By Zawn Villines

Reviewed by Dena Westphalen, PharmD

Fish oil is available as a supplement that manufacturers produce from fish. However, the benefits of fish oil are not always clear, and it may have unexpected side effects.

Certain fish and the oil from them contain healthful fats that health experts recommend people regularly consume in their diets.

The omega-3 fatty acids from fatty fish and shellfish may have a role in:

• reducing the risk of heart disease
• easing symptoms of some forms of arthritis
• lowering the risk of dementia

The United States Department of Agriculture’s Dietary Guidelines for Americans 2015–2020 recommend eating at least 8 ounces of seafood per week because of these benefits.

Research shows a link between eating fish and health, but studies of fish oil supplements often fail to find such clear benefits.

Read on to find out more about fish oil, the side effects of fish oil supplements, how much is too much, and some potential risks.

Side effects of fish oil

The side effects a person may experience from fish oil depend on several factors.

These include the person’s overall health, whether they take any medications, and if they have any risk factors for fish oil complications.

Most people who take fish oil supplements do not experience any serious side effects.

It is best to talk to a doctor before taking fish oil supplements, especially if using it for a specific medical condition.

Bad taste or smell

Fish has a distinctive odor, and so does fish oil. Some people report that fish oil tastes bad or leaves an unpleasant taste in their mouth. Others say it causes bad breath or makes their sweat smell bad.

These side effects are the most common ones that people may associate with fish oil, though there is no evidence that they cause lasting harm.

Bleeding

Fish oil is a natural anticoagulant, which means it can prevent the blood from clotting.

This property may help explain some of its heart health benefits, since thinning the blood may improve cardiovascular health.

Omega-3’s may increase bleeding risk when a person takes them with specific anticoagulant or medication.

However, a 2017 systematic review of 52 previous studies found that fish oil did reduce blood clotting but did not increase bleeding risk in healthy people.

So, people using blood thinners, such as warfarin, should not take fish oil or other omega-3 fatty acid supplements because of the increased risk of dangerous bleeding.

Gastrointestinal symptoms

As with many other supplements and medications, some people experience gastrointestinal problems after taking fish oil. Symptoms might include:

• nausea
• diarrhea
• constipation
• vomiting

Sometimes, lowering the dosage or taking fish oil with food can help. In other cases, a person may need to stop using fish oil supplements.

Less frequently, fish oil may cause bleeding in the stomach or intestines and may cause or worsen ulcers. This could be because fish oil tends to thin the blood, increasing bleeding.

These serious side effects are more likely with high doses of fish oil, or when a person takes the supplement with other drugs.

A 2014 case study focuses on a 60-year-old amateur athlete who consumed 20 grams (g) of omega-3 fatty acids daily. After adding antibiotics and cortisone to the regimen, they developed a bleeding ulcer, even though they had no previous gastrointestinal issues.

The authors of the study said that further work was needed to prove the cause.

Allergic reactions

A person may develop an allergy to any food or supplement, including fish oil.

People with fish or shellfish allergies may be more vulnerable to allergic reactions to fish oil. They should consult their doctor before taking fish oil supplements.

Prostate cancer

There is mixed evidence about fish oil and prostate cancer.

Some studies have suggested there may be a link between fish oil and prostate cancer risk, while others have come to the opposite conclusion.

A 2013 study of 2,268 older men found that fish oil might slow the progression of prostate cancer. On the other hand, men who ate significant amounts of salted or smoked fish were more likely to develop prostate cancer.

Overall, the researchers found no correlation between eating fish in midlife and a person’s prostate cancer risk.

Fish oil dosage and safety

There are no specific recommendations on the amount of omega-3 fatty acids a person should take. It depends on a variety of factors, such as their age and their state of health.

Most studies of fish oil have looked at small doses of a few grams (g) per day. Larger doses, such as 20 g, per day, may cause more side effects.

People can start with a small amount each day and talk to a doctor before increasing the dosage.

If someone notices an unpleasant smell or other minor side effects, they may want to decrease the dosage to see if that helps with the issue.

Anyone who develops serious complications, such as an allergic reaction, rash, vomiting, or breathing difficulties, should stop taking fish oil and seek emergency help.

Summary

A 2015 National Institutes of Health study estimate that 7.8% of people in the U.S. take fish oil supplements. Most experience no serious side effects. Some may even experience significant health improvements.

As well as the cardiovascular and brain health developments fish oil may offer, some research suggests that fish oil may support the development of fetuses during pregnancy. A 2018 study links fish oil supplements during pregnancy to a lower risk that a child will develop allergies.

While data pointing to the benefits of fish oil may seem positive, it is not always conclusive. People who want to improve their health with omega-3 fatty acid supplements should consider adding fish to their diet instead, as there is more research on the benefits of fresh fish.