This page is part of: Iron
Short answer
Under the WHO 2024 guideline, hemoglobin below 120 g/L (12 g/dL) in non-pregnant adult women and below 130 g/L (13 g/dL) in men is anemia. If the anemia is due to iron deficiency, the diagnosis is confirmed with iron tests such as ferritin. The 2021 British Society of Gastroenterology (BSG) guideline recommends oral iron as first-line treatment and intravenous iron where appropriate, and stresses investigating the cause, particularly in men and postmenopausal women. The treatment plan is made by a physician. [1] [3]
What is iron deficiency anemia?
The NIH describes iron deficiency as progressing in three stages: first the stores decrease, then the iron going to blood production decreases, and in the final stage the stores are exhausted and hemoglobin and hematocrit fall. At this last stage the red blood cells become small and poor in hemoglobin (microcytic, hypochromic anemia). [2]
Iron deficiency is the most common cause of anemia, but not the only one. Folate and vitamin B12 deficiency, as well as chronic infection and inflammation, can also cause different types of anemia or worsen its severity. Anemia of chronic disease (inflammation) is the second most common type after iron deficiency anemia, and its treatment is directed primarily at the underlying disease. [2]
What hemoglobin level counts as anemia?
In its 2024 guideline, the WHO updated the hemoglobin cutoffs that define anemia. The cutoffs are based on the 5th percentile of hemoglobin distributions in apparently healthy, ethnically diverse populations. [1]
| Group | No anemia | Mild | Moderate | Severe |
|---|---|---|---|---|
| 6–23 months | 105 g/L or above | 95–104 | 70–94 | below 70 |
| 24–59 months | 110 or above | 100–109 | 70–99 | below 70 |
| 5–11 years | 115 or above | 110–114 | 80–109 | below 80 |
| 12–14 years (girls and boys) | 120 or above | 110–119 | 80–109 | below 80 |
| 15–65 years, non-pregnant women | 120 or above | 110–119 | 80–109 | below 80 |
| 15–65 years, men | 130 or above | 110–129 | 80–109 | below 80 |
| Pregnancy, first and third trimesters | 110 or above | 100–109 | 70–99 | below 70 |
| Pregnancy, second trimester | 105 or above | 95–104 | 70–94 | below 70 |
The values in the table are in g/L; dividing by 10 converts them to g/dL (120 g/L = 12 g/dL). The WHO recommends adjusting hemoglobin values for the altitude of residence and for smoking; it states that no adjustment should be made for infection and inflammation or for genetic background, ethnicity or race. Venous blood, automated hematology analyzers and strict quality control are recommended for measurement. [1]
In clinical practice, the BSG recommends defining anemia as hemoglobin below the lower limit of normal for the relevant population and for the laboratory performing the test. [3] For this reason, the reference range on a report can differ slightly from the WHO cutoffs.
How is the diagnosis made?
Low hemoglobin shows that anemia is present, not what causes it. The BSG recommends confirming iron deficiency with iron tests before moving on to investigation. According to the guideline, serum ferritin is the most useful single test; if ferritin is suspected to be misleadingly normal, tests such as transferrin saturation help. A rise in hemoglobin of at least 10 g/L within 2 weeks of iron treatment strongly suggests absolute iron deficiency even when iron tests are inconclusive. [3]
Detailed guideWhat ferritin level is normal? Meaning of low and high ferritinWhy is investigating the cause part of treatment?
According to the BSG, the common cause of iron deficiency anemia in premenopausal women is menstrual bleeding; in men and postmenopausal women it is often due to chronic blood loss from the digestive tract. In this group, about one third of patients have an underlying disease, and iron deficiency anemia can be the first sign of colon or esophagogastric cancer. [3]
The guideline recommends taking a detailed history at the first assessment; and, in confirmed iron deficiency anemia, urinalysis, celiac disease screening and, in appropriate cases, endoscopic examination of the upper and lower digestive tract. Celiac disease is found in 3–5% of iron deficiency anemia cases. According to the guideline, having had stomach or bowel surgery (including bariatric surgery) should not prevent the investigation of other causes. [3]
How is it treated?
Treatment is planned by a physician; the following describes the general approach of a guideline and is not a personal treatment schedule.
The main recommendations of the 2021 BSG guideline are as follows: [3]
- Unless colonoscopy is imminent, iron treatment is not delayed while test results are awaited.
- Initial treatment is with oral iron salts (ferrous sulfate, fumarate or gluconate). If these are not tolerated, less frequent dosing, another oral preparation or intravenous iron is considered.
- Intravenous (parenteral) iron is considered at an early stage when oral iron cannot be used, is ineffective or is not tolerated, and when correction is urgent or oral treatment is not expected to be effective.
- In severe symptomatic anemia, a limited blood transfusion is occasionally needed; in this case iron treatment is still required afterwards.
Why do side effects make treatment difficult?
The most common problem with oral iron is gastrointestinal side effects. In the meta-analysis by Tolkien and colleagues covering 43 randomized trials (6,831 adults), ferrous sulfate increased the risk of gastrointestinal side effects compared with placebo (OR 2.32; 20 trials) and compared with intravenous iron (OR 3.05; 23 trials). In the meta-regression, no significant relationship between side effects and the amount of iron could be shown. [6] Because side effects can lead to discontinuation of treatment, the preparation, dosing frequency and route are reviewed together with the physician.
What is known about alternate-day use?
