This information is for people who have Crohn's disease. It tells you about fish oil supplements, a treatment used for Crohn's disease. It is based on the best and most up-to-date research.
Do they work?
We're not sure. Different studies give different answers. One study found that fish oil capsules helped to prevent flare-ups of Crohn's disease symptoms. But in other studies, fish oil didn't help.
What are they?
Fish such as trout, mackerel, sardines, and salmon have a lot of oil in them. You can get oil from these fish as capsules. Fish oil contains substances called omega-3 fatty acids.
You can buy fish oil capsules over the counter from pharmacies, health food shops, or supermarkets.
How can they help?
We don't know whether fish oil supplements can help. There has been one positive study, but other research has found that supplements don't prevent flare-ups of Crohn's disease.
The positive study found that fish oil capsules helped people to go longer without getting symptoms of Crohn's disease. After a year: 
One review of the research ( systematic review) found there wasn't enough evidence that fish oil capsules helped people to go longer without getting symptoms of Crohn's disease.  Another study also found that fish oil capsules didn't help.  About 3 in 10 people had no symptoms of Crohn's a year after the start of the study. It didn't matter whether they'd taken fish oil or a placebo. Two more recent studies have also had negative results. 
How do they work?
Some studies have found that people with Crohn's disease have lower amounts of omega-3 fatty acids in their bodies.  So, it's possible that taking supplements to boost your levels could have benefits.
It may also be that omega-3 fatty acids have an effect on your immune system, which could help to reduce the inflammation in your bowel. 
However, at the moment these ideas are just theories. There's no evidence to prove that fish oil can help people with Crohn's disease.
Can they be harmful?
In one study, 1 in 10 people taking fish oil capsules got diarrhoea. 
In another study, 14 in 100 people got fishy-smelling burps or heartburn.  Some fish oil capsules have a coating that only breaks down in your bowels, not in your stomach. These capsules, called enteric-coated capsules, might be less likely to cause fishy burps.
How good is the research on fish oil capsules?
A few studies have looked at whether fish oil supplements help people with Crohn's disease. There's been one positive study.  However, several other studies have had negative results.    So, it's hard to say yet whether or not fish oil capsules are worth taking.