Vaginal progesterone and the vaginal first-pass effect.

Vaginal progesterone is an effective alternative to systemic administration by oral or intramuscular use. The first-pass effect is reviewed, as are the most common uses for this route of delivery. This includes use in hormone replacement therapy, luteal support particularly in assisted reproduction, and avoidance of side-effects of oral progestins and progesterone. Vaginal progesterone represents a unique therapeutic solution to a number of clinical problems.