How long it takes for iron supplements to work will depend on an individual’s iron needs, diet, supplement tolerance, and the cause and severity of iron depletion. For someone with moderate to severe iron deficiency, it can take anywhere from 2-6 months to fully replete iron stores, though symptoms may start to improve sooner than that [1,2]. In some cases, such as individuals who have trouble absorbing iron, supplemental iron may be needed on a regular basis to achieve and maintain adequate iron levels.