I’m looking for an iron supplement. What’s this “ferrous” stuff?

The iron in iron supplements comes in several different forms, each one composed of elemental iron (the kind of iron your body actually uses) coupled with an organic acid that makes the iron easy to absorb.
The iron compounds commonly found in iron supplements are:

• Ferrous citrate (iron plus citric acid)
• Ferrous fumarate (iron plus fumaric acid)
• Ferrous gluconate (iron plus a sugar derivative)
• Ferrous lactate (iron plus lactic acid, an acid formed in the fermentation of milk)
• Ferrous succinate (iron plus succinic acid)
• Ferrous sulfate (iron plus a sulfuric acid derivative)

In your stomach, these compounds dissolve at different rates, yielding different amounts of elemental iron. So supplement labels list the compound and the amount of elemental iron it provides, like this:

Ferrous gluconate 300 milligrams
Elemental iron 34 milligrams

This tells you that the supplement has 300 milligrams of the iron compound ferrous gluconate, which gives you 34 milligrams of usable elemental iron. If the label just says “iron,” that’s shorthand for elemental iron. The elemental iron number is what you look for in judging the iron content of a vitamin/mineral supplement.