Fish oils

What is it?

Fish body oil and fish liver oil are rich in omega-3 essential fatty acids, which can help control your immune system and fight joint inflammation. Fish liver oil is also a rich source of vitamin A (a strong antioxidant) and vitamin D (which is important for maintaining healthy joints)

Evidence suggests that fish body oil can improve the symptoms of rheumatoid arthritis. Unconfirmed evidence also suggests a combination of fish body and liver oils might also be useful in the long term, particularly in reducing the use of non-steroidal anti-inflammatory drugs (NSAIDs). There isn’t enough evidence for the use of fish liver oil for osteoarthritis.

  • Family: Nutritional supplement
  • Scientific name: Fish oil (fish body oil and/or fish liver oil)

Fish body oil is made from tissues of fatty fish like sardines, sprat, salmon, and mackerel. Fish liver oil is made by pressing the cooked liver of halibut, shark or, most commonly, cod. Both types are available from high-street retailers and over the internet.

How does it work?

Fish oils are rich in omega-3 essential fatty acids, which have strong anti-inflammatory properties:

  • They significantly reduce the release of several elements that play a part in inflammation from your white blood cells.
  • They form the building blocks for prostaglandins, which regulate your immune system and fight joint inflammation.

Omega-3 fatty acids also play a role in lowering cholesterol and triglyceride levels in your blood, so they can reduce the risk of heart disease and stroke in people with inflammatory arthritis.

Fish liver oil contains high levels of vitamins A and D. Vitamin A is a strong antioxidant (meaning it can prevent cell damage in your body by interacting with harmful molecules called free radicals which are produced within the cells). Vitamin D plays an important part in the production of proteoglycan in cartilage as well as helping to maintain a healthy musculoskeletal system.

Is it safe?

In the UK, dietary guidelines recommend eating two portions of fish a week, including one oily. This works out at about 0.45 g per day of omega-3 fatty acids. Fish oil is considered to be well tolerated at this dose. However, certain environmental chemicals such as methylmercury and polychlorinated biphenyls (PCBs) can contaminate fish supplies and there’s a concern that taking very high doses of fish oil can cause a build-up of these chemicals in the body. This is also a concern for people who eat fish frequently.

At the correct doses, side-effects are usually minor and uncommon. The most common is stomach upsets, but flatulence and diarrhoea may also be experienced. You shouldn’t use fish oil if you take anticoagulants because fish oil can interfere with blood clotting.

It’s important not to take large amounts of fish liver oil because it can give you more than the recommended dietary allowance of vitamin A. Taking too much vitamin A can lead to liver problems and hair loss. It may also harm unborn babies, so you should avoid fish liver oil and vitamin A supplements if you’re pregnant. Fish liver oil that hasn’t been well purified can contain some contaminants (like mercury, and dioxins), which can lead to health problems, but most supplement companies test fish liver oil for purity before it become publicly available.

Fish oil trials for rheumatoid arthritis

Fish body oil

Data from 10 trials from 1985 onwards have been combined in a report to assess the potential therapeutic effect of fish body oil in rheumatoid arthritis. A more recent review article gave an overview of results from 17 RCTs into the same subject.


The quality of the trials included in this report ranged between low and moderate, and results were combined because of the small number of participants.

  • Fish body oil significantly decreased the number of tender joints and shortened the duration of morning stiffness compared to the placebo treatments.
  • It failed to make a significant change in a number of other disease measures (for example grip strength, blood tests for disease activity and the overall disease severity).

Review article

Trials included in this article used daily doses of between 1.6–7.1 g (average 3.5 g) omega-3 fatty acids

The evidence suggests that fish oil supplements were generally well tolerated and significantly reduced the following:

  • joint pain
  • the duration of morning stiffness
  • fatigue time
  • the number of tender or swollen joints
  • the use of painkillers.

Fish liver oil

In this 9-month trial, 97 people with rheumatoid arthritis were randomly selected to receive either 10 capsules of SSMO1, which contain both fish liver oil (1 g per capsule) and fish body oil, or 10 placebo capsules once a day.

  • There was no difference in outcome after 12 weeks of the trial, but participants given SSMO1 had a modest improvement in pain after 24 and 36 weeks compared to the placebo group.
  • 39% of the active treatment group reported a significant reduction in their daily NSAID need compared to just 10% in the placebo group (this was a significant difference).
  • Approximately 65% of participants in the fish liver oil group completed the trial, compared to 54% of the placebo group. Withdrawal from the trial wasn’t put down to side-effects, but it might have been related to the large number of capsules participants were asked to take every day.
  • In those who completed the trial, there was no significant difference in the number or type of side-effects reported, most of which were mild and gastrointestinal in nature.
  • Because we can’t tell whether the results were caused by the fish liver oil, the fish body oil or the combination of the two, we haven’t been able to make a conclusion about the use of fish liver oil to treat rheumatoid arthritis based on this RCT alone.

Fish oil trials for osteoarthritis

In this trial of fish liver oil, 86 people with osteoarthritis were randomly allocated to receive 10 ml of either cod liver oil or olive oil once a day for 24 weeks. Participants in both groups were asked to continue taking their regular NSAIDs through the trial period.

  • There was no significant difference between the two treatment groups in the amount by which the participants’ pain and disability changed during the study.
  • Both treatments failed to significantly reduce pain and disability symptoms of osteoarthritis.
  • Approximately 70% of participants in the fish liver oil group completed the 24-week trial (compared to 79% who were given olive oil), but side-effects from treatments were not the main reasons for withdrawal; similar proportions of participants in the fish liver oil (30%) and olive oil (24%) groups reported minor side-effects, including stomach upsets and dry skin.