One can experience numb lips for a number of reasons, such as exposure to cold temperatures or as a result of a mild allergic reaction, according to Heathgrades. Some prescription medicines may also cause numbness in the lips.
More serious causes of numb lips include neuropathy or nerve damage, according to Healthgrades. With neuropathy, the nerves between the lips and brain are damaged to the extent that they cannot transmit signals back and forth. Neuropathy has many causes, including exposure to toxic chemicals and nutritional deficiencies. Numb lips are also a sign of a severe allergic reaction called anaphylaxis. Anyone who experiences numb lips along with trouble breathing or swelling of the face or throat should seek immediate emergency care.