Fish oil is well known for its wealth of health-promoting properties.
Rich in heart-healthy omega-3 fatty acids, fish oil has been shown to reduce blood triglycerides, relieve inflammation and even ease symptoms of conditions such as rheumatoid arthritis (1).
However, more fish oil is not always better, and taking too high a dose may actually do more harm than good when it comes to your health.
Here are nine potential side effects that can occur when you consume too much fish oil or omega-3 fatty acids.
Some research shows that supplementing with high amounts of omega-3 fatty acids could increase blood sugar levels in people with diabetes.
One small study, for example, found that taking 8 grams of omega-3 fatty acids per day led to a 22% increase in blood sugar levels in people with type 2 diabetes over an eight-week period (2).
This is because large doses of omega-3s can stimulate the production of glucose, which can contribute to high levels of long-term blood sugar levels (3).
However, other research has turned up conflicting results, suggesting that only very high doses impact blood sugar.
In fact, another analysis of 20 studies found that daily doses of up to 3.9 grams of EPA and 3.7 grams of DHA — the two main forms of omega-3 fatty acids — had no effect on blood sugar levels for individuals with type 2 diabetes (4).
Summary Taking high doses of omega-3 fatty acids can stimulate glucose production, which may lead to increased blood sugar — though the scientific evidence is not conclusive.
Bleeding gums and nosebleeds are two of the hallmark side effects of excess fish oil consumption.
According to one massive review of 52 studies, fish oil can inhibit blood clot formation in healthy adults, which may increase the risk of bleeding (5).
A study of 56 people had similar results, finding that supplementing with 640 mg of fish oil per day over a four-week period decreased blood clotting in healthy adults (6).
Additionally, another small study showed that taking fish oil may be linked to a higher risk of nosebleeds, reporting that 72% of adolescents taking 1–5 grams of fish oil daily experienced nosebleeds as a side effect (7).
For this reason, it’s often advised to stop taking fish oil prior to surgery and to talk to your doctor before taking supplements if you’re on blood thinners like Warfarin.
Summary Taking large amounts of fish oil can inhibit blood clot formation, which may increase the risk of bleeding and cause symptoms such as nosebleeds or bleeding gums.
Fish oil’s capacity to lower blood pressure is well documented.
One study of 90 people on dialysis found that taking 3 grams of omega-3 fatty acids per day significantly decreased both systolic and diastolic blood pressure compared to a placebo (8).
Similarly, an analysis of 31 studies concluded that taking fish oil can effectively lower blood pressure, especially for those with high blood pressure or high cholesterol levels (9).
While these effects can certainly be beneficial for those with high blood pressure, it can cause serious problems for those who have low blood pressure.
Fish oil may also interact with blood pressure-lowering medications, so it’s important to discuss supplements with your doctor if you’re receiving treatment for high blood pressure.
Summary Omega-3 fatty acids have been shown to lower blood pressure, which may interfere with certain medications and cause problems for those with low blood pressure.
Diarrhea is one of the most common side effects associated with taking fish oil, and may be especially prevalent while taking high doses.
In fact, one review reported that diarrhea is one of the most common adverse effects of fish oil, alongside other digestive symptoms such as flatulence (10).
In addition to fish oil, other types of omega-3 supplements may also cause diarrhea.
Flaxseed oil, for example, is a popular vegetarian alternative to fish oil, but has been shown to have a laxative effect and may increase bowel movement frequency (11).
If you experience diarrhea after taking omega-3 fatty acids, make sure you’re taking your supplements with meals and consider decreasing your dosage to see if symptoms persist.
Summary Diarrhea is a common side effect of omega-3 fatty acid supplements such as fish oil and flaxseed oil.
Although fish oil is known for its powerful effects on heart health, many people report feeling heartburn after starting to take fish oil supplements.
Other acid reflux symptoms — including belching, nausea and stomach discomfort — are common side effects of fish oil due largely to its high fat content. Fat has been shown to trigger indigestion in several studies (12, 13).
