Months of tablets. Nothing.
Iron Tablets Not Working? Here's Why (And What to Do)
You've done everything your GP asked. Taken the tablets, pushed through the side effects, gone back for repeat blood tests. And your levels haven't budged. You're not imagining it — and you're not doing anything wrong. The tablets are the problem.
Why iron tablets don't work for everyone
Iron tablets are the standard first-line treatment for iron deficiency. Your GP prescribes them, you take them every day, and in theory your levels should climb. But for a significant number of people, it doesn't happen — and it's not because they're failing to take them properly.
The reality is that oral iron has one of the lowest absorption rates of any supplement. Only around 10% of each tablet is actually absorbed into your bloodstream. The other 90% passes straight through your digestive tract — which is exactly what causes the nausea, constipation, stomach cramps, and dark stools that make many people stop taking them altogether.
In the UK, an estimated 1 in 3 patients prescribed oral iron stops taking it due to side effects. Of those who persist, many still don't reach adequate ferritin levels — especially if they have an underlying absorption problem, ongoing losses from heavy periods, or a condition like coeliac disease or IBD. The result is months — sometimes years — of taking tablets that aren't solving the problem.
The 6 reasons your iron tablets aren't working
If this list looks familiar, you're not alone. These are the most common reasons we see at IronOx Clinic.
Poor absorption
Only ~10% of oral iron is absorbed. That's 20mg from a 200mg ferrous sulphate tablet. At that rate, restoring depleted stores takes months — if it works at all.
Stomach side effects force you to stop
Nausea, constipation, stomach cramps, diarrhoea, dark stools. Up to 70% of patients experience these side effects from iron tablets, and 1 in 3 stop taking them as a result.
Food and drink interactions
Tea, coffee, dairy, calcium supplements, wholegrain cereals, and even some vegetables block iron absorption. Taking tablets at the 'right' time is harder than it sounds.
Gut conditions reducing absorption
Coeliac disease, Crohn's disease, ulcerative colitis, H. pylori infection, and previous gastric surgery all impair your gut's ability to absorb iron from tablets.
Your losses exceed your absorption
If you have heavy periods, regular blood donation, or chronic bleeding, you may be losing iron faster than 10%-absorbed tablets can replace it.
Wrong type or wrong dose
Not all iron supplements are equally effective. Some over-the-counter formulations have very low elemental iron content, or use forms that are even harder to absorb.
The absorption problem — in numbers
To understand why iron tablets fail, you need to understand how little your body actually absorbs from each dose.
A standard ferrous sulphate tablet contains 200mg of ferrous sulphate — but only 65mg of that is elemental iron (the iron your body can use). Of that 65mg, your gut absorbs roughly 10% under ideal conditions. That's about 6–7mg of iron per tablet.
If your ferritin is 10 µg/L and you need to reach 70+ µg/L, you'd need to absorb hundreds of milligrams of iron. At 6–7mg per day, that's 5–6 months of perfect daily compliance — no missed doses, no food interactions, no stomach side effects — nausea, constipation, diarrhoea — forcing you to skip days.
An iron infusion delivers up to 1000mg in a single 30-minute session. No absorption barrier. No digestive system involved. Equivalent to over five months of tablets — in one visit.
Tablets vs infusion
Oral iron absorption rate
~10%
of each dose
IV iron bioavailability
100%
direct to bloodstream
Time to restore iron stores
5–6 months
Tablets
1 session
Infusion
What are your options?
Switch your supplement
Different forms of oral iron — ferrous fumarate, ferrous gluconate, liquid iron — suit different people. Taking tablets every other day (rather than daily) may actually improve absorption, according to recent research. Your GP can advise on alternatives.
Worth trying if: you haven't explored different formulations
Address the underlying cause
If heavy periods, a gut condition, or dietary factors are driving your deficiency, treating the root cause is essential. Iron supplements alone won't fix a problem where you're losing iron faster than you can replace it.
Worth investigating if: your levels keep dropping after treatment
IV iron infusion
A single 30-minute infusion delivers up to 1000mg of iron directly into your bloodstream. Bypasses the gut entirely — no absorption issues, no stomach side effects. It's what many haematologists recommend when oral iron has failed, but it can take months to be offered through the NHS.
Recommended if: tablets haven't worked after 3 months, or side effects make them impossible
How IronOx Clinic can help
Most of the patients we see have already tried tablets. Some have been taking them for months. Some were prescribed them years ago and stopped because of the side effects. Some were told to "keep going" by their GP even though their levels weren't improving.
We don't repeat that cycle. Dr Ros will review your full blood picture — ferritin, haemoglobin, transferrin saturation, inflammatory markers — and your clinical history. If an infusion is the right next step, it happens the same day. If it isn't, she'll tell you that too.
No waiting months for a haematology referral. No more persevering with tablets that aren't working. One appointment. One clear answer.
What happens at your appointment
Review
Dr Ros reviews your blood results, supplement history, and symptoms. If you don't have recent results, we can arrange testing.
Diagnosis
A proper assessment — not just 'your haemoglobin is fine.' She looks at ferritin, transferrin saturation, and the full picture.
Recommendation
Honest clinical advice on whether an infusion is the right next step for you. No pressure, no upselling.
Treatment (if appropriate)
A 30-minute iron infusion, supervised throughout. Up to 1000mg of iron — what would take 5+ months of tablets.
A patient's experience
"Very relaxed, comprehensive information and advice provided. Not rushed into making any decisions."
Iron tablets not working — common questions
Oral iron has very poor bioavailability — only around 10% of each dose is actually absorbed into your bloodstream. The rest passes through your digestive system unabsorbed. Factors like food interactions (tea, coffee, calcium, fibre), gut conditions (coeliac disease, IBD, gastric bypass), and even the type of supplement you're taking can reduce absorption further. If your losses exceed what you're absorbing — common with heavy periods — your levels may stay flat or even drop.
NICE guidelines suggest reviewing oral iron after 3 months. If your ferritin hasn't meaningfully improved after 3 months of consistent supplementation — or if side effects are preventing you from taking them — an iron infusion may be a more appropriate option. You don't have to suffer through months of tablets that aren't working.
They work differently. Iron tablets are absorbed slowly through the gut, with only about 10% reaching your bloodstream. An iron infusion delivers up to 1000mg directly into your bloodstream in a single 30-minute session — bypassing the digestive system entirely. For people with absorption issues, stomach side effects like nausea and constipation, or moderate-to-severe deficiency, an infusion is often faster and more effective. Dr Ros will advise which approach is right for your situation.
In most cases, yes. A single iron infusion can restore your iron stores to healthy levels, meaning you won't need to continue oral supplements for several months — sometimes longer. Dr Ros will advise you during your consultation based on your blood results and the underlying cause of your deficiency.
No. You can self-refer directly to IronOx Clinic. If you have recent blood results (within 3 months), bring them along — they'll help Dr Ros assess your situation. If you don't have recent results, we can arrange testing.
Done with tablets that don't work?
If iron supplements haven't raised your levels — or if the side effects have made them impossible — book a consultation with Dr Ros. No referral needed. Appointments available this week.