Why can't we take time released vitamins?

I know I read something on here a long time ago about not taking time relased vitamins. Can someone tell me why you cannot have time released vitamins. My iron is low and last week I took a time released iron tablet and my stomach was unhappy all day long.

We greatly appreciate your interest in helping us build our Q&A database. To discourage vandals from posting garbage, however, we require people to register before posting.