Fish oil is a common term used to refer to two kinds of omega-3 fatty acids: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These omega-3 fats are usually found in fish, animal products, and phytoplankton.
The fatty acids EPA and DHA are involved in regulating various biological processes such as the inflammatory response, various metabolic signaling pathways, and brain function. They can be synthesized in the body from alpha-linolenic acid (ALA), but in small amounts for most people.
Fish oil causes a potent reduction in triglyceride levels, and a more modest reduction in blood pressure in hypertensives. Despite this, long-term trials haven't found a reduction in the rate of cardiovascular events.
It appears to notably improve mood in people with major depression, though it's unclear if it has an effect in people with minor depression. EPA, in particular, seems to be the most effective omega-3 fatty acid for this purpose which suggests that the effects of fish oil are due to reducing neuroinflammation. Its anti-inflammatory benefits also seem to extend to reducing the symptoms of systemic lupus erythematosus. However, its benefits shouldn't be assumed to extend to inflammatory diseases in general.
Many fish oil supplements may contain harmful lipid peroxides (oxidized lipids that can damage cells), but it's unclear if this has notable consequences to health.
Fish oil reduces fasting triglycerides by reducing the amount of triglyceride-rich VLDL production done by the liver. It also reduces post-meal triglycerides by boosting the rate at which triglyceride-rich chylomicrons are removed. Fish oil may reduce blood pressure by increasing nitric oxide availability and relaxing smooth muscle. It also has anti-inflammatory effects partially due to its ability to affect the shifting balance between inflammatory and anti-inflammatory signaling, which, in turn, affects cell membranes.