Will cefdinir affect my progesterone only mini pill?

It should not: Commonly used antibiotics like cefdinir should not have any noticeable effects on oral contraceptives such as combination birth control pills or progesterone-only birth control pills. The antibiotic rifampin may have decrease the effectiveness of birth control pills that contain an estrogen hormone.

Cefdinir: The antibiotic Cefdinir is a wide spectrum antibiotic, and does not have appreciable effect on the progesterone "mini pill". The effect of the "mini pill" is on the lining of the uterus, and the effect creates an environment which prevents implantation of the fertilized egg. The effect on the uterine lining will last even if some of the progesterone is not absorbed.