While I wouldn't say there isn't an argument against allowing marijuana to be used for medical purposes, the FDA's arguments in its release (read the whole thing here), are ridiculous.
The FDA's fundamental argument is over whether marijuana is "safe and effective medication": marijuana "has a lack of accepted safety for use under medical supervision" and there is "sound evidence that smoked marijuana is harmful."
Gee, I guess if you're dying of AIDS--or cancer! you really need to worry about the harm a joint might do to your health!
As for the "effective" argument, I repeat: if a sick person says it helps, IT HELPS! If a nauseous person says a joint makes him feel better, that is, by my definition, "effective" -- regardless of what any large-scale study might find. Even if marijuana advocates' claims are grandiose, which for all I know they may be, the basic claim -- that marijuana can alleviate discomfort -- is valid if real, suffering people say it helps.
"But it has a placebo effect--they only think they feel better!" OK -- and your point is? Because my point is, "feeling better" IS THE POINT.