How do I repair a leaking spigot behind the toilet?

If its leaking at the threading, you could try putting some plumber's tape to see if that helps seals the gap. Are you sure its not the line that is leaking? If so replacing the line might be an inexpensive quick fix, too.

This site has a lot of good info but it takes a minute to load:

https://www.familyhandyman.com/plumbing/valves/how-to-fix-a-leaking-shutoff-valve/

It's sound like the valve seat is bad. This usually requires a valve replacement. They are around $10. You will need to shut the water off and make sure to have a bucket ready to catch drips.

Hi this is Peggy. So sorry you have a drip behind the toilet. Hope this helps you fix the problem. If the spigot is leaking around the valve stem when the water is turned on, it can usually be fixed by tightening the packing nut behind the handle 1/8 to 1/4 turn. If the faucet still leaks after tightening the packing nut, the washer on the end of the valve stem needs to be replaced.

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How to fix a dripping spigot | Staggs Plumbing