Iron comes in many forms. If taken with just water, all are about equally well-absorbed, so less expensive forms, such as ferrous sulfate, are fine. However, with larger doses, some people experience gastric discomfort and/or constipation. Taking with food may reduce discomfort, but also reduces absorption of certain forms of iron, such as ferrous sulfate, while other forms are better-absorbed in the presence of food, such ferrous bisglycinate (sold as "Gentle Iron") and ferrous glycinate.
Note that some supplements include vitamin C to increase iron absorption but this is unlikely have a significant effect, and there is concern that slow- or timed-release products may lead to reduced absorption of iron. For more details see the Forms of Iron and Avoiding Stomach Upset
section of our Iron Supplements Review
. Also see our overall Top Picks
among the iron supplements we tested.