Should I use teflon tape on compression fittings?

Should I use Teflon tape on compression fittings, generally no, you should not use Teflon tape on compression fittings, as compression fittings seal differently than threaded fittings and tape can actually prevent proper seal and cause leaks. Compression fittings work by compressing a brass or plastic ferrule or ring onto pipe as nut is tightened, creating mechanical seal between ferrule, pipe, and fitting body, not by threads sealing, so threads do not need sealing tape or pipe dope. Applying Teflon tape to compression threads can prevent nut from tightening properly, cause over tightening, and may allow tape to get into water line and block flow or contaminate. For angle stops and fixture shutoffs that use compression inlet, clean pipe, ensure ferrule is square and not over tightened, and tighten nut snugly then quarter turn more, no tape needed. Teflon tape is intended for tapered pipe threads like NPT where threads themselves form seal, such as when connecting threaded pipe to threaded valve. For compression fittings, if you have leak, it is usually due to damaged ferrule, scratched pipe, or not tightening enough, not lack of tape. Some plumbers use small amount of pipe dope on ferrule for lubrication, but manufacturers recommend dry installation. So do not use Teflon tape on compression fittings, use it only on threaded pipe threads.