Doctors Reveal: 7 Surprising Side Effects of Fish Oil

Fish oil supplements can offer a wide range of health benefits such as boosting brain function, improving heart health, reducing inflammation and more. But, like with any medication or supplement that you’re considering using, before beginning a new regimen you should consult your doctor to weigh the risks and benefits in your particular situation.

‘Yes, fish oil has omega-3 fatty acids and the daily recommended dose does come with some beneficial health effects, but there may be some undesirable side effects, just like any supplement.’ – Farhan Malik, MD, a board-certified doctor in family medicine and sports medicine and owner and senior medical director at Prometheus by Dr. Farhan Malik.

He points out that problems are likely to arise if you’re taking too much of it: ‘With supplements, more is not better; if you are suffering from side effects, it is probably because you are overdosing,’ Malik tells Best Life. ‘You have to be cautious about the dosage and freshness of what you are taking when it comes to fish oil. Always think moderation and remember that quality counts.

Curious what kind of sinister flipside to the fish oil glory might exist? Here are seven side effects you need to think hard about before you decide to take the supplement.

Side Effects of Fish Oil
Side Effects of Fish Oil

‘Fish oil supplements reduce platelet aggregation in people who have no circulatory problems, and there are also studies to see if they should be withdrawn before surgery if there is increased risk of bleeding,’ Jennie Stanford, MD, FAAFP, DipABOM, a triple-board-certified physician in family medicine, obesity medicine and clinical lipidology, tells me. ‘However, the evidence [since] then has not shown any increased risk of bleeding [needing to discontinue its use],’ she adds.

On the other hand, while Malik says bleeding and clotting problems have cropped up in his clinic, he emphasises that the best way to avoid them is to adhere to the recommended dosage listed on the supplement label.

Excess fish oil, he says, can cause bleeding problems. ‘I have had a patient experience bleeding in the nose, or bruising more from taking very high doses of fish oil for a long time.’

Taking fish oil supplements can, if you have a hypersensitive gastrointestinal tract, cause GI upset, says Stanford, a medical expert for Drugwatch.

For some people, fish oil can cause nausea, diarrhoea, bloating or belching with a fishy aftertaste, reports Best Life, adding that ‘these side effects are mild and occur less frequently when you take a coated fish oil tablet or capsule and if you take fish oil supplements with a meal’.

If you’re feeling well, taking high doses of fish oil will improve how your body manages blood sugar. If you’re unhealthy, it can harm your ability to regulate blood sugar.

‘Fish oil supplementation improves the lipid metabolism of patients with dyslipidaemia [an excess of lipids in the blood],’ Stanford told me. That’s important because ‘fasting blood glucose and/or insulin sensitivity are also improved by f​ish​ oil supplementation and are commonly abnormal with dyslipidaemia.’

But while improved glycaemic control is useful for most patients, ‘it may also alter the profile of glucose-lowering drugs by causing hypoglycaemia requiring more intensive monitoring and therapeutic dose adjustments’. Stanford adds.

Another risk of taking fish oil supplements that is not widely advertised is that, like most other supplements, they can interact with medications.

This can be dangerous as fish oil supplementation has been found to decrease clotting for those taking drugs, designers of the study note, and such medications include anticoagulants such as heparin, clopidogrel (Plavix), aspirin, warfarin (Coumadin), and other blood thinners and antiplatelet drugs. ‘For those taking blood thinning or antiplatelet medications, taking fish oil supplementation can enhance the anticoagulant effect of the drugs. Those who take aspirin, clopidogrel, warfarin, Coumadin, or other anticoagulant medications, should consult with their physicians before taking fish oil supplements,’ warns Stanford.

There’s also conflicting evidence regarding the risk and benefits of fish oil pills and the risk of atrial fibrillation and stroke.

‘Many large clinical trials suggest that fish oil supplementation raises the risk in healthy subjects of developing atrial fibrillation and then going on to have stroke,’ says Stanford. ‘In subjects with cardiovascular disease or predispositions to dysrhythmias (such as atrial fibrillation), fish oil seems to reduce the risk of atrial fibrillation and then going on to stroke.

Given the shelf-life of fish oil capsules (and the importance of storage conditions), your supplements might have gone rancid by the time you take them. Malik mentions that omega-3 supplements can spoil in just three months.

‘Rancid fish oil won’t be healthy, and you might wind up getting compounds in it that are toxic.’ Levin says he encourages patients to keep their fish oil in the fridge, check their labels for expiration dates, and sniff any pills before swallowing to make sure they’re fresh.

Unfortunately, the doctor is quick to point out that fish-oil supplements can easily become oxidised, thereby destroying the very compounds they’re meant to deliver and even forming harmful byproducts. The mouth is underappreciated for its intelligence In many ways, this source is what human babies are in the eyes of smartphones.

A lot of the fish oil supplements you take will already be oxidised by the time you get to take them. If you’ve ever put oil in an air fryer, you can imagine what it’s like. That’s going to promote oxidative stress and inflammation. So I only recommend pharmaceutical-grade fish oil brands which use appropriate processing and encapsulation techniques to not have oxidation.’

Besides, lots of people also take the pills instead of eating fish because they want to circumvent high levels of mercury and other toxins or contaminants – and Malik says research is showing that it’s still possible not to escape that risk when you take the pills.

Fish also tend to accumulate heavy metals, such as mercury, and other toxins in waters that might be polluted, and if the oil is not thoroughly purified and detoxified, these contaminants could also be concentrated in supplements. Longterm intake of contaminated fish oil could have neurological and developmental effects, Malik adds. ‘I recommend sticking to brands with strict quality-control measures,’ he says.

Our stories feature the newest and best advice from experts, peer-reviewed research and health organisations, but this is not a substitute for informed medical advice. Talk to your own doctor about your treatment regimen.

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