Medical-grade Omega-3 capsules could cause irregular heartbeats, European health chiefs have warned.
The heads of the European Medicines Agency (EMA) now say the side effect is 'common' after reviewing decades of evidence.
The safety warning applies specifically to medicines containing omega-3 ethyl esters, which are given to patients with unhealthy levels of fat in their blood to reduce the risk of heart attack or stroke.
Still, atrial fibrillation – the newly designated common side effect – can increase the risk of both conditions.
Some dietary supplements, sold for as little as 8 cents per pill, contain omega-3 ethyl esters. They claim to improve heart and brain health.
The heads of the European Medicines Agency (EMA) now say the side effect is 'common' after reviewing decades of evidence
But the doses are typically much lower than medical grade, which contains up to 4 grams and is very pure.
Experts said the risk from everyday supplements was 'probably quite small'.
Professor Jane Armitage, an honors consultant in public health medicine at the University of Oxford, told MailOnline: 'What you buy over the counter is a much lower dose, usually less than one gram.
'So if there is an effect, it is probably small. But we cannot say that this is not the case. It has not been proven in any way.”
She added: 'There have been several studies with smaller doses, many with less than one gram.
'The data from those studies are consistent with the fact that it is a dose-response effect.
'I don't think it is unreasonable for people to be aware of the fact that there may be an increased risk.
The drugs are given to patients with high levels of fat in their blood to reduce the risk of a heart attack. Survivors also receive the capsules, which can cost as little as 80 cents per capsule. Manufacturers of the drugs – which are different to the omega-3 pills sold on the high street and online – will be told to warn about the risk of atrial fibrillation in leaflets in the pack.
'If the risk is there, it is probably quite small. But we cannot say categorically how or what.'
EMA bosses said the risk of atrial fibrillation appeared highest in patients taking 4 grams a day.
They ruled that treatment should be 'permanently stopped' for anyone who develops an irregular heartbeat while taking it.
Manufacturers of the ethyl ester medicines will be told to warn about the risk in leaflets in the packaging.
The EMA's decision followed a review by a safety committee, which reviewed data from around 80,000 patients.
Omega-3 acid ethyl esters, in addition to dietary changes, can reduce very high triglyceride levels in patients with certain forms of hypertriglyceridemia – a common condition passed down through families.
The fat – the body's main source of energy – enters the blood through food and the liver.
Over time, they are thought to contribute to the hardening of the arteries, or the thickening of the artery walls, which can lead to heart disease.
According to the latest data, around 79,000 prescriptions for omega-3 acid ethyl esters were dispensed in England alone in 2022. NHS facts. Some are available without a prescription.
Dr. Martin Whyte, an expert in metabolic medicine at the University of Surrey, told MailOnline: 'The risk of atrial fibrillation should be taken into consideration when omega-3 fatty acids are prescribed or purchased over the counter.'
This is especially true in people who may be prone to developing this heart rhythm disorder, he added.
Omega-3 supplements have long been touted for improving heart, brain, joint, eye and skin health.
But a large body of research has shown that the evidence that the pills lower the risk of heart disease or stroke is generally insufficient or inconsistent.
Following the EMA's warning, the Medicines and Healthcare products Regulatory Agency (MHRA), which oversees the safety of medicines in Britain, confirmed that it is “carefully considering the evidence” referred to in the announcement.
Dr. Alison Cave, chief safety officer, told MailOnline: 'Patient safety is our top priority.
'We are aware that the EMA has announced that they are adding atrial fibrillation as a common side effect to medicines containing omega-3 acid ethyl esters.
'We want to reassure patients that the benefits of these medicines continue to outweigh the risks.
'We are closely monitoring the safety of all medicines and as part of this we are carefully considering the evidence referred to in the recent EMA announcement.
'We will provide further advice to prescribers and patients as necessary.'
She added: 'Report any suspected adverse reactions to the Yellow Card system. Patients should not stop taking these medicines without first consulting their doctor.”