Nothing as bad as most chemotherapy drugs do to the immune system, that’s for sure... If it does the same thing as other drugs and has fewer side effects, I don’t see why it shouldn’t be legalised for medical purposes. There are dozens of other medications that are more harmful and/or more addictive than marijuana (painkillers like Vicodin being among them) that are legal at various levels, even including cocaine if it’s a special circumstance and the doctor has the appropriate license to prescribe it.
Methamphetamine is also a Schedule II drug like cocaine that can be prescribed. It isn't used much, but it can be prescribed to treat narcolepsy, ADHD, exogenous obesity and a couple of other disorders. It is sold under the brand name Desoxyn. http://www.drugs.com/pdr/desoxyn.html If the feds would just allow marijuana to be prescribed and sold from pharmacies it would kill these medical marijuana initiatives in the states. People would be stuck with overpriced mediocre product from pharmacies. The feds are too stupid to do that though. They're sticking with their guns, keeping marijuana a Schedule I drug, claiming it has no medicinal value. Since the feds won't allow marijuana even by prescription voters in several states have voted to allow for medical marijuana programs where people can grow their own and in some cases the states are even allowing commercial growing and sales from dispensaries that are kind of like pot bars. In a couple of instances in recent years state legislative bodies have actually enacted these laws, and we're probably going to see more of that because the polls all show that the majority of Americans think that sick people ought to be able to get marijuana if it might help them. The majority are for it even in Southern states that have the lowest levels of support for marijuana legalization. With the voter initiatives and these instances where state legislatures pass medical marijuana laws I bet from now on that we see an average of one state a year passing medical marijuana laws until the feds allow for it to be sold from pharmacies by prescription.