I had a pretty heavy duty one and yes, I needed an Rx. Mine kept me from full range of motion, supported the tendons so my patella couldn't slip back out of place etc while I was doing physical therapy. It went from about mid-thigh to mid-calf. You can purchase more basic ones without one though, I think. That was the first one I have had that I bought outside of a big chain store like Walmart though. The only things I forsee you running into is that they may not want to sell you one for fear of lawsuit because you may blame them if it doesn't work and you need surgery. Not saying you will do that, but you know somewhere someone has most likely. Also, the type of brace you need often depends on the injury. if you don't know your exact injury they may not be able to get you in the right type of brace.