A GP has warned that taking popular supplements often promoted online could do 'more harm than good', potentially causing nerve damage, stomach problems and even distorting vital blood-test results. Supplements can be essential when treating a diagnosed deficiency or supporting particular life stages, but a GP says they should not automatically be treated as harmless simply because they are available without a prescription.
Why supplements can be risky
Dr Asiya Maula, private GP at The Health Suite, said: “Just because a supplement is fashionable or appears in an influencer’s morning routine does not mean you need it or that it is safe for you. More is not always better with vitamins and minerals. Some accumulate in the body, some cause harm at high doses and others can interfere with important medicines.
“Before copying somebody’s supplement routine, check what each product contains and whether you actually need it. Always include supplements when telling your doctor or pharmacist what you take.”
Taking several products at once can mean the same ingredient appears in a multivitamin, wellness blend and individual tablet, causing doses to quietly add up.
Six supplements to check before taking
1. Vitamin B6
Vitamin B6 is often found in multivitamins, B-complex tablets and products marketed for energy. The NHS warns that taking 200mg or more daily can cause peripheral neuropathy, producing numbness or loss of feeling in the arms and legs. Symptoms usually improve after stopping, but damage can sometimes be permanent.
It advises against exceeding 10mg daily through supplements unless directed by a doctor. The higher figure should not be interpreted as a safe threshold. Dr Maula said: “People may unknowingly combine several products containing B6. Persistent tingling, burning or numbness should not be ignored.
“There is insufficient evidence to say how long doses between 10mg and 200mg can be taken safely, so people should follow the NHS advice.”
2. Iron
Iron can be valuable for a confirmed deficiency, but tiredness alone does not prove somebody needs it. Adult men need 8.7mg daily and women aged 19 to 49 need 14.8mg. Treatment for diagnosed deficiency may legitimately involve higher doses under medical guidance.
Supplement doses above 20mg can cause constipation, nausea, vomiting and stomach pain. Very high doses can be fatal, particularly in children, so products must be kept out of their reach. “Speak to a healthcare professional rather than self-diagnosing,” Dr Maula advised. “Iron deficiency may require tests and its underlying cause may also need investigating.”
3. Vitamin A
Vitamin A may appear as retinol in multivitamins and fish liver oil. Because the body stores it, repeatedly taking too much may affect bone health. The NHS advises against averaging more than 1.5mg, or 1,500 micrograms, daily from food and supplements combined over many years because this may increase fracture risk later in life.
Pregnant women should avoid supplements containing vitamin A unless specifically advised otherwise. They should also avoid liver and liver products because excessive retinol can harm an unborn baby.
4. High-dose vitamin D
Adults and children aged four and over should consider taking 10 micrograms, or 400 IU, daily during autumn and winter. Some groups need supplementation throughout the year. However, taking excessive amounts over time can cause calcium to accumulate in the body, potentially damaging the kidneys and heart and weakening bones. Adults should not exceed 100 micrograms, or 4,000 IU, daily unless medically directed.
Dr Maula said: “The message here is not to stop taking vitamin D. It is to check the strength on the bottle. Some prescribed high-dose courses are taken weekly, so confusing that with a daily instruction can be dangerous.”
Vitamin K helps activate proteins involved in bone health, but evidence does not establish that everyone taking vitamin D needs additional K2. Anyone taking warfarin should speak to their clinician before starting vitamin K.
5. St John’s wort
This herbal product is used for low mood, but it can interact with numerous medicines and make some hormonal contraceptives, including pills and implants, less effective. Dr Maula said: “Natural does not mean interaction-free. St John’s wort can also interact with antidepressants, blood thinners and some medicines used for epilepsy, heart conditions and cancer.”
6. Biotin
High-dose biotin, widely sold for hair and nails, can interfere with certain laboratory tests, including some thyroid tests and tests measuring troponin, a marker used when investigating a suspected heart attack. Depending on the test, it can produce falsely high or low results. Falsely low troponin could contribute to a missed diagnosis, while misleading thyroid results could lead to inappropriate treatment.
Dr Maula said: “The concern here is that the supplement can distort the test result. Tell the clinician taking your blood about any hair, skin or nail supplements, including the dose and when you last took them.”
Patients should follow the testing team’s instructions about whether to pause biotin. Emergency care should never be delayed to allow a supplement to leave the body.
Check before throwing anything away
People should not suddenly stop a prescribed supplement or medicine. Those who are pregnant, vegan, living with absorption problems or treating a confirmed deficiency may have a genuine clinical need.
Dr Maula added: “The safest approach is not to bin everything. It is to understand why you are taking each product, check the combined dose and ask a pharmacist or doctor if you are uncertain.”
