The Institute of Medicine published in its Mar. 1999 report titled "Marijuana and Medicine: Assessing the Science Base":

“The movement disorders most often discussed as candidates for marijuana-based therapies are dystonias, Huntington’s disease, Parkinson’s disease and Tourette’s syndrome.

As a general consideration, it is important to note that stress and anxiety tend to worsen the symptoms of movement disorders. Thus, marijuana’s calming effect could be a primary reason why some patients claim that it brings them relief.”

Mar. 1999 - “Marijuana and Medicine: Assessing the Science Base”