Botulinum toxin is a poison produced by a certain type of bacteria (Clostridium botulinum). It is the same poison that causes a type of food poisoning (botulism).

Small amounts of the purified toxin (Botox) may be injected to weaken or relax muscles that are in spasm or are contracting uncontrollably, such as spasms in the eyelids or in the lower esophagus. It also may be used to rejuvenate the aging face by temporarily weakening or paralyzing the muscles, which has the effect of smoothing or eliminating wrinkles in the skin. The effect usually wears off within a few months. When injected in small amounts into muscles, the toxin does not get into the person's bloodstream and cause illness.