Well, I’ll give my opinion on the morality of drinking alcohol and then look at how the same concepts would apply to marijuana.
It seems to me that there are three main ways in which consuming alcohol could be sinful: illegality, irresponsibility, and gluttony.
Illegality is the easiest, which is why I put it first. If drinking alcohol, or drinking it in a particular way is illegal for you where you live (because you are under the legal drinking age, for example, or in a moving vehicle), then drinking it in violation of that law shows contempt for legitimate authority. It’s true we are not obliged to obey unjust laws, but denying classes of people alcohol or limiting the settings in which one may drink does not violate any moral law I am aware of. We might question whether the government should ideally get so involved in our eating and drinking habits, but I don’t think there’s any question that it has the authority to if it makes that decision.
Irresponsibility would mean consuming alcohol in a matter (quantity, timing, etc.) that puts you or others in some kind of danger, whether physical, moral, relational, etc. It would be irrational and thus immoral to drink and drive, or to drink in a socially inappropriate setting, or to drink to the point that you may well commit sins you never otherwise would have, or to seriously endanger your health through your drinking.
Gluttony is the most difficult of the three to pin down, because it is the most subjective. The primary consideration here is your motivation for drinking, though excessive quantity may well result from having bad motivations. Drinking a beer with your hamburger at the sports bar is a normal, rational, presumably non-gluttonous thing. You are using the beer as a beverage, for refreshment and to help down the burger, which in turn is being eaten for sustenance. When taking shots of liquor or shotgunning a beer one has, ordinarily at least, isolated drinking from its natural purpose of refreshment or aiding in eating and have made the incidental pleasures of the action the sole purpose for engaging in it. Even if you don’t actually drink an excessive amount in such a case, you have passed beyond the lines of reason in your manner of and motivation for drinking. Alcoholic beverages are first and formost beverages, and should be used as such, even if there are secondary reasons why you have chosen that particular beverage out of many possibilities, like the enjoyment you get from its taste and alcohol.
In the case of marijuana, in places it is illegal that closes the possibility of its morality right there. But this thread is about whether it should be made legal not whether it is legal, so we should look at the other factors.
The issue of irresponsibility would be essentially the same as for alcohol. Smoking the drug and then driving would still mean driving while ability-impaired, and so would be irrational and immoral risk taking. If consuming a given amount significantly increases the chances you will commit a sin (especially a mortal sin) that also could make using it immoral. But if it is possible to take in the drug in such a way as to avoid these dangers, then it’s time to move on to the next subject.
Again, gluttony is the most subjective of these criteria so it can be difficult. To discern whether something is gluttonous, we should first look at what, if anything, is the rational use for the thing which we might become gluttonous about. One rational use for marijuana would be to ease symptoms and spur appetites of cancer patients and other ill people. In this case it would act like many good, moral prescription drugs. As long as it is legal, actually effective, and the risks (both to the individual and to society) don’t outweigh the benefits, I see nothing wrong with the medical use of marijuana.
But what about the non-medical use of marijuana? What is the rational purpose for it that could be replaced by pure selfish pleasure-seeking? One poster mentioned using it to help him get to sleep, analogous to a sleeping pill. I suppose this could be a valid use, but it could ultimately be classified as a medical use. What about just smoking or otherwise consuming the drug for pleasure? It seems to me that, in the case of smoking at least, that the action cannot be done except in a gluttonous manner. Perhaps one could make an argument that adding it to food would be very closely analogous to having alcohol in beverages. Maybe so, but in those cases the person should be consuming the food primarily for the sake of nourishment, not as a drug. Is this the way it’s really often done in practice?