Williams was diagnosed with multiple sclerosis in 1999. In the following year, Williams created the MS Foundation, a nonprofit organization with a focus on research and education. Williams has openly stated that he uses medical cannabis, stating it helps to ease his multiple sclerosis–caused neuropathic pain. Williams has become a vocal advocate of cannabis, supporting efforts to pass medical cannabis laws in states, as well as calling for full legalization. He has also said that snowboarding is his "best therapy" for MS, commenting that "When I stand up I need first to hold on to something and think about the positioning of my legs. If I were to just start walking I would fall. I have to get my brain to find my legs and then I will usually take a test step, but I say something at the time to anyone who might be watching to distract from what I'm really doing. Then I'll find places to grab as I walk and talk, sometimes even walking backwards because I have more control that way. People have no idea that I'm doing this. But when I'm snowboarding and my feet are strapped in, my brain seems to have a direct connection to my legs. After snowboarding it's night and day for my balance and walking. There's a real physical change before I get up to the mountain and when I come down. The benefits last for days."