Cannabis is a prohibited substance in-competition in most combat sports jurisdictions, but only over a certain threshold. Only if an athlete tests positive for more than 150 ng/ml of cannabis in a sample collected in-competition will that athlete be sanctioned by the regulatory body.
Things are different in Illinois, though. The Illinois State Athletic Commission (ISAC) has a zero-tolerance policy for banned substances, including marijuana. There is no such threshold for cannabis, so if a fighter tests positive for even a small amount of the drug he or she can be suspended, fined and possibly have a win overturned.