The active ingredient in Latisse is bimatoprost. Bimatoprost is a structural prostaglandin analog. Prostaglandins are naturally occurring chemicals that function as local messengers. They are not hormones like estrogen or progesterone, but work locally to pass messages between cells and organs. It is not widely understood why bimatoprost results in longer, fuller, and darker eyelashes. One hypothesis is that the prostaglandin effect increases the growth phase of the hair cycle.
Like all hair on the body, our eyelashes follow a specific growth cycle before they are shed. The first phase is called anagen. In anagen, the hair is rooted to the follicle and receives nutrients to promote growth. The next phase is called catagen. In the catagen phase, the hair stops growing and the follicle begins to collapse, which leads to the telogen phase. The telogen phase is a resting stage and lasts 30-90 days. At the conclusion of the telogen phase, the hair is detached from the follicle and sheds. Individual hairs go through the stages of the growth cycle at different times. If every hair went through the growth cycle at the same time we would have periods with full eyelashes and periods with none. The average person loses 3-5 eyelashes per day due to the staggered stages of the growth cycle.
It is thought that bimatoprost, the active ingredient in Latisse, increases the time that the eyelashes spend in the anagen or growth phase.