Iron Supplements Ultimate FAQ (UK 2026)

Cheap supplements use ferrous sulfate. It is the most studied iron form, it is the cheapest by a wide margin, and it works. But it is also the worst-tolerated. Single-dose absorption is only 20-30%, so the unabsorbed iron sits in your gut, where it causes cramping, constipation, and dark stools. The dark colour is harmless. The constipation is the reason most people quit ferrous sulfate within a month.

Iron bisglycinate is a different beast. The iron atom is bound to the amino acid glycine, which means it gets absorbed via amino acid transporters instead of the standard iron uptake pathway. GI side effects drop dramatically. Most users on bisglycinate report no constipation, no nausea, no dark stools. Trade-off: lower elemental iron per tablet (typically 25mg vs 65mg for ferrous sulfate) and 2-3x the price.

If you have tried ferrous sulfate and quit, the answer is almost always to switch forms, not to give up on iron entirely. Thorne Iron Bisglycinate 25mg is one of the gentlest options on the UK market, with consistent reviews from people who failed on cheaper iron.

Absorption is highest on an empty stomach. First thing in the morning, 30-60 minutes before breakfast, is the textbook answer.

If empty-stomach dosing makes you nauseous, take it with a small non-dairy snack. You will lose some absorption (maybe 30-40%), but a dose you actually take every day beats a perfect dose you skip half the time.

Every-other-day dosing is a newer angle worth knowing. Trials on iron-deficient women show that taking iron every day can actually backfire because it spikes hepcidin (a hormone that blocks iron absorption) for about 24 hours. Dosing every other day gives hepcidin time to drop, and total iron absorbed over a week ends up similar or better, with fewer side effects.

No. This is the single biggest mistake people make. Coffee and tea contain tannins and polyphenols that bind to iron directly in your gut and stop it being absorbed. Wash an iron tablet down with your morning coffee and you can lose 60-90% of the dose.

Dairy and calcium supplements do the same thing through a different mechanism. Calcium competes with iron at the absorption site.

The 2-hour rule: keep at least 2 hours between your iron and any coffee, tea, calcium supplement, or high-dairy meal. Pair the iron with vitamin C instead. A glass of orange juice or a 100-200mg vitamin C tablet boosts absorption by 3-4x. That single change does more for your blood iron than swapping brands.

Iron recovery is slow. Faster than people expect at the start, slower than they expect at the finish.

In the first 2 weeks you might notice slightly less brain fog. By weeks 2-4, energy levels and breathlessness during exercise should ease as your body builds more haemoglobin. The dramatic 'I feel alive again' moment usually shows up around weeks 6-8.

Refilling your ferritin stores takes much longer: 3 to 6 months of consistent supplementation. Most people stop too early. They feel fine for a few weeks, then crash back into deficiency. Keep dosing until a follow-up blood test shows ferritin above 30 mcg/L.

No. Iron tablets are zero calories and have no direct effect on fat storage.

What sometimes happens: as anaemia resolves, your appetite returns to normal (severe iron deficiency suppresses appetite), so you start eating a bit more. That can add a kilo or two. It is a sign of recovery, not a side effect of the iron.

Mild fluid retention from severe anaemia can also shift as iron status improves, which occasionally shows up as a small weight bump. Plus, with restored energy you usually start moving more, which works in the other direction over a few months.