Viagra (Sildenafil) does not just aid in increasing erections but it also assists someone to gain muscle and to lose weight.
Since it lowers blood pressure it helps push nutrients into muscles faster. It increases a chemical called Nitric oxide which is important in bodybuilding.
It also helps metabolise unhealthy fats faster. Apparently cyclists use it to perform better in high altitude and it’s also being considered as a performance enhancing drug by doping bodies.