained their target range about 87% of the time, and there was no increase in minor or major bleeding events.⁹ That said, these studies mostly looked at fish oil alone, not the combination of fish oil and flaxseed oil in people on anticoagulants. If you are on blood thinners and want to take both supplements, mention it to your doctor. The risk appears low, but the specific combination has not been studied extensively in that population.

Practical Considerations for Taking Both

If you decide to take flaxseed oil and fish oil together, there are a few practical things worth knowing. Timing does not seem to matter much. Both are fat-soluble and absorb better when taken with a meal that contains some fat, so taking them with food is a good idea. There is no evidence that taking them at separate meals versus the same meal makes a difference in absorption or effectiveness.

Dose is more important than timing. A common supplemental dose of fish oil provides somewhere around 250 to 500 milligrams of combined EPA and DHA per capsule. Flaxseed oil capsules typically contain about 500 to 700 milligrams of ALA per capsule. Liquid forms of both deliver more per serving. In the study of young women mentioned earlier, fish oil raised EPA and DHA in a dose-dependent pattern, but the effects became statistically significant only at moderate-to-higher doses rather than the lowest dose tested.² Meanwhile, the flaxseed oil arm used about 6 grams of ALA daily (10 mL of flaxseed oil). Most people taking capsules get considerably less than that.

Digestive side effects are the most common complaint with either supplement. Fishy burps, loose stools, and mild nausea affect some people with fish oil, particularly at higher doses. Flaxseed oil can also cause loose stools or a laxative effect, especially when you first start taking it. Taking both at once may make these effects more noticeable. Splitting them between meals can help if stomach upset is a problem.

Oxidation and Storage Matter More Than You Think

Both flaxseed oil and fish oil are highly unsaturated, which makes them prone to going rancid. When omega-3-rich oils oxidize, they break down into compounds that may not only be ineffective but could actually be harmful. A review on the oxidation of marine omega-3 supplements noted that these oils are highly prone to degradation into lipid peroxides and secondary oxidation products, and that oxidized oils may have altered biological activity, making them either ineffective or potentially harmful.¹⁰

Flaxseed oil is particularly vulnerable because ALA is even more prone to oxidation than EPA or DHA when exposed to heat, light, or air. This is why flaxseed oil is typically sold in dark bottles and needs to be refrigerated after opening. Capsules offer some protection, but even encapsulated oils degrade over time if stored improperly. If your flaxseed oil tastes bitter or your fish oil capsules smell strongly fishy when you bite into one, they are likely oxidized and should be replaced. Buying smaller bottles that you use up within a few weeks, keeping them refrigerated, and checking expiration dates are simple steps that make a real difference in whether you are getting functional omega-3s or degraded oil.

Who Might Actually Benefit From Taking Both

For most people, taking one or the other is probably sufficient. If you eat fatty fish a couple of times a week, you are likely getting enough EPA and DHA directly, and adding flaxseed oil on top of that gives you extra ALA but may not change your health outcomes meaningfully. If you are vegetarian or vegan and cannot take fish oil, flaxseed oil is a reasonable plant-based alternative for boosting EPA, though it will not reliably raise DHA. In that situation, an algae-based DHA supplement is a more direct solution than hoping your body converts enough ALA.

The people most likely to benefit from combining the two are those who want the specific advantages each oil may offer. Fish oil reliably raises DHA, which flaxseed oil does not. Flaxseed oil appears to have some edge in reducing inflammatory markers like hs-CRP and may lower insulin levels in certain populations.⁴ ALA also has its own biological roles that do not depend on conversion to EPA and DHA, including contributions to cell membrane structure and energy metabolism. So the combination is not pure redundancy, even if the overlap is large.

People taking omega-3 supplements for dry eye symptoms may find the combination worthwhile. Both omega-3 types have been used as supportive treatments for dry eye, with evidence suggesting the anti-inflammatory properties of EPA and DHA help with the underlying irritation.¹¹ In this context, covering both ALA and EPA/DHA could theoretically provide a broader anti-inflammatory effect, though studies directly testing the combination for dry eye are lacking.

Omega-3s and Gut Health

An emerging area of research concerns how omega-3 fatty acids influence the gut microbiome. Early evidence suggests that omega-3s can shift the composition of gut bacteria in beneficial directions, increasing microbial diversity and promoting the growth of bacteria associated with good health. A review of the evidence found that omega-3 fatty acids influence gut microbiota composition, metabolite production, and the intestinal mucus barrier, though some studies have reported inconsistent results. The ratio of omega-6 to omega-3 fats in your diet appears to matter: a high omega-6-to-omega-3 ratio may blunt the beneficial effects of omega-3 supplementation on microbial diversity, while a more balanced ratio supports a healthier microbiome profile.¹²

Animal studies have started to tease apart whether ALA and EPA/DHA differ in their gut effects. In mice fed a high-fat diet, flaxseed oil supplementation restored some of the microbial diversity that the high-fat diet had destroyed, improving both the Shannon and Simpson indices of diversity.¹³ Different omega-3 types, including ALA, EPA, DPA, and DHA, appear to exert distinct effects on intestinal immune tolerance and gut microbiota maintenance.¹⁴ This is still early research, and no one should take both supplements purely for microbiome benefits based on what is currently known. But it does suggest that ALA and EPA/DHA are not interchangeable in every system, which adds another dimension to the question of whether combining them has value beyond what either achieves alone.

The Omega-6 Factor

One thing that rarely comes up in discussions about omega-3 supplements but probably should is the role your overall omega-6 intake plays. Most Western diets are heavy in omega-6 fatty acids from sources like soybean oil, corn oil, and processed foods. Omega-6 and omega-3 fatty acids compete for the same enzymes that convert them into their active forms. When omega-6 intake is very high, it can crowd out ALA’s conversion to EPA and DHA, which means your flaxseed oil supplement may be less effective than it would be in someone eating fewer omega-6-rich foods.

This competitive dynamic also has implications for gut health. As noted in the microbiome research, a high omega-6-to-omega-3 ratio may weaken the beneficial effects of omega-3 supplementation on microbial diversity.¹² In practice, this means that what you eat alongside your supplements affects how well they work. Adding fish oil and flaxseed oil to a diet that is already drowning in omega-6 fats may produce less benefit than the same supplements added to a diet where omega-6 intake has been moderated. Cutting back on processed vegetable oils and fried foods is arguably as important as choosing the right supplements, and it is free.

None of this means omega-6 fats are inherently bad. They are essential nutrients. But the balance between the two families of fats matters for how your body handles inflammation, enzyme activity, and now, it seems, the composition of your gut bacteria. If you are spending money on both flaxseed oil and fish oil capsules, it is worth at least thinking about whether your overall diet is set up to let those supplements do their job.