Pharmacist reveals the popular supplements that can fuel extreme bleeding and terrifying 'serotonin syndrome'

Britons are spending up to £4 billion a year on supplements that promise sharper focus, better mood and increased energy levels. 

But pharmacists have warned that some of the most popular pills carry risks of excessive bleeding, kidney damage and life-threatening heart attacks. 

These dangers can arise when supplements interact with prescription medications, or if recommended doses are exceeded. 

'Some supplements can change how well a prescription medicine works, or make its effects stronger,' Palvinder Deol, a pharmacist at Happy Pharmacy said.

'You don't necessarily need to stop taking supplements, but if you're on regular medication, have a quick chat with your pharmacist before adding something new.' 

Magnesium is having a moment, partly due to its supposed benefit for sleep problems, as reported widely on social media.

Videos carrying the hashtag magnesium were viewed more than a billion times on TikTok last year. 

Pharmacists warn that some of the most popular pills could be putting users at risk of excessive bleeding, kidney damage and life-threatening serotonin syndrome

Although most adults can get enough magnesium through their diet, supplements have surged in popularity amid claims they can help people achieve the coveted eight hours of sleep.  

Magnesium naturally occurs in a lot of foods, including dark leafy greens, wholegrains and nuts and seeds - and recommended daily intakes are relatively modest at 270mg for women and 300mg for men.

The mineral is essential for converting food into energy, regulating the nervous system and maintaining healthy bones. Research also suggests healthy magnesium levels are linked to better sleep quality and duration.  

However, Mr Deol warns that people may be taking more than they need without realising it.  

'The important thing is that taking extra magnesium isn't quite the same as simply getting magnesium naturally through your diet, especially when you're also taking regular medication,' he said. 

People who are taking medicines for osteoporosis, levothyroxine for an underactive thyroid and certain antibiotics need to be particularly cautious. 

'Magnesium can interfere with the absorption of certain medicines when they're taken too close together, meaning your body may not get as much of the prescribed treatment as intended.' 

Rather than simply taking medication in the morning and magnesium before bed, Mr Deol advises checking the patient information leaflet and speaking to a pharmacist about the best timing.  

Turmeric supplements can also pose risks, particularly because some products contain high doses of the active ingredient curcumin. 

Curcumin has been linked to benefits ranging from reducing indigestion and joint pain to supporting weight loss. It is also thought to possess antimicrobial and anti-inflammatory properties.  

But Mr Deol warns, high-doses can increase the risk of excessive bleeding, especially when combined with medicines that reduce blood's clotting ability. 

'This isn't a reason for somebody taking these medicines to suddenly remove turmeric from their normal diet,' he added. 

'The concern is much more about concentrated supplements taking regularly alongside medication.' 

The World Health Organisation (WHO) advises taking no more than 3mg of curcumin per kg of bodyweight of a day – but dosage can vary based on whether it is being used to treat a specific condition. 

Studies suggest 1,000mg per day for up to 12 weeks may ease arthritis symptoms, but such doses are not risk-free.  

High dose turmeric supplements have been shown to increase the risk of kidney stones, lessen the effects of painkillers and in some cases make allergies worse. 

Without iron, the body can't make enough haemoglobin, the protein in the blood responsible for carrying oxygen around the body

For that reason, health professionals generally recommend obtaining turmeric through food wherever possible. 

People struggling with fatigue are also urged not to reach for iron tablets without first checking whether they are deficient.  

Tiredness can have many causes and taking iron when it is not needed can trigger constipation, nausea and abdominal pain without addressing the underlying issue.

Other common symptoms include hair loss, headaches and cold extremities. And it is more common in those who don't eat meat or rely heavily on ultra-processed foods. 

'If you’re taking iron purely because you feel tired and nobody has confirmed that you’re deficient, find out what’s actually behind the tiredness rather than carrying on indefinitely,' the pharmacist said.  

Patients taking statins, antidepressants or chemotherapy drugs should also think twice before using herbal medicines like St John's Wort, experts warn.  

The herbal remedy, derived from a yellow-flowering  plant, is often used to treat mild to moderate depression. 

'This is probably the clearest example on the list of why herbal doesn't mean interaction-free,' Mr Deol said. 

'St John’s wort can change the way the body processes a surprisingly wide range of medicines, so it deserves more caution than its over-the-counter availability might suggest.' 

Research suggests that people who take the herbal remedy alongside hormonal contraceptives, antidepressants or blood thinners are at greater risk of adverse drug reactions. 

'Combining some antidepressants with St John's wort can push serotonin levels too high and potentially lead to serotonin syndrome, which can be serious,' he said. 

Symptoms include confusion, agitation, rapid heart rate, sweating and shaking and can begin within hours. 

Anyone experiencing symptoms should seek urgent medical advice by contacting their GP, specialist or NHS 111.  

Finally, Mr Deol warned that people switching to low-salt products may be consuming more potassium than they realise.  

While potassium is less commonly supplemented than things like iron and vitamin D – with many of us probably only thinking about our levels when we reach for a banana – it is crucial for muscle function. 

A 2025 study found that consuming more potassium while cutting back on salt has greater impact on blood pressure than simply reducing our consumption of salty foods alone. 

The NHS recommends adults consume 3,500mg of potassium a day – the equivalent of ten medium bananas. 

But given that whole grains, poultry, milk and many fruits and vegetables are abundant in the mineral, most people who eat a balanced, varied diet will be getting enough of it.  

However, many low-sodium versions of bread, cheese and processed meats use potassium-based salt substitutes instead, which can become an issue if your medication is already affecting how much potassium your body holds on to, Mr Deol says. 

'Certain medicines used to treat high blood pressure and heart conditions can increase potassium levels,' he said 

'The kidneys normally play an important part in keeping potassium within the right range, so the risk can be greater for somebody whose kidney function is reduced. 

'Adding extra potassium through a supplement or salt substitute can then push levels too high. This matters because very high potassium can affect the heart, and you can’t reliably tell what your potassium level is simply from how you feel.'

Symptoms of a potassium deficiency include muscle cramps, fatigue, constipation, heart palpitations, numbness or tingling and feeling light-headed. 

The good news is that, unlike many vitamins and minerals, potassium is rarely sold in high doses on the UK high street.

Even so, Mr Deol recommends checking labels on low-sodium foods and salt substitutes, particularly for people taking blood pressure medication or those with kidney problems.