Mouth numbing is a physical side effect of administering certain drugs sublingually (under the tongue) or buccally (via the cheeks and gum). The effect can be described as a distinct feeling of general numbness or tactile suppression around the tongue and mouth which can last for up to an hour after the drug has been administered.
The NBOMe series (25C-NBOMe, 25B-NBOMe, and 25I-NBOMe) cause this effect consistently and it is accompanied by a strong, unpleasant, metallic chemical taste immediately after sublingual absorption.
The stimulant known as cocaine also causes numbing of the tongue, gums, and mouth when administered sublingually. Many people test the purity of their cocaine by rubbing it in their mouth. This, however, is not a guarantee of the drug's quality as it is common for cocaine to be cut with various other numbing agents and local anaesthetics, such as novocaine, lidocaine, or benzocaine, which mimic or add to cocaine's numbing effect.