RATIONALE: Iron deficiency is a significant cause of deferral in people wishing to donate blood. If iron removed from the body through blood donation is not replaced, then donors may become iron deficient. All donors are screened at each visit for low haemoglobin (Hb) levels. Some deferred blood donors do not return to donate. Deferred first-time donors are even less likely to return. Interventions that reduce the risk of iron deficiency and anaemia in blood donors will therefore increase the number of blood donations. Currently, iron supplementation for blood donors is not the standard of care in many blood services. OBJECTIVES: To assess the effectiveness and safety of iron supplementation to reduce deferral, iron deficiency and/or anaemia in blood donors. SEARCH METHODS: We searched 10 electronic databases (including CENTRAL, MEDLINE, Embase, and Transfusion Evidence Library) and 4 ongoing trial registries on 28 January 2025. We did not restrict our search by language, date, or publication status. ELIGIBILITY CRITERIA: Randomised controlled trials (RCTs) of healthy blood donors, comparing iron supplementation versus no iron, iron in one form compared to another form (e.g. oral versus intravenous (IV)), different doses, or different formulations/preparations. We excluded autologous blood donors and plasma donors. OUTCOMES: Deferral rates due to low Hb, measures of blood indices and iron stores (Hb and ferritin as either serum or plasma ferritin), adverse events, adherence, and health-related quality of life. RISK OF BIAS: We used Cochrane's RoB 2 tool to assess risk of bias in all studies, including reassessing previously included studies using this updated tool. SYNTHESIS METHODS: We combined data using random-effects meta-analyses. We performed sensitivity analyses to assess the impact of trial quality on the results, and subgrouped by population and treatment characteristics to investigate heterogeneity and the impact of these characteristics on response. INCLUDED STUDIES: We identified 8 new RCTs, for a total of 38 RCTs (7475 participants) in the current update. The studies were conducted in 15 different countries; however, most took place in Europe (20 RCTs, 53%) and the USA (10 RCTs, 26%). Six studies were exclusively of male donors, and 13 were exclusively female donors. Three studies looked only at first-time donors, while 27 studies excluded them. Various iron preparations were used, including FE2+ and FE3+ (both with/out additives like vitamin C), with doses ranging from 7 mg of elemental iron per day, up to 1800 mg, although most studies used 25 to 105 mg/day. Treatment duration varied from four days to ongoing for a year (with multiple donations in that year), but most were one to three months. SYNTHESIS OF RESULTS: Risk of bias and certainty of the evidence Risk of bias was often high or with some concerns across multiple domains. High risk of bias was often related to missing outcome data, and some of the potential biases in other domains (some concerns) were likely due to historical reporting standards (lack of information, especially regarding randomisation processes, blinding, and trial protocol/registration). Overall, we downgraded the certainty of the evidence primarily for risk of bias, high heterogeneity (between-study inconsistency) that was not explained through subgrouping, and imprecision (wide confidence intervals (CIs)), largely due to very small sample sizes in some studies. Oral iron versus no iron (placebo/standard care): 23 RCTs, N = 5518 The deferral rate due to low Hb (critical outcome) is probably reduced (risk ratio (RR) 0.39, 95% CI 0.20 to 0.76; 7 studies, 1647 participants; moderate-certainty evidence) with oral iron supplementation at the first donation since commencement of treatment when compared to no iron. This is a clinically important effect. Hb and ferritin may also be improved with oral iron supplementation compared to no iron (Hb: mean difference (MD) 2.52, 95% CI 0.77 to 4.26 g/L higher; 14 studies, 1940 participants; ferritin: MD 12.39, 95% CI 8.16 to 16.62 ng/mL higher; 8 studies, 1335 participants), but the evidence is very uncertain. More participants may experience adverse events with oral iron compared to no iron, including abdominal pain, constipation, and diarrhoea (RR 1.69, 95% CI 1.15 to 2.47; 8 studies, 2641 participants), but the evidence is very uncertain. There may be no difference in reported non-gastrointestinal symptoms, except a feeling of "weakness", which was reduced with oral iron supplements. IV iron versus no iron (placebo/standard care): 3 RCTs, N = 569 Deferral was not reported by the IV supplementation studies, but Hb is probably higher with IV iron compared to no iron at the first donation since commencement of treatment (MD 8.02, 95% CI 3.22 to 12.83 g/L higher; 3 studies, 558 participants; moderate-certainty evidence). Serum ferritin may also be higher with IV iron at the first donation since commencement of treatment (MD 78.16, 95% CI 37.20 to 119.12 ng/mL higher; 2 studies, 479 participants), but the evidence is very uncertain. There may be no difference between IV iron and no iron in adverse events, but the evidence is very uncertain. AUTHORS' CONCLUSIONS: Iron supplementation may lead to improved blood indices and iron stores, but we are uncertain about the effect due to heterogeneity in the population and different methods and doses of iron supplementation. Subgrouping (by sex, baseline iron stores, daily elemental iron dose, supplementation duration, and iron preparation) showed some 'low credibility' subgroup differences. Visual inspection of the data (forest plots) suggests more high-quality data may highlight potential differences further in some populations, to target those most in need. Future studies should assess the impact of supplementation on potentially more vulnerable populations through the stratification of participants by both sex and baseline iron stores; investigate the impact of total and daily doses of iron; and conduct a longer-term assessment of adverse events associated with oral iron supplementation. FUNDING: This review had no dedicated funding. REGISTRATION: Protocol and previous versions available via DOIs: 10.1002/14651858.CD009532 and 10.1002/14651858.CD009532.pub2.