Taking omega-3 supplements offers "little to no" benefit against cancer, say experts.
Consumption of fish oil supplements is promoted as having a wide range of positive impacts on health.
These are purported to include lowering the risk of strokes, as well as diseases such as cancer and dementia.
But researchers at University of Anglia (UEA) found taking daily supplements will likely have no significant impact on a person's health.
The research relates specifically to supplements, rather than omega-3 derived from eating fish, with experts still suggesting the latter is good for the heart, as well as general health.
More than 100,000 participants were randomised to either consume more omega-3 fats in supplement form, or maintain their usual intake, for at least a year.
Researchers found that if 1000 people took supplements for approximately four years, the actual effects on their health, both positively or negatively, would be minimal at best, the BBC reported.
What is omega-3?
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, brussels sprouts, nuts and seeds and is good source for those on vegan diets
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.
The global market for omega-3 supplements was estimated to be worth around $33bn (£25.6bn) in 2018, according to the National Health Service (NHS).
Dr Lee Hooper, from UEA's Norwich Medical School, said that the study - funded by the World Health Organisation (WHO) - adds to mounting evidence that omega-3 supplements are failing to offer consumers the benefits they advertise.
"Our previous research has shown that long-chain omega 3 supplements, including fish oils, do not protect against conditions such as anxiety, depression, stroke, diabetes or death," he said.
"In fact, we found that they may very slightly increase cancer risk, particularly for prostate cancer."
More than 80 studies in 2019 found no evidence that omega-3 offers health benefits to those with type 2 diabetes.
Similar studies have also suggested that supplements offer little to no protection against heart disease.
With the harmful environmental factors associated with over-fishing, Dr Hooper added that it seems "unhelpful" for people to continue taking supplements that give" little or no benefit".
Dr Hooper added that the environmental damage associated with decreasing fish stocks is not comparative with the minimal health benefits associated with supplements.
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. Some fish contain small amounts of chemicals that may be harmful if eaten in large amounts.
