Supplement shelves can give the impression that almost everyone needs something. Yet for generally healthy adults, a review of 84 studies found that routine vitamin and mineral supplements offered little or no protection against cardiovascular disease, cancer or early death.

Taking a supplement without a clear reason may also cause harm. High doses of some water-soluble vitamins can cause harm, and too much vitamin B6, for example, has been linked to nerve damage. Some fat-soluble vitamins, particularly vitamin A, can accumulate and cause toxicity when taken in excess.

A diagnosed deficiency is one reason to take a supplement. Other evidence-based reasons include folic acid before and during early pregnancy, vitamin B12 for people who cannot obtain enough from food and the ten micrograms of vitamin D that UK guidance recommends people consider during autumn and winter.

Once there is a sound reason to take a supplement, timing can sometimes affect absorption. Usually, “timing” concerns meals, drinks and other medicines rather than a perfect hour on the clock.

Iron: the fussiest supplement

Iron supplements are used to treat iron deficiency anaemia. Because iron deficiency can be caused by blood loss and poor absorption, including conditions affecting the gut, it should be confirmed and its cause considered rather than self-treated on the basis of tiredness........

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