Charlie Sheen Drunk a Guns N' Roses Concert?

Sources close to Sheen tell TMZ -- the Warlock realized people were concerned he had fallen off the wagon ... but he is adamant ... he NEVER gave up booze.

Sheen is telling friends he does drink occasionally ... and had a few drinks at the concert ... but they hit him "pretty hard" ... because he simply doesn't have the tolerance he had in his heyday.

Charlie told one friend, "This is what happens when you don't drink very often."