I was tired and pale so I thought I was iron deficient, but I felt WORSE when I started supplementing with iron. Then I found out that B vitamins are really important to red blood cells and I started supplementing with those instead, and THAT helped.
I don't think it's iron most people need. It's difficult to be iron deficient, iron is in just about every food you eat. And your body doesn't need a huge amount.
I think B vitamin deficiency is more common, and it can have the same symptoms.