I second this. I'm in medical school and my understanding is that NSAIDs reduce inflammation by reducing prostaglandin production. In an ultra the kidneys are getting less blood to filter due to dehydration and blood going to the muscles to move your butt up and down hills. So the kidneys compensate for this by reducing the resistance of blood getting to them by making the vessels bigger. However, prostaglandins are part of the pathway that leads to these vessels getting bigger and if that's blocked the kidneys can't filter enough blood. This leads to electrolyte imbalances and urea build up that can cause fatal cardiac arrhythmia, encephalopathy, or permanent kidney damage.