A tic is a sudden, rapid, repetitive movement (motor tic) or vocalization (vocal tic). Tics sometimes change over time from one simple type of tic to another or from a simple to a complex tic. Some tics are slow and sustained rather than brief and rapid. Some involve the lower body. Tics are thought to be inherited neurological disorders that affect the body's motor system. They also can be caused by head injury or certain drugs, such as stimulants.  
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