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Fish Oil Side Effects & Adverse Reactions

Fish oil is among the most popular dietary supplements in the world. Everyone making an attempt to optimize their physical health and brain performance has at least considered supplementing omega-3 fatty acids. The most common way to supplement the omega-3s is by taking fish oil. The fact that fish oil supplements are able to lower triglycerides, improve cardiovascular health, and possibly prevent neurodgeneration – has made them a hot commodity.

Although fish oil has become a popular supplement, many people have learned that it can elicit unwanted side effects. A common misconception is that because fish oil is a supplement, that it cannot possibly have negative side effects. The reality is that many people can experience significant side effects, and may actually be damaging their health with fish oil by ignoring any signs of an adverse reaction.

Factors that influence fish oil’s side effects

Before blaming the fish oil for the side effects that you experience, it is important to consider several influential factors. Factors that can influence side effects include: the fish oil quality, ingredients, dosage, how long you’ve taken it, whether you’re taking other drugs (or supplements), and your individual physiology.

1. Quality

Taking low quality fish oil is a good way to set yourself up for both side effects and adverse reactions. You may actually be damaging your physical health and mental health if you’re taking a supplement that is expired, rancid, or doesn’t pass quality control tests. If you aren’t sure whether your fish oil is a quality brand, check with the IFOS (International Fish Oil Standards Program).

The problem is that most people haven’t done any diligent research on their fish oil and assume that it’s good even if they got it from their local convenience store. The problem is that it may not have been properly stored (it should be refrigerated and kept out of the sun). If you’re ordering the best fish oil supplements, you won’t have to worry as much about quality concerns.

2. Contents

Most fish oil products contain varying amounts of omeaga-3 fatty acids (DHA and EPA). Some contain more DHA than EPA, while others contain more EPA than DHA. Certain products are actually created with 100% pure DHA, and others contain 100% pure EPA. If you are taking a product that is heavily weighted in one particular omega-3 over the other, you may be having a bad reaction to that particular fatty acid.

Some people have reactions to one fatty acid more than the other. To determine which is more problematic, you may want to do some experimentation. Also consider the fact that the specific type of fish (and any added ingredients in the product) may be leading to unwanted side effects.

3. Dosage

When taking any fish oil supplement, it is important to take the minimal effective dose. Taking “more” than necessary is not always better. The dosage you’re taking could be responsible for any side effects that you’re experiencing. In general, the higher the dose you take, the more likely you will experience some sort of side effects.

Larger doses of fish oil will have a greater effect on your physiological and brain functioning. If you are taking a high dose and side effects have become noticeably uncomfortable, consider cutting back. You may be getting too many omega-3s per day. While higher doses are generally necessary when taking for mental health conditions, side effects may be problematic with a high dose.

4. Duration

The time span over which you’ve been taking fish oil may influence whether you experience side effects. Those that have been taking it for a longer period of time may be adapted to the supplement and may have found an optimal dose that works. For some people an adjustment phase occurs where they experience minor side effects, and eventually all side effects subside.

For others, side effects may take awhile to become noticeable. You may notice side effects that emerge after several weeks or a couple months of taking fish oil. For some people it may take awhile for the fish oil’s effect to build up and side effect to emerge. Another major red flag is when side effects occur immediately and progressively worsen throughout supplementation.

5. Interactions

Another very important element to consider is that of drug and/or supplement interactions. If you’re taking a variety of drugs or a stack of supplements, who’s to say that the fish oil is the cause of your side effects? Couldn’t the side effects have emerged as an interaction or contraindication with another drug that you’re taking?

It is always important to proceed with medical caution whenever taking fish oil and another supplement (or even another drug). Talk to your doctor to verify the safety of the combination that you’re taking. Without verification of safety, you may be setting yourself up for adverse effects (without knowing it).

