Thanks Alisia and Ambah,

they are designed to put your body into ketosis, which can have toxic byproducts and isn't safe for BF.
I think I'll wait. I want to breastfeed for 18 months though so that's another year before I can start. It's not worth taking the risk. I thought it may just diminish milk supple and if it started doing that I would of stopped but it sounds as though it is not safe.