First of all Viagra does not increase libido. It is a vasodilator. Someone with no libido who takes it won’t suddenly have an increased desire for sex. What it does, is allow persons who have a normal libido to be able to engage in normal sexual relations, at least in the cases where it is applicable. Not all causes of impotence will be corrected by Viagra, and Viagra (and the other similar drugs) can have side effects, some of them dangerous in certain cases.
Secondly there are drugs that reduce libido. One of the treatments for prostate cancer is hormone treatments. These drugs reduce or eliminate the body’s production of testosterone. The problem is that they have serious side effects, one of which is osteoporosis, because they have the same effect as menopause on women, the elimination of sex hormones from the body. They also have side effects like breast growth, possibly irreversible sterility, depression, reduced concentration, reduced muscle mass, increased body fat, etc.
In short the side effects would be worse than the benefits unless you have prostate cancer for which the alternative is death.
It seems to me to be a very dangerous way to achieve what should really be achieved through learning self-control. We are already an over-medicated society.