6. Individual

Finally consider any lifestyle changes or stressors that may cause psychogenic symptoms. For example, if you are working late, have had relationship problems, and haven’t gotten good sleep, and start to notice a headache, it may not be from the fish oil. Always be conscious of things going on in your life that may affect how you feel and may lead you to wrongfully pin the effects of stress on the fish oil. Other factors such as genetics and physiology will certainly play a role in determining how you respond.

Fish Oil Side Effects: List of Possibilities

Below is a list of potential side effects that you may experience while taking fish oil. Keep in mind that not everyone will experience every side effect listed and that some of these side effects are non-harmful (e.g. burps). Realize that the severity of each will be subject to individual variation.

  • Agitation: Not everyone feels calm and relaxed when they take fish oil. You may become increasingly agitated and/or fidgety, unable to sit still. Noticing an increase in agitation may be a good reason to cut back and/or discontinue the supplement.
  • Anxiety: While there is some evidence that omega-3 increases lead to decreased cortisol (a stress hormone), not everyone feels less anxious from fish oil. One side effect that people have reported is an increased level of anxiety. This may stem directly from an increase certain neurotransmitters like acetylcholine.
  • Blood thinning: A common side effect is that of blood thinning, which results in slower clotting should you suffer a cut. This blood thinning effect isn’t generally dangerous unless it becomes excessive, but could be something you monitor if it’s a cause of concern.
  • Burping: Nearly everyone taking fish oil supplements will experience fishy burps throughout the day. This is a result of your body processing the oil and may be more common when taking the supplement on an empty stomach. While these burps may not be pleasant, most people don’t consider them a big deal.
  • Constipation: Some people end up constipated when they take fish oil, especially at high doses. This can be highly uncomfortable and lead to stomach pain. Should you experience constipation from your fish oil, it is recommended to reduce the dosage to see if it improves. If the constipation doesn’t improve, discontinuation may be a smart idea.
  • Depression: There is considerable research supporting the usage of fish oil for depression; EPA-dominant formulations tend to be effective both as a standalone treatment as well as an augmentation strategy. That said, some people notice that instead of helping their mood, fish oil makes depression worse. If you notice your depression is worsening, it may be due to heightened acetylcholine. Discontinue the supplement if you end up feeling more depressed.
  • Diarrhea: Another reaction that some people experience is diarrhea after taking fish oil. This may be a sign of spoiled or a rancid product. Diarrhea is generally a sign that your body doesn’t like whatever you’re giving it. You may want to try a better brand of product or simply stop taking it if you’re running to the toilet all day.
  • Gas: For some people, the fish oil affects their stomach acid and digestive process. This can lead to excessive passing of gas. If you find yourself farting more than usual, recognize that it may be related to the fish oil. Obviously to pin this on the fish oil, it should be occurring consistently rather than the night after you ate a burrito.
  • Headaches: Another side effect that you could experience is a mild or moderate headache. Most people don’t end up with full-blown migraines from their fish oil. A headache could stem from increased stimulation from the omega-3 fatty acids, but it could also be a sign that the fish oil is making you sick.
  • Heartburn: In some people, fish oil exacerbates heart burn, which is highly uncomfortable. If you notice that your heartburn worsens or occurs directly after taking a fish oil supplement, you may want to cut back or avoid it altogether.
  • High energy: Although most people aren’t going to complain about “high energy,” it can be problematic if the energy makes you feel agitated or anxious. Sometimes the increased stimulation from the fish oil makes people feel uncomfortable or edgy.
  • Insomnia: The increase in energy and perhaps anxiety as a result of the supplementation may lead to anxiety. Increases in concentrations of stimulatory neurotransmitters may play a role in detrimentally affecting your sleep cycle. Some people have reported that their sleep quality declined after they began fish oil.
  • Nausea: Another side effect that some people experience is that of nausea. While nausea can easily result from taking rancid fish oil or an expired product, it may also be a natural response to disliking the taste of the oil. If you start to feel increasingly nauseous, fish oil in another format (e.g. capsules) may be preferred over the liquid.
  • Stomach aches: You may notice that fish oil causes you to develop a stomach ache. This is due to the fact that it may affect your digestive process. Taking it on an empty stomach is more likely to cause a stomach ache than taking it with food. Stomach aches can be minimized by reducing the amount of fish oil that you’re taking and making sure the product is quality.
  • Taste changes: If you notice a fishy or unpleasant taste in your mouth, it may be related to the supplementation. While fish oil doesn’t generally taste pleasant, some people find that even after swallowing, their taste seems funky for the rest of the day. This effect can be mitigated by eating other foods around the time you’re taking the oil. If the fish oil tastes noticeably bad – understand that it may be rancid (if it smells bad, it’s rancid).
  • Tiredness: If you feel fatigued as a result of taking fish oil, it could be that you’re taking a low quality brand or that your product is spoiled. Excessive tiredness and lethargy aren’t commonly associated with fish oil. Also consider that you may feel tired in early stages of supplementation as your brain adjusts. Obviously if the tiredness becomes chronic, discontinue the product; if the fish oil caused it, your energy level should bounce back.

