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Fish Oil Side Effects: What To Know About Boosting Your Omega-3 Intake With Supplements

Korin Miller
Contributing writer
By Korin Miller
Contributing writer
Korin Miller is a freelance writer specializing in general wellness, relationships, and lifestyle trends with a master’s degree from American University. Her work has appeared in Women’s Health, Prevention, Self, Glamour, and more.
Ashley Jordan Ferira, Ph.D., RDN
Expert review by
Ashley Jordan Ferira, Ph.D., RDN
mbg Vice President of Scientific Affairs
Ashley Jordan Ferira, Ph.D., RDN is Vice President of Scientific Affairs at mindbodygreen. She received her bachelor's degree in Biological Basis of Behavior from the University of Pennsylvania and Ph.D. in Foods and Nutrition from the University of Georgia.

Be honest: Do you really get in those two recommended servings of heart-healthy fatty fish per week? No judgment—most people don't! And that's just the foundational, baseline recommendation. The starting line, if you will. Even if you make it a habit to regularly top salads with tinned sardines or swap the usual beef for salmon burgers, there are still certain cases in which your doctor or nutritionist might recommend upping your omega-3 game (often with a supplement like fish oil) to make sure you're getting enough of these healthy fats in your system on the regular. If you know seafood isn't your jam, perhaps you've taken it upon yourself to add a supplement to your routine!

Whether you've been taking fish oil for years or are just now considering an omega-3 supplement, there are—of course—a few things you'll want to keep in mind. From the benefits you can reap (think cardiovascular wellness, joint health, and cognitive function) to precautions you should take note of, here's what you should know about fish oil's potential side effects—and how to make your supplement a winning part of your healthy routine.*

What to know about fish oil.

The reason people pop gelcaps filled with fish oil: The stuff, which comes from fatty fish like anchovies, sardines, mackerel, salmon, and tuna, is loaded with healthy fats called omega-3s, explains dietitian Lauren Kelly, R.D., head of community and nutrition for Sound. In addition to supporting your whole-body antioxidant status, plus cardiovascular and immune systems, omega-3s' anti-inflammatory properties also support your brain, eyes, and more—and since the body can't produce these healthy fats on its own, it's essential that you get them from food or a high-quality omega-3 supplement, she says.*

Another reason the omega-3s found in fish oil are so important, especially these days, is that the average American diet is (too) high in omega-6 fats, which have inflammatory properties. While omega-6s are found in healthy foods like pumpkin seeds, sunflower seeds, and walnuts, they're also found in high amounts in vegetable oils (aka soybean, corn, canola, etc.) and processed foods, Kelly explains. And when people eat lots of processed, vegetable-oil-containing foods, they consume too many omega-6s compared to omega-3s, which creates an omega fats imbalance and is counterproductive for omega-3s' anti-inflammatory actions.*

For these reasons (among others) a quality fish oil supplement can be incredibly helpful—especially if you don't eat fatty fish like salmon or tuna at least two to three times a week, says Kelly.

Can you take too much fish oil?

Just how many omega-3s you need—and thus how much fish oil would best support your health—depends on individual factors and health priorities. The Dietary Guidelines for Americans recommends adults get an average of 250 to 500 milligrams of EPA and DHA per day (that's about one to two servings of fish a week) for starters.

The American Heart Association echoes this two-fish-serving baseline and then recommends a higher daily dose of these marine omega-3s—1,000 milligrams per day—for optimizing heart health.*

Meanwhile, research1 indicates that pregnant women need 650 milligrams of omega-3s per day, with at least 300 milligrams of that coming from DHA (a must-have for baby's brain, eye, and neural development), specifically.*

That said, clinical research indicates that up to 10,000 milligrams (aka a whopping 10 grams) of EPA and DHA can be taken safely without adverse effects. In fact, you're probably more likely to run into issues eating tons of fish than taking high amounts of fish oil, according to Andrea Paul, M.D. "The more likely negative health outcome is exposure to high levels of environmental toxins like mercury through very high levels of fish consumption," she says. Since quality supplement brands purify and test their products for contamination for things like heavy metals, you would actually face greater risk by eating lots of tuna than by taking a high-quality supplement for a concentrated dose of omega-3s.*

The potential side effects of too much fish oil.