In Switzerland, Stoffel and colleagues conducted two open-label randomized trials using ferrous sulfate labeled with stable isotopes in women aged 18–40 with ferritin of 25 µg/L or below. In the first trial (40 women), giving the same amount every day for 14 days was compared with giving it on alternate days for 28 days; the cumulative absorption fraction was higher in the alternate-day group (21.8% vs 16.3%). In the second trial (20 women), splitting the daily amount into morning and evening doses did not increase absorption and raised hepcidin levels. [4] These women were not anemic; the authors noted that the results need to be confirmed in patients with iron deficiency anemia.
In the same team’s 2020 study of 19 women with iron deficiency anemia, hepcidin measured on the consecutive second day was higher than on the first day and on the alternate-day dosing day. The fractional absorption of iron given on the first day and on the alternate day was 40–50% higher than that of iron given on the consecutive second day; there was no significant difference between the absorption of iron given on the first day and on the alternate day. [5] The BSG likewise reports that alternate-day use increases absorption and lists it among the options for those who cannot tolerate daily use. [3]
These studies measured short-term absorption in a small number of women; they did not measure clinical outcomes such as the speed of hemoglobin recovery or symptoms. The dosing frequency is determined by the physician on an individual basis.
Detailed guideWhen to take iron? Fasting vs fed, tea and medication intervalsWhat does follow-up after treatment look like?
The BSG recommends monitoring the hemoglobin response to oral iron within the first 4 weeks and continuing treatment for about 3 months after hemoglobin normalizes so that bone marrow iron stores are adequately replenished. According to the guideline, hemoglobin returns to normal with iron treatment in most cases, but in long-term follow-up anemia recurs in some patients. In people with good-quality normal upper and lower endoscopy who respond inadequately to treatment or whose anemia recurs, investigation of the small bowel and the kidney-urinary tract is also recommended; where oral iron is ineffective, intravenous iron comes into consideration. [3]
The NIH notes that, in iron deficiency anemia, physicians can sometimes prescribe amounts above the upper limit set for healthy people (45 mg per day in adults) in order to replenish stores. This does not mean that using high amounts on one’s own without a prescription is safe. [2]
Iron deficiency anemia in pregnancy
Anemia cutoffs in pregnancy differ from those in non-pregnant women: according to WHO 2024, below 110 g/L in the first and third trimesters and below 105 g/L in the second trimester is anemia. [1]
In the 2015 Cochrane review (44 trials, 43,274 women), preventive daily iron supplementation reduced maternal anemia at term (RR 0.30; low-quality evidence), iron deficiency anemia (RR 0.33; 6 trials) and iron deficiency (RR 0.43). No statistically significant difference was found for low birth weight, preterm birth and neonatal death. [7]
In Türkiye, the Ministry of Health runs the Iron Supplementation Program for Pregnant Women under circular No. 2020/4, “Iron and Vitamin D Supplementation Practices for Pregnant Women”. According to the General Directorate of Public Health, except where iron is not to be given, all pregnant women receive iron supplementation for a total of 9 months, starting from the 16th week of pregnancy, comprising 6 months during pregnancy and 3 months after birth; it is prescribed for those with social security coverage and provided free of charge from local sources for those without. [8] This is a preventive program; if anemia is detected in pregnancy, diagnosis and treatment are planned separately by a physician.
This page does not make a diagnosis or recommend personal treatment. Interpretation of hemoglobin, ferritin and other tests, choice of preparation and follow-up are done by a physician.
Detailed guideIron: functions, daily requirement, excess and interactions Detailed guideIron deficiency symptoms: fatigue, hair loss, restless legs Detailed guideWhich foods contain iron? Serving table and absorptionFrequently asked questions
What hemoglobin level counts as anemia?
Under the WHO 2024 guideline, hemoglobin below 120 g/L (12 g/dL) in non-pregnant women aged 15–65 and below 130 g/L (13 g/dL) in men is anemia. In pregnancy the cutoff is 110 g/L in the first and third trimesters and 105 g/L in the second.
How is iron deficiency anemia diagnosed?
Diagnosis rests on low hemoglobin together with tests showing iron deficiency; the most useful single test is ferritin. In inflammatory conditions, where ferritin can be misleadingly normal, additional tests such as transferrin saturation are used.
How many months does iron deficiency anemia take to correct?
The time varies by person and treatment. The British Society of Gastroenterology guideline recommends monitoring the hemoglobin response in the first 4 weeks and continuing treatment for about 3 months after hemoglobin normalizes so that stores are replenished.
Is iron taken on alternate days?
The physician decides. Stable isotope studies showed that iron given on alternate days is absorbed better than iron given daily, and the British guideline lists alternate-day use among the options for people who cannot tolerate daily iron. These studies measured absorption; they did not measure long-term clinical outcomes.
Sources
- Institution World Health Organization — Guideline on haemoglobin cutoffs to define anaemia in individuals and populations (2024)Back to text ↑
- Guideline / review NIH Office of Dietary Supplements — Iron: Health Professional Fact Sheet (2025)Back to text ↑
- Institution Snook et al. — British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults (Gut) (2021)Back to text ↑
- RCT Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. (2017)Back to text ↑
- Human study Stoffel et al. — Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic women (Haematologica) (2020)Back to text ↑
- Tolkien et al. — Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis (PLoS One) (2015)Back to text ↑
- Peña-Rosas et al. — Daily oral iron supplementation during pregnancy (Cochrane Database Syst Rev) (2015)Back to text ↑
- Institution T.C. Sağlık Bakanlığı Halk Sağlığı Genel Müdürlüğü — Gebelere Nutrisyonel Destek ProgramıBack to text ↑
Method: Institutional clinical guidance and relevant studies were compared. Each finding is presented with its population, outcome, sources and limitations. Independent medical review has not yet been completed.
Updated
This content is for information only and is not medical advice. It does not recommend doses, products or vendors. Talk to your clinician before making any health decision.
