Alright, my answer may surprise some people. I answer as follows:
In the
Groundwork of the Metaphysics of Morals, Kant tells us that we should "act so as always to treat humanity, whether in your own person or in the person of another, always at the same time as an end in itself, never merely as a means." To treat a person as an end-in-himself is to respect his rational autonomy. Whenever you get drunk/high, you are treating yourself merely as a means to an end (namely, whatever the recreational effects of getting high are). Getting drunk/high impairs your ability to exercise your rational autonomy, and therefore it is contrary to the Moral Law. Of incidental note: A Kantian professor of mine in college consciously abstained from alcoholic beverages.
So, is smoking marijuana immoral? Yes, probably to the same sense that getting drunk is, if we ignore the fact that the health effects of alcohol abuse are substantially worse than smoking marijuana, and consider getting drunk and smoking marijuana only
qua relieving oneself from sobriety.
That said, there's still a question of degree. Some acts are substantially worse than others. Killing your father is much worse than stealing a dollar from Wal-Mart. Since exercise of your rational autonomy, although impaired, is still quite possible while high (when I smoked incense that one time, I still was able to walk around and talk and stuff...though I smiled like an idiot at everyone, waved and acted silly overall). In fact, you're much more functional high than drunk.
Therefore, to whatever degree getting drunk is immoral
qua getting drunk, marijuana is less than that.
Then, I don't think that the health effects are non-negligible. Marijuana abuse is much less harmful to the body than alcohol abuse is.
But most importantly: Should it be illegal?
The answer is no. Criminal law ought to concern itself solely with the transgression of rights. I have committed an intrinsically criminal act if and only if I intentionally have transgressed someone's life, liberty or property. Since marijuana involves none of these, it ought not to be illegal, since there is no victim. A crime must have a victim.
Simply put, and here I address Inzl Kett directly:
What I do in my personal time is
none of your business. In fact, and here I quote Metallica, "why don't you worry about yourself instead?"