In the event that you decide to consume a Painkiller before exercise, it is absolutely vital that you watch your levels of hydration. Always remember to keep your water in close proximity and give yourself plenty of breaks to take some sips of it. NSAIDs inhibit the production of a hormone known as “prostaglandin,” which regulates blood flow to the kidneys. This combined with the fact that as you dehydrate you begin to receive less blood volume through the vessels, overwhelms the kidney.  When the blood flow going to your kidney starts to taper off, they do what they can to maintain proper function. Therefore, in order to avoid damage, they constrict the vessels in an attempt to sustain flow to the area. This does not always end in kidney failure; for some it just causes damage, but for others it is fatal.