It's a trick that's been around for ages, with many variations - I've even heard that the use of certain bodily fluids gives good results, but it isn't something I'm about to try.
The main reason to do this is to save money, but you should ask yourself whether the savings are worth it.
It's generally a better idea to keep your strings in good shape - wiping them clean of grime and sweat after playing and giving them a rubdown with string-cleaner every now and again.
Boiling is a short-term remedy and if you've been taking care of your strings on a regular basis, by the time it becomes an option, you really should think about changing them.
The key drawback of boiling is that you must take the strings off and put them on again - which, unfortunately, exposes them to certain risks.
For a start, once they've been wound onto the tuning pegs, the wire is permanently disfigured.
This isn't a problem as long as they remain wound on, but removing them and putting them back on may make it a factor in tuning, consistent string vibration and overall strength - bend and straighten a wire too many times and it breaks.
You can take care in putting the strings back on again in order to achieve an near-identical string position as originally, however, it's likely that you will end up with some inconsistency.
String tension will take care of it to an extent, but the string won't perform as well as when it was originally put on.
With wound strings, an additional problem might be distortion of the windings for the exact same reason.
My suggestion would be to resort to boiling only when left with no other option.
It really shouldn't be seen as a standard method of extending string life.
music.stackexchange.com/users/None/
I used to do this with bass strings, and it does make a difference.
They'll brighten up and sound like new, but not for as long as they did right out of the package.
It's a bit of a pain, though, because of course to remove the strings, you have to fully unwind them rather than simply cut them, so it takes longer.
I wouldn't try this with guitar strings, mostly because the marginal cost savings doesn't seem worth it.
Also, I'm not sure if it would make much difference with plain strings---the idea behind boiling is that it clears out dead skin stuck between the windings, which of course plain strings don't have.
music.stackexchange.com/users/36/Alex Basson
I wouldn't worry about boiling, etc unless absolutely necessary due to very limited finances.
Because out of all honesty, nothing sounds like a new fresh set of strings!
music.stackexchange.com/users/1635/5StringRyan
For some reason, when I put a new set of guitar or bass strings (roundwound) on, I only get about 2 sets worth (1.5 hours) of playing and they are dead!
I tried all brands and none lasted any longer than the D"Adarrio's.
I definately have a body chemical issue!
I played Bass in a hard rock band for 20 years, and I did boil each set of strings between each gig
and it did work.
Ocassinally, they would last longer after boiling and then rubbing alcohol wiping then
when new?
Nobody ever let me touch their bass because they knew what I would do to their strings....
music.stackexchange.com/users/6790/Ron Hallman
Don't boil old strings; once they're gone they're gone.
I boil my new set right out of the package, it makes them softer and more pliable.
That's an old John Entwisle trick, it works.
music.stackexchange.com/users/18975/user18975
I've read that some people boil their strings from time to time to make them sound again like they were new (and save some money).  Is this really worthwhile, or are there any disadvantages to this?
Boiling/washing used guitar/bass strings - is it worthwhile?
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<guitar><strings><bass-guitar>
music.stackexchange.com/questions/2948
music.stackexchange.com/users/580/phoibos
S3: Boiling is a short-term remedy and if you've been taking care of your strings on a regular basis, by the time it becomes an option, you really should think about changing them.  Boiling is recommended only as a last resort. The general consensus is do not boil strings, buy new ones. Some people claim that boiling old strings is worthwhile and gives good results, although only in the short term. Other say it is Not worthwhile unless you have limited finances - Once they're gone, they're gone!
Some people claim that boiling old strings is worthwhile and gives good results, although only in the short term. Not worthwhile unless you have limited finances. Once they're gone, they're gone.