Filing the nut on my guitar

So my friend bought me some strings for my birthday, which I am only now just getting round to putting on. However, the gauge of the string is too thick for the groove on the nut, so I've been told I should file it. However, I'm worried there might be unwanted side effects?

Told by who?

" So my friend bought me some strings for my birthday, which I am only now just getting round to putting on. However, the gauge of the string is too thick for the groove on the nut, so I've been told I should file it. However, I'm worried there might be unwanted side effects? "

Yeah I wouldnt go filing the nut. Especially if you might go back to playing a lighter gauge again.  
 
Is it electric or acoustic? Did you make sure your guitar is setup for the heavier gauge?  Sometimes using a higher gauge can cause tension on the neck and cause other problems if the truss rod isn't adjusted.  Depends on the guitar.

You should try to keep that gauge of your guitar the same as the older ones, or lighter. One major side effect will be that a thicker gauge will start to bend the hell out of your neck.

As others have said, if you're worried about damaging the guitar, take it to somebody that knows how to do it.
 
@CrazyManAndy said:

" Edit: I just re-read your post. All this for one set of strings? If you don't plan on using that heavier gauge regularly, just go and get yourself some strings of the same gauge that you normally use. Not really worth it to do all that for one set of strings. "