well , i liked ur question of decreasing libido. first of all anyone should try the nonmedical method, like for example :-
1. according to basics of psychology the more you think abt a disease , more u r going to stay wid dat. ofcourse increased libido is not disease bt its a condition which is causing problems for you, so here r the suggestions like , do not over indulge in these activity llike viewing blue films or having more of sex , if u r married try to decrease ur freq. of doin it.
if u r not married so decrease down ur freq of masterbation , or do not think of such things . some people of this variety think about havin incest relationship because it gives them more pleasure. so keep these things away.
medically you can try
sildenafil
that's funny i'm trying to increase my libido and testosterone. This isn't really a good option, but i've heard weed can decrease testosterone levels..............