Shutoff valve leaks

My bad luck with plumbing strikes again...

I replaced a bathroom faucet and the hot water supply line drips now with the value fully open. The valve does shut off completely, just drips when open. The cold water line is fine. Is it just a matter of tightening the value with an adjustable wrench?

Can I close the vavlue, take off the nut, and add some pipe dope to fix it?

Quite often I've found the nut to slightly loosen when opening valves that haven't been opened for while. Try closing the valve, tighten the nut and re-open it. If this doesn't work, I just replace the valve - they're inexpensive.

If you do have a compression fitting on the supply line, and if that's where the leak is, and CAREFULLY tightening it just a tad doesn't work, it may be better to take the fitting apart, make sure the tubing goes straight into the fitting, and then use just a drop of oil (yes, oil) on the threads.

That's one of many things I've learned on this site. Using pipe dope or even Teflon tape may work, but may also work against properly tightening a compression fitting. Teflon tape and pipe dope are intended for use on fittings where the threads do the sealing. On a compression fitting, it's the brass ferrule seating inside the valve fitting which does the sealing. The threads, on the other hand, serve to pull the mating parts together and are not responsible for being watertight themselves.

Since I've switched to using a drop of 3-in-1 oil on threads of compression fittings, I haven't had a leaking compression fitting.