Fish Oil Adverse Reactions

As with any supplement, it is important to be aware of potential adverse reactions. These tend to occur among individuals with allergies to seafood, but may also occur if the individual is taking another drug or supplement along with the fish oil. To minimize risk of adverse effects, talk to your doctor about contraindications (such as with anticoagulants).

  • Appetite loss: If you started taking fish oil and completely lost your appetite, this may be a sign of an adverse effect. While some people have claimed that fish oil helped them lose weight, they don’t generally report a drastic change in appetite.
  • Bloody nose: Due to the blood thinning properties of fish oil, people have an increased susceptibility to bleeding. If you notice that you’re getting more bloody noses than usual, you may want to stop taking the fish oil. This is a relatively uncommon reaction, but can occur.
  • Body aches: Some people claim that fish oil acts as a lubricant for joints and helps ease pain. However, some people actually report a counterintuitive reaction of body aches. You may notice aches in your lower back, chest, or muscles from taking fish oil.
  • Chest pain: If you notice chest pain after you start taking fish oil, seek immediate medical attention. This is not something to ignore as some people find that fish oil causes changes in heart beats. Heart beat dysfunction is associated with chest pain and negative health outcomes.
  • Chills: If you’ve become noticeably chilled or feel like your body temperature is fluctuating while on the fish oil, you should probably discontinue. Although most people don’t notice any major shift in body temperature, chills have been reported as an adverse response.
  • Difficulty breathing: Those that have breathing problems from supplementation may have an allergic reaction to the contents of the fish oil. Should you ever have a tough time getting proper oxygen or notice that something feels “off” with your breathing process, the fish oil may be to blame.
  • Dizziness: Some people become dizzy when they start taking fish oil. This may be due to neurological changes, but may also be a result of an adverse reaction (e.g. getting sick). If dizziness or vertigo becomes extreme, the supplementation should be discontinued.
  • Fever: Another rare reaction that some people experience is a fever. If your temperature shoots up after you began taking fish oil, it may be more than a coincidence. People who aren’t able to tolerate fish oil may develop a minor (or modest) fever.
  • Flu-like symptoms: Experiencing dizziness, headaches, nausea, and temperature changes can make it seem like you have the flu. Some people report “flu-like” symptoms as a reaction to fish oil. This is an extremely uncommon reaction, but most often occurs among those who are allergic or are taking a contaminated product.
  • Irregular heartbeat: Many consider fish oil to be helpful to the heart, but in rare cases it can cause uneven heartbeats. If you notice an irregular heartbeat after you started taking fish oil, it is important to realize that others have had this same experience.
  • Itchiness: Generally itchiness is a sign that you are allergic to something within the product. Itching is caused with increased production of histamine, which is a signal from your body that it isn’t able to tolerate whatever you ingested. This may be accompanied by the appearance of hives.
  • Low blood pressure: Another adverse reaction is that of low blood pressure. If you notice a significant dip in your blood pressure, you may be prone to lightheadedness, dizziness, and/or fainting.
  • Significant blood thinning: While a little blood thinning may not be detrimental to your health, significant blood thinning can be problematic. If you are on high doses of fish oil, it will thin your blood. The blood thinning effect can cause clotting to slow should you ever endure a cut; potentially contributing to a medical emergency.
  • Skin rash: Another sign that you should discontinue fish oil is if you develop a skin rash. A rash resulting from fish oil supplementation may be mild or severe. Some people experience itchiness along with the rash.
  • Sweats: If you notice that you’re breaking out in hot or cold sweats more than usual, this may be a sign of an adverse reaction. Generally an increased in sweats is accompanied by other unwanted effects.
  • Swelling: Those who are allergic to shellfish and fish oil products may notice that certain parts of their face are prone to swelling. Swelling is an obvious sign of an allergic reaction to the contents of your fish oil supplement.
  • Vomiting: You may find yourself vomiting if you aren’t able to tolerate fish oil. Those who end up vomiting likely experienced an allergic reaction, were taking a rancid product, or one that contained contaminants. It is important to verify the purity of your fish oil and make sure it hasn’t expired before supplementing.

