Naproxen (and Ibuprofen) can both cause kidney damage, particularly when you are physically active and/or not hydrating well (where hydrating well means getting enough sodium and glucose in addition to water). After experiencing kidney pain after taking naproxen on a hike and learning about this possible side-effect I am now very careful about naproxen use.

http://www.gssiweb.org/en/sports-science-exchange/article/ss...

http://www.mdpi.com/1424-8247/3/4/1279/pdf