Dear Susan, My understanding is that the calcium in the milk blocks the absortion of the iron. During my last pregnancy, I was taking my Ca with my prenatal vitamin and my iron tested low. As soon as I split the two up, my iron came right back up. This is why, if you read a vitamin bottle, you'll see 100% of the RDA of most things but only 10-20% of the RDA for calcium. I don't know about iron and antibiotics. Lezlie Densmore LLLL in MA