Black tea, green tea, and oolong tea naturally contain compounds called tannins. Tannins give tea their color and characteristic astringent taste.
Tannins can also inhibit the absorption of iron, however, especially iron from plant sources, such as peas, beans, nuts, leafy green vegetables, enriched pastas and breads, and fortified cereals. Plant sources of iron are called non-heme iron.
The absorption of iron from animal sources, such as red and dark meat, is generally not affected by tannins.
Herbal teas often contain tannins, which means they could partially reduce the absorption of non-heme iron if it's taken together with a meal. A cup of chamomile tea or peppermint tea contains far fewer tannins, however, than a cup of black tea.
Adding lemon, which is rich in vitamin C, may partially counteract this effect.
You may notice that the longer you leave a teabag in water, whether it's black tea or herbal tea, the more of an astringent, bitter taste it takes on due to the tannins. Being careful not to oversteep tea by immediately removing the tea bag or leaves after steeping can reduce the amount of tannins in tea.
But keep in mind that unless a person has iron-deficiency anemia or does not eat meat, one cup of herbal tea is often acceptable.