Experts suggest that those with iron deficiency are most likely to reap the benefits of an iron supplement (reduced fatigue), while those without a notable deficiency may find the results negligible. Most iron supplements tend to hover between 15mg and 30mg, but it’s always best to check the nutritional information on the back of the product. How soon after iron supplementation should you feel the difference? How to know if your iron levels are improvingThe most straightforward way is to take a blood test before and during iron supplementation. According to Packham, signs that your iron levels are rising (in those with a deficiency) are: