This information is for people who have high blood pressure. It tells you about fish oil supplements. It is based on the best and most up-to-date research.
Do they work?
If you have high blood pressure, taking a fish oil supplement every day may help to lower it.
You will need to take 3 to 4 grams of fish oil every day to have a small effect on blood pressure. This is a high dose and you might find it difficult or unpleasant to take this much. Too much fish oil can cause stomach problems and bad breath. It can also make your mouth taste fishy.
Having high blood pressure increases your chances of having a heart attack or a stroke. We don't know if taking fish oil supplements will reduce this risk.
What are they?
Fish oil supplements are capsules that contain the omega-3 fatty acids called DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid). These are the same as the fatty acids found in oily fish such as trout, mackerel, sardines, and salmon. Omega-3 fatty acids are also called n-3 fatty acids.
How can they help?
Taking about 3 to 4 grams of fish oil supplements every day may lower your blood pressure. Studies show that taking these supplements can lower the top number of your blood pressure (the systolic pressure) by an average of 3.7 points and the bottom number (the diastolic pressure) by 2.5 points.  (To find out more about the numbers, see Understanding your blood pressure reading.)
Studies don't show whether taking fish oil supplements will reduce your risk of getting heart disease or having a stroke.
The studies used different mixtures of oils. So we can't say for sure if one type of fish oil supplement works better than another.
How do they work?
We're not sure how fish oils work in the body to lower blood pressure. But doctors became interested in fish oils after they noticed that people who eat lots of fish have lower blood pressure and are less likely to have heart disease.
Fish oils may help keep blood vessels stretchy. And this may make it easier for blood to flow through them.
Can they be harmful?
About a third of people taking 3 grams of fish oil a day get stomach pain, wind, bad breath, or an unpleasant fishy taste in their mouth. 
How good is the research on fish oil supplements?
We found one big summary of the research (called a systematic review) that looked at 36 studies.  Together, these studies looked at about 1,000 people with high blood pressure. Taking an average of 3.7 grams of fish oil did help to reduce blood pressure. But it might be hard to take this amount of fish oil for a long time.