Sticking to a moderate dose and taking supplements with meals can often effectively reduce acid reflux and relieve symptoms.
Additionally, splitting your dose into a few smaller portions throughout the day may help eliminate indigestion.
Summary Fish oil is high in fat and may cause acid reflux symptoms such as belching, nausea, indigestion and heartburn in some people.
Hemorrhagic stroke is a condition characterized by bleeding in the brain, usually caused by the rupture of weakened blood vessels.
These findings are also consistent with other research showing that fish oil could inhibit blood clot formation (5).
Further human studies should be conducted to determine how omega-3 fatty acids may impact the risk of hemorrhagic stroke.
Summary Some animal studies have found that a high intake of omega-3 fatty acids could increase the risk of hemorrhagic stroke while other human studies have found no association.
Many people start taking fish oil supplements seeking to shed extra pounds and boost fat burning.
In moderation, some studies have found that fish oil could be beneficial for weight loss.
One study compared the effects of aerobic exercise and fish oil on weight loss and found that both factors independently helped reduce body fat and improve heart health in overweight people (18).
This is because fish oil is high in fat and relatively high in calories, with just one teaspoon (4.5 grams) packing in about 40 calories and 4.5 grams of fat (21).
While this may not seem like much, consuming excess amounts can cause the pounds to pile on quickly.
Summary Fish oil is high in fat and calories, which can contribute to weight gain when consumed in high amounts.
Certain types of omega-3 fatty acid supplements are high in vitamin A, which can be toxic if consumed in large amounts.
Vitamin A toxicity can cause side effects such as dizziness, nausea, joint pain and skin irritation (23).
Long term, it could also lead to liver damage and even liver failure in severe cases (24).
For this reason, it’s best to pay close attention to the vitamin A content of your omega-3 supplement and keep your dosage moderate.
Summary Certain types of omega-3 fatty acid supplements, such as cod liver oil, are high in vitamin A, which can be toxic in large amounts.
Some studies have found that taking moderate doses of fish oil could enhance sleep quality.
One study of 395 children, for instance, showed that taking 600 mg of omega-3 fatty acids daily for 16 weeks helped improve sleep quality (25).
In some cases, though, taking too much fish oil may actually interfere with sleep and contribute to insomnia.
In one case study, it was reported that taking a high dose of fish oil worsened symptoms of insomnia and anxiety for a patient with a history of depression (26).
However, current research is limited to case studies and anecdotal reports.
More research is needed to understand how large doses may affect sleep quality in the general population.
Summary Although moderate doses of fish oil have been shown to improve sleep quality, one case study found that taking large amounts caused insomnia.
Although recommendations can vary widely, most health organizations recommend an intake of at least 250–500 milligrams of combined EPA and DHA, the two essential forms of omega-3 fatty acids, per day (27, 28, 29).
However, a higher amount is often recommended for people with certain health conditions, such as heart disease or high triglyceride levels (30).
For reference, a typical 1,000-mg fish oil softgel generally contains about 250 mg of combined EPA and DHA, while one teaspoon (5 ml) of liquid fish oil packs in around 1,300 mg.
According to the European Food Safety Authority, omega-3 fatty acid supplements can be safely consumed at doses up to 5,000 mg daily (27).
As a general rule of thumb, if you experience any negative symptoms, simply decrease your intake or consider meeting your omega-3 fatty acid needs through food sources instead.
Summary Up to 5,000 mg of omega-3 fatty acids per day is considered safe. If you experience any negative symptoms, decrease your intake or switch to food sources instead.
Omega-3 is an essential part of the diet and supplements like fish oil have been associated with a number of health benefits.
However, consuming too much fish oil could actually take a toll on your health and lead to side effects such as high blood sugar, insomnia and weight gain.
Stick to the recommended dosage and aim to get the majority of your omega-3 fatty acids from whole food sources to get the most nutritional gain.