Note: Should you experience any adverse reactions or suspect a potential adverse reaction, seek immediate medical attention and discontinue the fish oil.

Fish Oil: Weighing the Benefits vs. Side Effects

Those with an allergic reaction to seafood should avoid taking fish oil – this is common sense. However, others may notice that they don’t respond well to fish oil supplements or products containing omega-3 fatty acids. For some people, the increase in both DHA and EPA can result in adverse neurological reactions such as: heightened levels of anxiety and increased depression. Therefore if you notice that fish oil is making you feel “worse” than before taking it, discontinuation should be advised.

Others may have a more difficult time deciding whether it is healthy to supplement fish oil. Your best bet is to get a panel of blood tests done along with other medical evaluations to determine your current state of health and get a “baseline.” After supplementing fish oil for awhile, you’ll want to compare yourself to the original baseline to determine whether any measures of health have improved (or regressed).

Some people don’t experience any side effects from fish oil supplements, but don’t seem to get much benefit. In this case, the person may want to consider continuing to supplement to maintain healthy omega-3 to omega-6 dietary ratios. In other cases, a person may not feel that supplementation is necessary and may wish to discontinue after experiencing no noticeable benefit.

There will also be others who experience one or two unpleasant side effects such as “fishy burps” or agitation, but the fish oil may help improve their mood or lower triglycerides. In this case, the minor side effects may be worth putting up with because a more important measure of health has improved. Regardless of your situation, always weigh the benefits you’ve gotten from fish oil with any side effects you’ve experienced.

If you have a difficult time determining whether to continue supplementation, talk to your doctor and get a professional opinion. Have you experienced any side effects from taking fish oil? If you’ve experienced noticeable side effects that you can pinpoint directly to the fish oil, be sure to share them with others in the comments section below. Feel free to discuss how much you were taking, how long you took it, and the brand (to help others get an accurate depiction of your situation).

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{ 11 comments… add one }
  • Ana August 7, 2018, 9:40 pm

    I been taking ON Fish Oil. I think I overdid it and was taking 4000mg a day. Well the side effect I am getting is my body sweat smells like rotted fish. I smell like fish when I sweat. I stopped taking fish oil entirely since my body isn’t processing it. It’s really embarrassing. Any solution to this?

  • Jake July 5, 2018, 2:24 am

    Fish oil seems to raise acetylcholine levels and cause AFIB.

  • Barbara May 29, 2018, 10:46 pm

    After I declined cataract surgery, the eye guys recommend DHA Xtra 1660 but when I started to experience dull headaches, weakness some blurry vision, and yet I thought it was my BP but BP was normal, I had no idea, then it occurred to see what side effects Omega 3 had! I discovered I was having the same side effects.

  • Jennifer May 25, 2018, 2:43 am

    I had heard that fish oil was good for your heart so I bought some over the counter fish oil. At first I did not experience any side effects, and in fact, I noticed any sporadic hearth related pain went away once I started taking the fish oil.

