Does Fish Oil Have Side Effects?

Fish oil is one of the most commonly used supplements. It is a potent source of omega-3 fatty acids, nutrients associated with many anti-inflammatory, health-promoting benefits. 

Specifically, fish oil contains docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), two omega-3 fatty acids with health benefits, including supporting a healthy heart and brain.  

However, like all supplements and medications, fish oil supplements can produce side effects.

Common Side Effects

Although generally safe, fish oil supplements can have side effects, some of which may impact your health. Some side effects may depend on the dosage of fish oil, with more intense effects at higher doses.

Common, more mild side effects of fish oil supplements include the following:

• Fishy-smelling sweat: Due to compounds in the fish oil excreted through your sweat glands.
• Gastrointestinal symptoms: Burps, fishy or unpleasant taste in the mouth, heartburn, nausea, vomiting, constipation, and diarrhea.
  
• Headaches: Possibly due to compounds in fish oil supplements and their effects on the vascular system. Notably, studies show that omega-3 supplements may also prevent headaches.

Other side effects include the following:

• Blood sugar changes: Studies yield mixed results on the influence of fish oil on blood sugar (blood glucose). One analysis observed increases in fasting blood sugar with high-dose fish oil supplements. However, it may improve insulin sensitivity in specific populations.
• Decreased blood pressure: One large analysis observed reductions in blood pressure with 2 to 3 grams (g) of fish oil daily. While beneficial for those with high blood pressure, it may cause problems for those with low blood pressure.
• Delayed blood clotting: How quickly your blood clots is essential for stopping bleeding and healing from cuts, scrapes, and other injuries. Fish oil supplements may delay blood clotting, especially with blood thinner medications. You may also need to consider this if you're undergoing surgery.
• Increased low-density lipoprotein (LDL) cholesterol: Some studies suggest some fish oil supplements raise LDL cholesterol. Although a modest increase, it may make the difference between normal and elevated cholesterol values.
  

Less Common but Serious Side Effects

Fish oil supplements may cause more severe side effects in some individuals, including the following:

• Atrial fibrillation (A-fib): Studies suggest regular fish oil supplementation may be associated with a higher risk of atrial fibrillation in healthy people, those at high risk of A-fib, and those with current, specific heart conditions. Higher dosages may carry a higher risk. On the other hand, regular fish oil use may delay heart disease and reduce major cardiac events (ex., heart attack) in people with specific heart issues. Further study is needed to confirm these findings. 
  
• Prostate cancer: Some research indicates higher circulating levels of omega-3 fatty acids from supplements may increase the risk of prostate cancer. Eating lots of fatty fish is not associated with this side effect.
• Reduced immune function: Taking high doses of fish oil for extended periods (more than several weeks) may reduce immune function because of the anti-inflammatory effects of omega-3 fatty acids. High doses are greater than 900 mg of EPA and more than 600 mg of DHA.
  

Factors Influencing Side Effects

Several factors influence side effect risk from fish oil, primarily specific pre-existing heart conditions and how much fish oil you take (dosage).

The U.S. Food and Drug Administration (FDA) deems 5 g daily of omega-3 supplements, like fish oil, safe for most people. However, specific doses of fish oil may contribute to more or worse side effects and may differ if you live with specific types of heart disease.

For example, the risk of A-fib associated with fish oil may be dose-dependent, with higher doses increasing risk. Some research suggests daily dosages of over 4 g may increase the risk of A-fib in people with specific heart conditions or at high risk of A-fib. 

Also, consuming over 3 g of fish oil a day may increase the risk of excessive bleeding.

Keep in mind that while higher doses may be harmful to some people, they may benefit others.

To minimize adverse effects, start with a small dosage and gradually increase to the amount recommended by your healthcare provider or registered dietitian nutritionist. 

Precautions

Specific populations may need to use additional caution when considering fish oil.

You should also avoid fish oil if you're allergic to it or its components (parts).

Your healthcare provider may suggest that you stop taking fish oil supplements before surgery due to their potential blood thinning effects. Interestingly, though, recent research contradicts this long-standing recommendation.

Fish oil and fish liver oil are two different types of supplements. Although both are significant sources of omega-3s, fish liver oil naturally contains vitamins A and D. High doses of either can be toxic. Additionally, taking high doses of vitamin A from fish liver oil or other sources when trying to get pregnant or during pregnancy can harm your fetus.

DHA and EPA are recommended during pregnancy to support fetal brain development and reduce the risk of preterm birth. Taking regular fish oil supplements and avoiding fish liver oil may be a safer alternative.

Check with your healthcare provider for additional guidance.

Drug Interactions

Fish oil may interact with the following medications:

• Blood-pressure-lowering medications: Since fish oil can lower blood pressure, taking them with medications used to treat high blood pressure may enhance these effects.
• Blood-sugar-lowering medications: Given that fish oil has mixed results on blood sugar, it's best to use it cautiously if you have diabetes and are taking medications that impact your blood sugar.
• Blood-thinning medications (anticoagulants) and nonsteroidal anti-inflammatory medications (NSAIDs): Taking these medications with fish oil can cause problems with bleeding. If you regularly take anticoagulants or NSAIDs, speak with your healthcare provider before starting a fish oil supplement.
• Medications to reduce cholesterol levels (statins): These medications may influence the side effects of fish oil supplements. Moreover, statins may influence the role of fish oil in preventing or reducing the risk of heart disease.

Consult your pharmacist or healthcare provider for more information about fish oil interactions.

Eating Fish as a Safe Alternative

As long as you are not allergic, eating fish is a safe and effective way to consume omega-3 fatty acids. Unlike fish oil supplements, there are minimal safety concerns when eating fish.

Good sources of omega-3-rich fish include salmon, sardines, mackerel, and anchovies. Aside from omega-3s, fish provide other essential nutrients like selenium, iodine, and zinc. Regular fish oil supplements may lack these additional vital nutrients.

Avoid consuming mercury-heavy fish like king mackerel, shark, tilefish, and swordfish. 

Summary

Fish oil is among the most commonly used supplements for its purported health benefits, namely its effects on heart health.

Its side effects range from gastrointestinal issues to more severe effects like delayed blood clotting and reduced immune function. However, for most individuals, the side effects of fish oil supplements are minimal.

So, any benefit may outweigh the risks associated with fish oil supplements. Still, always consult a healthcare provider before picking a new fish oil supplement to ensure it is safe and effective for you.