That said, there are a few potential side effects that can come with consuming high amounts of fish oil, particularly if you have certain health considerations:


Upset stomach

Though research2 suggests that a teeny-tiny proportion of people (just about 1%) experience digestive upset after taking fish oil, it is a potential side effect those with sensitive stomachs might notice. This could be simply because fish oil is fat, which some people have trouble digesting. "Fat may relax the lower esophageal sphincter [a set of muscles at the bottom of the esophagus, near the entrance to the stomach] and promote acid reflux or slow the flow of digestion," explains dietitian Isa Kujawski, MPH, RDN.

But as Ashley Jordan Ferira, Ph.D., RDN, mbg's in-house nutrition scientist, warns: "A negative GI experience with an omega-3 supplement is more commonly tied to poor-quality ingredients, in other words, lower-quality fish oil or fish oil that's been allowed to oxidize and become rancid." So, it's of utmost importance to consider these "fishy" factors.


Blood thinning

If you have a bleeding disorder or take medications classified as blood thinners, out of an abundance of caution, Kelly recommends talking to your doctor about any supplements, including omega-3 fish oil supplements, you take (or are interested in taking), since they can have a blood-thinning effect at extremely high doses. Again, 10,000 milligrams of EPA plus DHA daily (that's 10 grams) has been shown to be safe according to research. The cautionary linking of fish oil to blood thinning started decades ago simply because of the way omega-3s interact with platelets3, which are cell fragments in the blood that play an integral role in clotting, Kujawski says.

Ferira says this potential side effect has been blown out of proportion. "Way too much of any good thing, even water, can have potentially negative effects on our health. But in this case, a textbook physiological mechanism, that omega-3s can impact platelets, has been used to broadly fearmonger against completely safe doses of omega-3s found in supplements." She goes on to say that, "In reality, the blood-thinning effect of fish oil actually occurs at absurdly high levels of EPA plus DHA, 10 grams plus, which by the way, no supplement even comes close to containing. Omega-3 supplements are playing in the 200- to 1,800-milligram range of EPA plus DHA 99% of the time."

Ferira explains that the scientific case for omega-3s' safety is a strong one. "I'm talking about 80-plus research studies demonstrating the safety of low, medium, and high levels of daily EPA + DHA consumption. It's time somebody debunked this myth that is genuinely scaring people away from using a truly helpful tool (i.e., fish oil supplementation) for whole-body health."*


Blood sugar changes

There's conflicting science here, but some research does suggest that very high doses of omega-3 can affect blood sugar in some people, according to Kujawski. (Newer studies4 actively question this.) To be safe, she recommends checking in with your doctor if you have any blood sugar concerns.

Ferira shares that, "to be clear, omega-3 supplementation research has been shown to be highly effective for promoting a healthy profile of lipids, with particular benefit to triglyceride levels. That same research has found a modest beneficial trend for glucose, insulin, and HbA1c measures of glucose control5, while other studies have reached statistical significance, demonstrating omega-3s' ability to improve insulin sensitivity6."*

For most people, this means that omega-3s are actually helpful for your blood sugar balance, but Ferira explains, "For those with true hypoglycemia considerations, partner with your doctor to optimize your omega-3 supplement approach."


Too much vitamin A

Though not a concern with the large majority of fish oil options (i.e., in gelcaps or softgels), "high doses of certain sources of omega-3s such as cod liver oil also contain high amounts of vitamin A," Kujawski explains.

One teaspoon of cod liver oil, she says, contains 4,500 I.U. of vitamin A7—so if you take multiple teaspoons per day (which some people might), you may exceed the recommended maximum (i.e., tolerable upper intake level) of 10,000 I.U. for vitamin A. A few signs of too much vitamin A8 may include dizziness, queasiness, or even joint discomfort.


Low blood pressure

While fish oil supplements' impact on blood pressure9 is exactly what makes them appealing to certain people, it may be a consideration for those who take blood pressure medications or who already have low blood pressure. So partner with a health care practitioner to be sure.

Ferira shares that, "the fact that omega-3s have been shown in a large number of research studies to promote healthy blood pressure levels is compelling, so compelling that a couple of years ago, the FDA issued a claim10 for omega-3 food and supplement products containing at least 800 mg of EPA plus DHA per serving, saying those products may reduce blood pressure and the risk of hypertension, which is, in turn, a major risk factor for coronary heart disease."†


Trouble sleeping

Research is quite limited here, with just individual case studies11 (i.e., one person) suggesting a link between taking high doses of omega-3s (specifically a very high-dose EPA supplement in this singular case12) and trouble sleeping—so Kujawski says that more scientific investigation is needed to better understand any potential relationship between high amounts of omega-3s and sleep.

