Blimey, what happened there then? The wide differences in mgs per tablet is because they're different kinds of iron, with different absorption.
Say you want to absorb 25/30mgs a day. If you take 30mgs of basic ferrous sulphate (the cheapest, that cause all those symptoms), you'll only actually absorb about 3-5 mgs.
So you need to take a 325mgs tablet, to get just over 100mgs of elemental iron, of which you'll absorb maybe 20-30%.
Apologies if my sums are off, still sick!
If you take something which is more BIO-AVAILABLE, easier and more complete absorption, you need less of it, and a big factor in less symptoms.
100mgs of elemental iron in your gut is noticeable.
So stop obsessing about actual numbers if they're confusing, and just look at whether they're described as high potency, or gentle on the stomach.
Also do not take anywhere near:
Tea, coffee
Eggs
Spinach, chard or rhubarb,
Peas, lentils aka legumes incl. soy protein
High fibre, bran etc
Calcium esp dairy
But WITH vit c , on an empty stomach.
Honestly, it's not difficult, and makes a huge difference.
Horrified to hear that iron levels are sil being dismissed in UK, such a debilitating issue, so easy to fix.