    After two weeks of taking the fish oil (either daily or every other day) I noticed that my legs got real fatigued and sore. It got to the point where after work I had to lie down and prop my legs ups on pillows to get relief. I take other medications and have been for years, so I figured it was the fish oil that caused my leg fatigue.

    After I stopped taking it, after about a week to a week and a half, the fatigue and soreness went completely away. I’m figuring it either was a reaction purely from the fish oil only or the fish oil interacted with existing medication that I am on. I stopped taking the fish oil.

  • Karen March 16, 2018, 9:38 am

    I started taking Omega 3 fish oil capsules 3 weeks ago. In the last week I have suffered with diarrhea, severe headache, nausea causing lack of appetite and increased anxiety, fatigue but then poor sleep at night and struggling to get to sleep.

    I had also started taking Vitamin D supplements just over a week ago. Two days ago I stopped taking both as I could not link any other reason why I was suffering with these symptoms. How long will it take for these symptoms to cease if they are caused by the vitamin supplement?

  • brian karchut December 19, 2016, 9:12 pm

    I am having dreadful symptoms of withdrawal from escitalopram. I am 72, and was on a dose very much higher than appropriate for my age for around four years. I weaned off it, stopping two weeks ago but the flu-like symptoms and brain zaps are unrelenting, if not actually getting worse. My reading indicates that these symptoms could take months or even years to go away.

    I also have meibomian gland dysfunction which blurs my vision. My ophthalmologist recommended 2 gm of 3- omega fish oil daily, and I use a very high quality source. Now I read that it can have numerous neurological and psychiatric side effects, and wonder if it may be aggravating symptoms of escitalopram withdrawal. Should I stop the fatty acid supplement all at once?

  • Cecelia Ray September 17, 2016, 3:32 am

    I have been told by my NP that I needed to take high dose of fish oil due to my bloodwork. I have noticed that I am sweating more and have more body odor than normal. This site has helped me understand why.

  • David Dennison August 2, 2016, 4:19 am

    I’m overweight and my VA. Dr. Has me on Fish Oil. 1,000mg a day. I am not a normal sweater… Like if I don’t work out then I don’t sweat. Since I have been on these pills. I sweat like I just got out of a shower. Glad I found out why.

    • CJ July 22, 2018, 10:01 am

      Lack of regular sweating can also be due to Dysautonomia (when your autonomic nervous system isn’t working properly). Dysautonomia can present with many different symptoms & treatments, although “extra salt & water” is a tried and trusted stand-by.

      Dysautonomia is usually only well-known in the neurology circle of medicine, so many GPs/PCPs may/do not know much about the subject. Maybe give it a good ol’ Google search and see if any of the signs and symptoms relate to your own experiences.

      Disclaimer: I am *NOT* a licensed medical professional, so the “salt & water” comment I made is something you should discuss with your GP/PCP BEFORE commencing any form of treatment should you believe to have some form of Dysautonomia. I am simply relating my own life experiences to this particular case.

  • Raquel May 16, 2016, 5:15 am

    Hi, This article is so informative and helps me understand that the agitation and worsening insomnia, gas, and diarrhea are probably from the fish oil I started 10 days ago. I recall it happened also more then a year ago when I was also trying fish oil. Thanks a lot for this information!

  • Linda November 17, 2015, 3:29 am

    I have been taking fish oil for 22 days and from day 2 I have had constant horrible gas, bloating, itching and diarrhea. 3 days ago the effects got worse and have continued. Contacted my doc for something to help and he said stop the fish oil immediately. I already had the day the symptoms increased. Does anyone know how long it takes for the symptoms to go away?

    I will never take this stuff again. Doc put me on it to lower cholesterol and lipids but it’s not worth it. I can’t take statins because it gave me a muscular disorder that lasted for 15 years and finally healed by Jesus in February this year. Praise God! Beware of fish oil!

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