Actually, the pharmacist is right. Any pill that contains prostaglandin (and almost all do) acts to prevent implantation of the "products of conception" (read "baby" in the red states).
It's really quite the opposite. Synthetic estrogen and progestin (a form of progesterone) in the pills travel through the bloodstream to the hypothalamus and the pituitary gland. The estrogen suppresses production of FSH in the pituitary gland so follicle maturation doesn't occur. Without a mature follicle, no egg is released and fertilization does not occur. Other than the 'morning after' pill and 'IUDs' there are very very very few post-fertilization, noninvasive methods of 'birth control'.