The principal fatty acids in fish oil are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Fish oil has been reported to have anti-inflammatory and immunosuppressive effects, and EPA competitively inhibits synthesis of thromboxane A2. Both EPA and DHA interfere with prostaglandin synthesis in platelets and blood vessels, which results in decreased platelet activity. The beneficial effect of dietary fish oil on plasma triglycerides is thought to be related to the high quantities of omega-3 polyunsaturated fatty acids found in many types of fish.

Australia's Heart Foundation recommends consumption of fish (either fresh or canned) two or three times a week for all patients, whether for primary or secondary prevention, as part of an overall healthy eating plan.

For patients with hypertriglyceridaemia, fish oil capsules can be used. Given the variability between fish oil products, there is a need to confirm the dosage based on the content of fatty acid in each preparation. The usual strength fish oil contains 180 mg EPA and 120 mg DHA per mL or 1000 mg. The usual dose of this fish oil concentrate is 4 to 12 capsules per day (half this number for double strength capsules).

EPA and DHA have anti-inflammatory properties when taken in doses greater than 2.7 g daily, and are of proven benefit in rheumatoid arthritis—including reduced morning stiffness, decreased tender joint count, and a lessened need for treatment with NSAIDs.

The required dose of omega-3 fish oil can be taken using a large number of fish oil capsules (approximately 14 standard fish oil capsules containing oil from eviscerated fish bodies [30% omega-3] daily); however, the cost of these capsules can be prohibitive. A less expensive alternative is to take about 15 mL of bottled fish body oil, daily.

Palatability of fish oil can be improved by using a ‘two-glass technique’ [Note 1]. This involves floating the dose of fish oil on 30 mL of fruit or vegetable juice in a small glass. The contents are swallowed without contact with the lips, avoiding the fish oil taste. This is followed by slowly sipping a further 40 mL of juice (from a separate glass) to rinse the mouth. Reflux of the fish oil can be avoided by taking it immediately before a solid meal—and without any extra fluid—to encourage mixing of the fish oil with the food and its transport from the stomach into the small intestine. Passage of the oil from the stomach can also be facilitated by lying on the left side for 15 minutes after the dose, or by dividing the daily dose and having each half immediately before the morning and evening meals.

Other benefits of using fish oil in arthritis include amelioration of renal toxicity and hypertension in patients taking cyclosporin, and cardiovascular protection (in the setting of potentially increased risk).

Cod liver oil is not recommended as a source of omega-3 because it contains cholesterol and, in the dose required, potentially toxic amounts of vitamins A and D.