I take them for my back. They can take a while to have an effect. Apparently if you're already in lots of pain when you first take it it takes longer to work. I've been recommended to take them at the first onset of soreness and not wait until I'm in agony.
They can upset the stomach if you take them without food though.
I have chronic back pain from a twist in my spine. My spine never grew properly, so I'm a short arse. It actually sounds worse than it usually is.
I have phases in the year when I can't walk upright properly, normally due to having done stuff that I know are not good for me! My lower back is sore a lot of the time so by making allowances and judicious use of naproxen I can manage ok.