Multivitamins are marketed as a harmless top-up to a busy diet, and for most people, taken at standard doses, they are. But “vitamin” is not a synonym for “safe at any amount.” Several of the nutrients sold in general-purpose supplements have official safe upper limits in the UK, and going over them for months or years at a time is genuinely linked to nerve damage, bone problems and organ strain. Before you decide which supplement to buy, it is worth knowing where those ceilings actually sit.
Why some vitamins are riskier to overdo than others
The nutrients worth watching fall into two groups. Fat-soluble vitamins — A, D, E and K — are stored in the body’s fat tissue and liver rather than flushed out in urine, so they can build up over weeks of daily supplementing. Water-soluble vitamins, like most of the B group and vitamin C, are generally excreted more readily, which is why very high one-off doses tend to just produce expensive urine rather than lasting harm. The exception that trips people up is vitamin B6, a water-soluble vitamin that has still been linked to nerve damage at sustained high doses — a reminder that "water-soluble" is not the same as "no upper limit." Minerals sit in their own category again: the margin between a useful daily dose and one that causes side effects is often much narrower than with vitamins.
Vitamin A and vitamin D: the fat-soluble risks
Vitamin A is the one with the clearest long-term warning. NHS guidance is that consuming more than an average of 1.5mg (1,500 micrograms) a day over many years may affect bone health and increase fracture risk, particularly in older people and postmenopausal women. It’s specific enough to include a practical warning: if you already eat liver about once a week, you’re advised not to also take a supplement containing vitamin A, since liver alone can push intake well past the limit. The European Food Safety Authority’s more recent 2024 advice sets a slightly higher ceiling of 3,000 micrograms a day. Chronic overuse is associated with coarse hair, hair loss, cracked lips, dry skin, headaches, joint pain and liver damage; in pregnancy, high vitamin A intake carries a risk of birth defects, which is why pregnancy multivitamins deliberately keep it low or leave it out.
Vitamin D gets more attention because so many UK adults are advised to take it in winter, but it has its own ceiling. NHS and EFSA guidance is not to take more than 100 micrograms (4,000 IU) a day from supplements unless a healthcare professional has told you otherwise. Sustained very high doses — guidance points to prolonged intake above 10,000 IU a day — can raise blood calcium to the point of hypercalcaemia, with symptoms including nausea, fatigue, confusion, excess thirst and, over time, kidney damage. The standard 10-microgram maintenance dose most adults are advised to take is nowhere near this; the risk is specifically from people stacking multiple high-strength D products at once, not from following the ordinary recommendation.
Vitamin B6: the one that catches people out
Vitamin B6 is the most-cited example of a vitamin causing real harm at a dose many people wouldn’t think twice about. NHS advice is that taking 200mg or more a day can lead to peripheral neuropathy — a loss of feeling in the arms and legs. The picture in the 10–200mg range is genuinely unclear, with the NHS stating there isn’t enough evidence to say how long doses in that band can safely be taken, while the Food Standards Agency’s Expert Group on Vitamins and Minerals has separately suggested a safe upper level of 100mg a day for supplemental use. Long-term intake above roughly 250mg a day is linked to nerve symptoms including tingling, numbness and pain in the hands and feet, and in some reported cases the damage has been permanent. B6 turns up in multivitamins, B-complex products and some energy or mood-support supplements, so it’s worth checking the label if you’re taking more than one supplement that lists it.
Iron and zinc: minerals with less room for error
Minerals tend to have a tighter gap between a useful dose and an uncomfortable one. UK Department of Health and Social Care advice is that 17mg or less of iron a day is unlikely to cause harm for most people, and side effects such as constipation, nausea and stomach pain are already reported at doses above 20mg. The European Food Safety Authority has set a more conservative total intake ceiling of 40mg a day from all sources, including food, precisely because there isn’t enough data to establish a firm upper limit for iron the way there is for some vitamins. Iron is also one of the few supplements where a large overdose can be genuinely dangerous in a short space of time, particularly for children, which is why it’s kept in child-resistant packaging.
Zinc has a similar shape to its safety profile. NHS guidance is not to exceed 25mg a day from supplements, though other clinical references put the practical adult ceiling closer to 40mg. Short-term high doses commonly cause nausea, vomiting, cramps and headaches. The longer-term issue is less well known: sustained intake of 100–150mg a day or more interferes with how the body handles copper, and can eventually cause a copper deficiency of its own, with knock-on effects including anaemia and neurological symptoms. Zinc lozenges taken frequently for colds, on top of a daily multivitamin that already contains zinc, is a realistic way people end up stacking doses without noticing.
Choosing a supplement without guessing at the ceiling
The practical takeaway isn’t to avoid vitamins and minerals — for most adults eating an ordinary diet, a standard-dose multivitamin sits nowhere near these limits. It’s to avoid stacking. The most common way people accidentally exceed a safe upper limit isn’t from one product but from several: a multivitamin, a separate high-strength vitamin D capsule, a B-complex, and an occasional zinc lozenge, each reasonable on its own but adding up when combined daily. Before buying a second or third supplement to layer on top of an existing one, it’s worth totting up how much of each nutrient you’re actually taking across everything in the cabinet, and checking that combined figure against the limits above rather than assuming each product is safe in isolation. If you’re on prescribed medication or have a health condition, that same stacking question is worth raising with a pharmacist or GP before adding anything new, since several of these nutrients can also interact with existing treatment.