Britons are spending up to £4 billion a year on supplements promising sharper focus, better mood and increased energy, but pharmacists have warned that some of the most popular pills carry risks of excessive bleeding, kidney damage and life-threatening heart problems.
The dangers can arise when supplements interact with prescription medications or when recommended doses are exceeded, according to Palvinder Deol, a pharmacist at Happy Pharmacy.
"Some supplements can change how well a prescription medicine works, or make its effects stronger," Deol 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 and medication timing
Magnesium has surged in popularity, partly for its supposed sleep benefits, as reported widely on social media. Videos carrying the hashtag magnesium were viewed more than a billion times on TikTok last year.
Most adults can get enough magnesium through their diet. The mineral occurs naturally in dark leafy greens, wholegrains, nuts and seeds, and recommended daily intakes are relatively modest at 270mg for women and 300mg for men. It is essential for converting food into energy, regulating the nervous system and maintaining healthy bones, and research suggests healthy magnesium levels are linked to better sleep quality and duration.
Deol warned 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 taking medicines for osteoporosis, levothyroxine for an underactive thyroid, or certain antibiotics need to be particularly cautious, since magnesium can interfere with the absorption of some medicines when taken too close together, meaning the body may not get as much of the prescribed treatment as intended. Deol advised checking the patient information leaflet and speaking to a pharmacist about the best timing, rather than simply taking medication in the morning and magnesium before bed.
Turmeric and bleeding risk
Turmeric supplements can also pose risks, particularly because some products contain high doses of the active ingredient curcumin, which has been linked to benefits ranging from reducing indigestion and joint pain to supporting weight loss, and is thought to have antimicrobial and anti-inflammatory properties.
Deol warned that high doses can increase the risk of excessive bleeding, especially when combined with medicines that reduce the blood's clotting ability. "This isn't a reason for somebody taking these medicines to suddenly remove turmeric from their normal diet," he said. "The concern is much more about concentrated supplements taken regularly alongside medication."
The World Health Organization advises taking no more than 3mg of curcumin per kg of bodyweight a day, though dosage can vary depending on whether it is being used to treat a specific condition. Studies suggest 1,000mg a 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, worsen allergies. For that reason, health professionals generally recommend getting turmeric through food wherever possible.
Iron, St John's wort and serotonin syndrome
People struggling with fatigue are also urged not to reach for iron tablets without first checking whether they are actually deficient. Tiredness can have many causes, and taking iron when it isn't needed can trigger constipation, nausea and abdominal pain without addressing the underlying issue. Other symptoms of deficiency include hair loss, headaches and cold extremities, and it is more common in people 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," Deol said.
Patients taking statins, antidepressants or chemotherapy drugs should also think twice before using herbal medicines like St John's wort, a remedy derived from a yellow-flowering plant that 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," 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 people who take the herbal remedy alongside hormonal contraceptives, antidepressants or blood thinners face a 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," Deol 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.
Potassium in salt substitutes
Deol also warned that people switching to low-salt products may be consuming more potassium than they realise. Potassium is less commonly supplemented than iron or vitamin D, but it is crucial for muscle function. A 2025 study found that consuming more potassium while cutting back on salt has a greater impact on blood pressure than simply reducing salty food alone.
The NHS recommends adults consume 3,500mg of potassium a day, equivalent to ten medium bananas. Whole grains, poultry, milk and many fruits and vegetables are abundant in the mineral, so most people eating a balanced, varied diet will get enough. But many low-sodium versions of bread, cheese and processed meats use potassium-based salt substitutes instead, which Deol said can become a problem if medication is already affecting how much potassium the body retains.
"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 potassium deficiency include muscle cramps, fatigue, constipation, heart palpitations, numbness or tingling and feeling light-headed. Potassium is rarely sold in high doses on the UK high street, but Deol still recommends checking labels on low-sodium foods and salt substitutes, particularly for people taking blood pressure medication or those with kidney problems.