Ferira concludes that "these case studies are informative, but n-of-1 reports are not something to base population info on. In fact, recent clinical trial research in healthy young adults found omega-3 supplements to be helpful13 for sleep architecture and efficiency13."*

The positive effects of fish oil.

Of course, omega-3s (and the fish oil supplements that contain them) can benefit your health and well-being in a number of ways. Here is a bird's-eye view of some of the positive side effects of fish oil that experts and researchers have uncovered to date:


Immune support

Because omega-3s have been shown to have anti-inflammatory properties14, combat oxidative stress, and directly support our immune response in the body, taking an omega-3-rich fish oil might be beneficial to the immune system.* All types of omega-3s help signal the immune system15 to activate our natural defense pathways.*


Cardiovascular wellness

One of the most well-known positives of omega-3s: cardiovascular health. In addition to promoting healthy blood circulation and pressure, these good-for-you fatty acids also support optimal levels of lipids, particularly triglycerides16 (a type of fat in the blood).*

As Ferira alluded to before, the FDA believes the science collectively supports omega-3-rich supplements' ability (i.e., at least 800 milligrams of EPA plus DHA per serving) to help reduce coronary heart disease risk.†


Bone & joint health

Because of their (lesser-known) role in helping to regulate calcium absorption, omega-3s support bone health, studies17 indicate. Those anti-inflammatory properties also come in handy for supporting joint function and comfort18.*

Ferira concludes, "If omega-3s were not top of mind for musculoskeletal health, they should be. The science is exciting and growing."*


Brain development & cognition.

The omega-3 DHA, in particular, is crucial for brain development19, as well as for supporting cognition20 (including things like memory and mood) throughout your lifetime.*

Ferira shares, "Indeed, these omega-3s are literally intrinsic to the cellular membranes of cells throughout our central nervous system, from our brain and retina to neurons throughout the body. They are ensuring healthy neuronal signaling via synaptic transmission, which translates practically into mental acuity, memory, vision, and more."*

She caveats these potential benefits with an important message: "These central nervous system benefits are all predicated on the assumption that, of course, you are regularly consuming a meaningful amount of omega-3s EPA and DHA in the first place."*


Balanced mood

More recently, science has started digging more into another exciting perk of omega-3s: Their role in mental well-being. Researchers are still exploring21 exactly how EPA and DHA (the two omegas being touted for their mood benefits) have a positive impact here, but there is certainly evidence to suggest they have an important part to play in supporting a balanced mood22.*

"Given omega-3s' extensive neuroprotective roles in the body, it's not surprising that they have been linked to mood balance and health,"* concludes Ferira.


Healthy eyes & vision

In addition to brain development, DHA is also a must-have for eye development early on and function throughout life. It both works to ensure that the retina functions23 as it should and helps the eyes combat oxidative stress24.*

Explaining the science a bit more, Ferira shares that, "We know DHA supports functioning & regeneration of rhodopsin, a visual pigment that plays a critical role in the visual system that converts light into visual images. In other words, DHA helps you see25, that's pretty important stuff!"*

The bottom line on fish oil and its effects.

Taking high-quality fish oil supplements that boast purity and potency, especially if you aren't much of a fish eater, can be a great move for everything from immune and cardiovascular health to cognition and joint health.* (For specific recommendations, check out our omega-3 supplement roundup.) And though scientific evidence has shown that incredibly high amounts of fish oil can be tolerated, it's never a bad idea to check in with your doctor or nutritionist before making changes to your supplement routine, especially if you have certain health concerns or needs.

†Consuming EPA and DHA combined may reduce blood pressure and reduce the risk of hypertension, a risk factor for CHD (coronary heart disease). However, FDA has concluded that the evidence is inconsistent and inconclusive. One serving of omega-3 potency+ provides 1.5 grams of EPA and DHA.
If you are pregnant, breastfeeding, or taking medications, consult with your doctor before starting a supplement routine. It is always optimal to consult with a health care provider when considering what supplements are right for you.
Korin Miller author page.
Korin Miller
Contributing writer

Korin Miller is a freelance writer specializing in general wellness, relationships, and lifestyle trends with a master’s degree from American University. Her work has appeared in Women’s Health, Prevention, Self, Glamour, and more. She lives by the beach, and hopes to own a taco truck one day.