There's an abundance of research that conclusively shows that taking NSAIDs (ibuprofen or acetaminophen) before during and right after exercise inhibits exercise induced muscle breakdown and repair. There's a natural system in which our body reacts to exercise stress and breaks down tissue and then repairs it so we actually progress athletically. NSAIDs work by inhibiting a hormone called prostaglandin, which is vital component involved in the protein synthesis that occurs post-exercise. The anabolic process of repairing tissue is dependent on levels of prostaglandin. Some endurance athletes actually have nicknamed it Vitamin I and an alarming number of athletes pop it before during and after an event or in training. I had a friend whom was a great marathoner and he would buy ibuprofen in the largest containers available. It was literally a small jug. At times he would consume 800 to 1000 milligrams at once if something felt a little "tweaked". To say the least we would have many discussions about it. It not only halts athletic gains but regulates blood flow to the kidneys. During exercise, it's vital that the kidneys have adequate blood and oxygen in order to function normally. Basically it reduces the kidneys’ ability to flush out the byproducts of exercise, putting athletes at increased risk for rhabdomyolysis. In this extreme form of muscle breakdown, myoglobin is released into the bloodstream, where it can cause severe kidney damage. Another side-effect is gastrointestinal damage that can lead to colonic seepage into the bloodstream. I've used NSAIDs myself from time to time, heck we all have. I really try to limit any usage to maybe once or twice a month and only if the situation really requires it and never before or during or right after exercise.