Scientists found no evidence that the capsules had any effect on progression of the disease.
Some MS patients take omega-3 fatty acids in the belief that they might protect their brain neurons from further damage.
Some multiple sclerosis sufferers take omega-3 to help prevent further damage
Researchers studied 92 patients, half of whom were given omega-3 supplements and half a 'dummy' placebo.
After six months all the patients received beta inferferon injections, a standard MS treatment, which was continued for another 18 months.
Magnetic resonance imaging (MRI) brain scans were used to measure disease activity.
The scientists, led by Dr Oivind Torkildsen, from Haukeland University Hospital in Bergen, Norway, wrote in the journal Archives of Neurology:
'The results from this study did not show any beneficial effects of omega-3 fatty acid supplementation on disease activity in multiple sclerosis as a monotherapy or in combination with interferon beta.'
The findings contrasted with two other studies which reported a possible beneficial effect.
The researchers added there was no evidence that omega-3 supplements were harmful to MS patients or that they interfered with beta interferon treatment.