Iron supplement labels can look straightforward until one product says “14mg iron”, another says “65mg”, and a third uses a compound name such as ferrous bisglycinate. Those numbers are not always directly comparable.
Iron is important for making red blood cells, but it is also one of the supplements where taking more “just in case” can be a bad idea. The first step is understanding what the label actually means — and whether you need supplemental iron at all.
Why iron matters
Iron contributes to normal formation of red blood cells and haemoglobin and to normal oxygen transport. Low iron can contribute to iron deficiency anaemia, but tiredness alone does not prove that iron is the cause.
The NHS guide to iron deficiency anaemia lists symptoms including tiredness, shortness of breath, palpitations, headaches and paler skin. These symptoms can have other causes, so a blood test is usually more useful than guessing.
Elemental iron vs the iron compound
This is the most important label distinction. Iron supplements use compounds such as ferrous sulfate, ferrous fumarate or iron bisglycinate. The total weight of the compound is not the same as the amount of elemental iron it supplies.
A good UK supplement label should tell you the amount of iron contributed by the daily serving. Use that number when comparing products, rather than comparing the chemical weight of two different compounds.
Common forms of iron
Ferrous sulfate
Ferrous sulfate is widely used to treat diagnosed iron deficiency and is available as a medicine. It is effective but can cause digestive side effects in some people.
Ferrous fumarate
Another commonly used ferrous salt. As with sulfate, the relevant comparison is the elemental iron dose, not the total compound weight.
Iron bisglycinate
Iron bisglycinate is a chelated form that is common in food supplements and is often marketed as “gentle iron”. Some people may find particular formulations easier to tolerate, but “gentle” does not mean side-effect-free or appropriate without a reason to supplement.
Our Iron Bisglycinate 14mg tablets show a lower-dose supplement format, while our Slow Release Iron 65mg is a much higher-strength product. These are not interchangeable choices simply because both contain iron.
How much iron do adults need?
The NHS iron guidance gives daily dietary reference amounts of 8.7mg for men aged 19 and over, 14.8mg for women aged 19–49, and 8.7mg for women aged 50 and over. Individual needs can differ, particularly with heavy menstrual bleeding, pregnancy, dietary restrictions or medical conditions.
Those dietary requirements are not the same as a treatment dose for diagnosed deficiency. Therapeutic iron can be substantially higher and should be guided by a clinician.
Why more iron is not automatically better
The NHS notes that high supplemental doses can cause constipation, nausea, vomiting and stomach pain. Very high doses can be dangerous, especially for children. Keep iron products securely out of children's reach.
Most people should not take a high-strength iron supplement indefinitely without understanding why they need it and whether blood levels have normalised.
Iron and food
Iron comes from animal and plant foods. Meat provides haem iron, which is generally well absorbed. Beans, lentils, fortified cereals, nuts, seeds and leafy vegetables provide non-haem iron.
Vitamin C can improve absorption of non-haem iron when consumed in the same meal. Tea and coffee around meals can reduce absorption because of their polyphenol content.
Iron and medicines
Iron can interact with or reduce absorption of several medicines. The NHS ferrous sulfate interaction guidance specifically highlights medicines including some antibiotics, levothyroxine and bisphosphonates, as well as supplements containing calcium, magnesium or zinc.
If you take prescription medicine, ask a pharmacist how much time to leave between doses rather than inventing your own spacing schedule.
What does slow release mean?
Slow-release products release iron over a longer period. This may alter tolerability, but it does not make a high dose automatically more appropriate or guarantee better absorption. Compare elemental iron first.
Who is more likely to need an iron check?
- people with heavy periods;
- pregnant people or those recently pregnant;
- people with unexplained blood loss;
- people following diets with limited iron sources;
- people with gastrointestinal conditions that affect absorption;
- people with persistent symptoms compatible with anaemia.
A blood test can distinguish iron deficiency from many other causes of fatigue.
Browse our Women’s Health collection for relevant products, but do not use category placement as evidence that you personally need iron.
Frequently asked questions
Is 65mg iron stronger than 14mg?
If both numbers refer to elemental iron, yes. But always check the nutrition panel because product names sometimes use compound weights or other front-label numbers.
Is iron bisglycinate better absorbed?
Chelated forms are widely used and may be well tolerated, but treatment choice should consider dose, deficiency severity, side effects and clinical advice rather than form alone.
Why does iron make stools dark?
Dark stools are a common effect of oral iron. However, black tarry stools with illness, pain or suspected bleeding need medical assessment rather than being assumed to come from a supplement.
Can I take iron with coffee?
Tea and coffee can reduce iron absorption. If iron is prescribed or recommended, follow the timing advice on the product or from your pharmacist.
Should I take iron for tiredness?
Not automatically. Tiredness has many causes. If iron deficiency is suspected, testing is a better starting point.
The bottom line
Read iron supplements by elemental iron per daily serving. Do not compare compound names or front-label numbers in isolation. High-strength iron is not a general energy supplement; it is something to use for a clear reason, with appropriate testing or professional advice when needed.