I cannot tolerate oral iron supplements and throw them up instantly. So I took multi vitamins with iron in them and ate iron rich foods and after months of my body still not obsorbing iron and my iron levels dropping lower and lower I talked with my doctor about getting iv iron treatments. He sent me to a hematologist and although the referal process took months I finally got in. They fit me in quickly and I ended up needing a few treatments but my iron levels have increased to a low normal which is amazing considering low low it was. They will do three month Checkups or as needed from here on out. I have PCOS and have heavy and long periods. They think that is the cause of my iron deficiency. I hope this helps some! It's definitely something to look into.

If it’s not one of the many plausible things other people mentioned, it could also be the form of iron. I take a ferrochel supplement that helps me a lot more than a normal iron supplement. I have no idea what the difference is but it works for me! Might be worth considering

Same, when I started taking iron supplements, one 200% tablet helped a lot. Despite that I kept going down and my doctor kept increasing the dose every year until I hit 1100% the daily value. I asked at that point if that could be getting too much and was referred for iv iron. Definitely works better but wears off for me after around a year unfortunately