WATCH: Man with Parkinson's uses marijuana for the first time -- and the results are amazing

Larry Smith is a former cop who lives in South Dakota. He’s had Parkinson’s for 20 years, which gives him symptoms including severe dyskinesia (uncontrolled movements), loss of voice and tremors. This clip from Ride With Larry, a documentary about him and his extraordinary efforts to address his illness and raise awareness, has gone viral on social media—and it’s easy to see why.

This article was originally published by The Influence, a news site that covers the full spectrum of human relationships with drugs. Follow The Influence on Facebook or Twitter.

Smith is shown taking medical marijuana for the first time—it is still not legally available in South Dakota. After he puts a drop of cannabis oil under his tongue, the unedited, sped-up film shows the rapid change he undergoes. His involuntary movements end, his hands are steady and he can speak normally.

Dr. Daniele Piomelli of UC, Irvine explains what we know about the science of what’s going on, as well as frustrations about the political obstacles to wider study and use of medical marijuana. For many, the sight of this dramatic positive change in a person with this debilitating disease will speak louder than words.

This article was originally published by The Influence, a news site that covers the full spectrum of human relationships with drugs. Follow The Influence on Facebook or Twitter.