callus's will strengthen your fingers. it's normal.
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Topic title.

It means you are getting a musicians fingers. What did you expect? Cancer? :-)
You fret with your thumb? Cool i guess. It's good to have callouses, they're really useful, it makes playing more comfortable.
A callus (or callosity) is an especially toughened area of skin which has become relatively thick and hard in response to repeated friction, pressure, or other irritation. Rubbing that is too frequent or forceful will cause blisters rather than allow calluses to form. Since repeated contact is required, calluses are most often found on feet because of frequent walking. Guitarists often also develop calluses on their fingertips due to the frequent friction on the strings. Bowlers will often develop calluses on the thumb as well as their middle and ring finers due to the friction caused in releasing the ball. Bicycle riders can also get calluses especially if they are not wearing gloves. Calluses are generally not harmful, but may sometimes lead to other problems, such as skin ulceration or infection.

****, you should stop playing guitar :P
Omfg. I meant my thumb behind the neck has a callus. lol. I don't fret with my thumb.
try playing as lightly as possible on your fretting hand;
e.g. fret a dead note. then slowly press down until you JUST fret the note CLEANLY.
this is the pressure you want to play with, so practice it with scales, chords, arpeggios etc.

if you already do this, or have good fretting habits, I suggest breaking out on the moisturizer...
They are a sign you play a lot so that is good,
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