Taking omega-3 fish oil supplements is often touted as a simple way to protect your heart - but experts say the evidence that it does any good is flimsy at best.
Cochrane researchers looked at trials in over 100,000 people and found little proof that it prevented heart disease.
They say the chance of getting any meaningful benefit from taking omega-3 is one in 1,000.
Eating oily fish, however, can still be recommended as part of a healthy diet.
The review mainly looked at supplements rather than omega-3 from eating fish. Experts still believe the latter is good for the heart as well as general health.
The NHS says people should try to eat two portions of fish per week, one of which should be oily fish, such as salmon, fresh tuna or mackerel, to get enough "good" fats.
Omega-3 is a family of fats that includes:
- ALA (alpha-linolenic acid) - which the body can't make for itself but is found in vegetable oils, nuts and seeds
- EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) - which the body can make from ALA but are also present in oily fish and fish oils, including cod liver oil
Some brands of milk, yoghurt, bread and spreads have extra omega-3 (usually ALA) added to them.
But when it comes to fish oil supplements, Cochrane lead author, Dr Lee Hooper, from the University of East Anglia, said: "We can be confident in the findings of this review which go against the popular belief that long-chain omega-3 supplements protect the heart.
"This large systematic review included information from many thousands of people over long periods.
"Despite all this information, we don't see protective effects.
"The review provides good evidence that taking long-chain omega-3 [fish oil, EPA or DHA] supplements does not benefit heart health or reduce our risk of stroke or death from any cause.
"The most trustworthy studies consistently showed little or no effect of long-chain omega-3 fats on cardiovascular health."
Some fish contain small amounts of chemicals that may be harmful if eaten in large amounts.
Shark, marlin and swordfish may contain small amounts of mercury and should be avoided by women who are pregnant or planning a baby and by all children under 16.
Other groups should eat no more than one portion of these fish each week.
Prof Tom Sanders, a nutrition expert at King's College London and honorary director of Heart UK, said: "Current dietary guidelines to prevent cardiovascular disease encourage fish consumption, rather than taking supplements.
"This study provides no evidence to suggest that this dietary advice should change."
Prof Tim Chico, a cardiologist from Sheffield University, said: "There was a period where people who had suffered a heart attack were prescribed these on the NHS. This stopped some years ago.
"Such supplements come with a significant cost, so my advice to anyone buying them in the hope that they reduce the risk of heart disease, I'd advise them to spend their money on vegetables instead."
Dr Carrie Ruxton, from the Health and Food Supplements Information Service, said early studies of omega-3 fats had found a protective benefit for the heart, but it wasn't always easy to pick up the modest effects of dietary change, particularly in older people on medication.
"For those who won't eat mackerel, salmon or herring, a daily fish oil supplement is a useful way of meeting recommendations," she said.
"Omega-3s are also used by the body to maintain the health of the eyes, immune function and brain so it's not all about the heart."