On Monday, American judo competitor Nick Delpopolo was expelled from the
Olympics for doping with cannabis. (He says he accidentally ate a pot
brownie.)
Caffeine, a known stimulant is NOT on the banned substance list, but pot is... duh!
Here's how they justify it: Cannabis is considered to be in violation of all three main
disqualifying criteria. It violates the first (performance enhancement)
because of its ability to decrease anxiety and fear, and potentially to
improve some types of oxygenation and concentration. It violates the
second (health risk) because it can result in, among other things,
“decreased cognitive performance” and “pulmonary toxicity.” It violates
the third (spirit of sport) because of the drug’s widespread illegality
and conflicts with the “role model of athletes in modern society,” along
with “negative reactions by the public, sponsors, and the media.”
http://www.popsci.com/technology/article/2012-08/fyi-why-cannabis-olympic-banned-substances-list