Lidocaine is a medicine most commonly used by dentists. It comes as a jelly rubbed on the gums, and causes a lack of feeling during dental procedures. You will not feel pain or any other sensations until Lidocaine wears off, which usually takes about 90 minutes. As a precaution during that time, you should avoid hot drinks or situations in which you could injure your mouth. Physicians sometimes use Lidocaine as a local anesthetic before minor surgeries or to numb pain from cuts and other injuries. A few people are allergic to Lidocaine and may experience severe symptoms such as faintness, difficulty breathing, and swelling of the face and mouth. In very rare circumstances, Lidocaine can cause a serious blood problem called methemoglobinemia. Makers of illegal drugs frequently add Lidocaine to cocaine as a filler because it intensifies numbness. After they taste the product, cocaine users believe they are buying a high-quality variety of cocaine, when they are actually getting Lidocaine.