Well in a sense, it is already starting to. According to a study by the University of British Colombia in Canada, most medical marijuana users are substituting prescription pills for marijuana. This is the largest study of medical marijuana usage to date, and the results led them to surmise the following:
“Our study shows that more than 80 per cent of medicinal cannabis users reported substituting cannabis for prescription drugs including opiate pain killers."
The study also looked into the reasons for this and surprisingly (or not, depending on your viewpoint) it was because marijuana was seen as less addictive than prescription drugs. Now, this brings up a whole other conversation about the supposed role of marijuana as a gateway drug, but I'll save that for another time. Instead, I'll post just a few examples of where marijuana can be effectively used as a substitute for a prescription drug.
This one is pretty obvious, but marijuana can certainly be used as a substitution for the like of Vicodin.
ANTI ANXIETY MEDICATION
This one is a bit more complicated because certainly some people can have anxiety fuelled my marijuana. That said, it can also really help people calm down and relax. I suppose this depends more on the individual's response to the plant.
I actually am especially interested in this one as I had a friend who became addicted to Ambien. He was struggling to get to sleep and was wrongly (I believe) prescribed this. It was fine at first but his tolerance built up to the drug and he basically couldn't ever sleep without it. Marijuana could definitely be used as an effective substitution here.