Medical-grade omega-3 capsules may cause irregular heartbeats, European health chiefs have warned.
The European Medicines Agency (EMA) now says the side effect is 'common' after reviewing decades of evidence.
The safety alert applies specifically to medicines containing omega-3 ethyl esters, given to patients with unhealthy amounts of fat in their blood to slash their odds of suffering a heart attack or stroke.
Yet atrial fibrillation - the newly-designated common side effect - can increase the risk of both ailments.
Some nutritional supplements, sold for as little as 8p a pill, contain omega-3 ethyl esters. They claim to boost heart and brain health.
European Medicines Agency (EMA) chiefs now say the side effect is 'common' after reviewing decades of evidence
But doses are typically much lower than medical-grade varieties, which have up to 4g and are highly purified.
Experts said that, if there was a risk from everyday supplements, it was 'probably quite small'. Professor Jane Armitage, an honorary consultant in public health medicine at the University of Oxford, told MailOnline: 'What you buy over the counter is a much lower dose, typically less than one gram.
'So if there is an effect, it's probably small. But we can't say that it doesn't. It's not proven one way or another.'
She added: 'There have been several trials with smaller doses, many of them with less than one gram.
'The data from those trials is consistent with it being a dose response effect. 'I think that it's not unreasonable for people to be aware of the fact that there could be an increased risk.
The drugs are given to patients with high levels of fat in their blood to cut their risk of suffering a heart attack. Survivors are also given the capsules, which can cost as little as 80p a capsule. Manufacturers of the drugs — which are different to omega-3 pills sold on the high street and
Theanex Test online — will be told to warn of the risk of atrial fibrillation in leaflets put inside packaging