Iron

Iron deficiency is genuinely common in menstruating women, and correcting it can transform energy, concentration and exercise tolerance. But the symptoms - tiredness, low mood, poor concentration - overlap with a dozen other things, including thyroid disease, perimenopause, sleep disruption and depression. Taking iron on a hunch means you might mask something else while fixing nothing.

Ferritin is the usual first test, with a full blood count. Ferritin rises with inflammation, so a normal-looking result in someone unwell can still hide deficiency. This is exactly the kind of nuance a clinician should interpret, not an algorithm on a supplement site.

Unexplained iron deficiency in a woman who is not menstruating heavily needs investigation. It can be the first sign of gut blood loss or coeliac disease. Supplementing without asking why is the mistake that delays diagnoses.

Absorption improves with vitamin C - a glass of orange juice or a vitamin C tablet with the dose is a genuinely useful trick. Tea, coffee, calcium and dairy all reduce absorption and should be separated by an hour or two.

Alternate-day dosing is now often preferred to daily dosing. Taking iron every day raises hepcidin, a hormone that blocks further absorption, so every-other-day dosing can deliver as much iron with fewer side effects. This is a change worth knowing about if you have struggled with iron in the past.

Gentler forms such as bisglycinate cause less constipation and nausea than ferrous sulfate, though they typically contain less elemental iron per tablet. If standard iron has been intolerable, this is a reasonable route to raise with your GP or pharmacist.