Finasteride is a hormone therapy drug which prevents the synthesis of dihydrotestosterone (DHT) from testosterone, a powerful androgen (male sex hormone) which the prostate normally uses (5-20 times more powerful than testosterone). Unlike the prostate cancer hormone therapy drugs, Finasteride doesn't remove (or block) testosterone, only DHT.
Some of the body functions which we loosely say depend on testosterone actually depend on DHT. Without it, the prostate becomes much less active. DHT is also used by hair folicles. It causes baldness, which you might see reverse. Conversely, it has the opposite effect on other body hair, causing growth from puberty, and you may see this thin down or vanish. It is partly responsible for libido, so that will reduce although not as much as when on prostate cancer hormone therapy, and a reduction in libido can make erections more difficult, although erections and orgasm do not require either testosterone or DHT. You are likely to have reduced semen or dry orgasms, but I really doubt that would be caused in as little as 5 days.
As Lyn pointed out, when you have PSA tests for prostate cancer, you must remember to double the readings while you are on Finasteride.