You should not take phentermine while you are breast feeding. Phentermine is a sympathomimetic amine, which is similar to an amphetamine, and is not suitable for women who are breast feeding. No doctor would ever prescribe this for you if you informed him you were breast feeding an infant. He would recommend waiting until you were finished breast feeding.
Many things pass through your breast milk to your baby, from the tastes of certain foods to alcohol to drugs.
Phentermine is not save for infants. As a general rule when breast feeding, if you wouldn't put it into your babies mouth, do not put it into your mouth.