So guess what? Turns out some researchers in the Netherlands think more countries should legalize pot.

For those who think flying to Amsterdam (or Colorado, etc.) to indulge in the herb is just fine and dandy, let’s do some thinkin’.
Principle 1: Creation is good.
Principle 2: Not all creatures are equally good.
Principle 3: We ought to avoid evil.
Principle 4: Rastafari is a false religion.
Humans are ontologically higher than rocks, plants, and animals. We can use them, even to their detriment, if they are beneficial enough to us. Jesus was not a vegetarian. And yes, Brother Carrot and Sister Lettuce are okay to kill, unless it is out of sheer disdain and spite for their existence as creatures of God.
But Uncle Bud is a little different, because when we harvest him, it’s usually for the sake of affecting our bodies in a way that suspends our intellect.
Eat-ay ad Thomam:
The sin of drunkenness, as stated in the foregoing Article, consists in the immoderate use and concupiscence of wine. Now this may happen to a man in three ways. First, so that he knows not the drink to be immoderate and intoxicating: and then drunkenness may be without sin, as stated above (Article 1). Secondly, so that he perceives the drink to be immoderate, but without knowing it to be intoxicating, and then drunkenness may involve a venial sin. Thirdly, it may happen that a man is well aware that the drink is immoderate and intoxicating, and yet he would rather be drunk than abstain from drink.
That’s from the Summa Theologica, II-II, Q. 150, art. 2. Wine stands here for any intoxicating substance… One might not know a substance to have intoxicating effects, and so there is no sin in such drunkenness (unless its use was immoderate for other reasons). But if one knows something to be potent, it is another story. But just how drunk is “drunk?”
[The third kind of man] is a drunkard properly speaking, because morals take their species not from things that occur accidentally and beside the intention, but from that which is directly intended. On this way drunkenness is a mortal sin, because then a man willingly and knowingly deprives himself of the use of reason, whereby he performs virtuous deeds and avoids sin, and thus he sins mortally by running the risk of falling into sin. For Ambrose says (De Patriarch. [De Abraham i.]): “We learn that we should shun drunkenness, which prevents us from avoiding grievous sins. For the things we avoid when sober, we unknowingly commit through drunkenness.” Therefore drunkenness, properly speaking, is a mortal sin.
So there is still a mystery… How intoxicated must one be before he “deprives himself of the use of reason?” Let’s remember a few things though before we shame the Angelic Doctor for being obscure. First, he expects a student to have read all the text which precedes this Article. That would give one a better idea of what he means. Second, the Summa really is just a beginner’s crash-course. It is not meant to be exhaustive. In some articles, this is more evident than in others. Third, it’s unlikely St. Thomas had much firsthand experience with drinking to provide us with more subtlety… When Albertus Magnus is your professor and Bonaventure lives on your hall, you’re inspired to “rise above the influence,” as it were.

However, we know that some of the more austere saints occasionally indulged, such as Charles Borromeo and John Vianney. And of course, the Lord did as well, as He so famously pointed out in Matthew 11:19. Since potent substances will technically have some effect no matter how little is taken of them, we can say from this alone that it is not evil in itself to use intoxicating substances.
Then there is the principle of totality to keep in mind. Later in the same Question, Thomas alludes to this by saying a physician might tell one to use drink to induce vomiting – but since lukewarm water works too, that should be used instead. However, if it didn’t (and we hadn’t discovered Ipecac) then it would be fine. This is because the greater health of the body is worth the temporary loss of reason… That’s also why it’s not a sin to plan on going to sleep each night! And while there is violence done to the body and soul when, for instance, a gangrenous limb is removed, it is for the sake of the entire person. But this too should be moderated by wisdom, since not every ailment is worth doing violence to yourself. If you get occasional leg pain, that doesn’t mean you should cut off your leg.
So anyway, how drunk is drunk? How high is high? It is so difficult to say because of the problem in trying to quantify a quality. “It’s when you feel like… you know, drunk.”

We won’t solve the issue of exactly where “the line” is today – maybe another post with some ¡HARDCORE SCIENCE! – but perhaps we can lay down some guideposts based on Thomas and basic research.
Certain drugs act far more quickly than others. THC (the active ingredient in marijuana) acts more quickly than alcohol, based on the popular conventions of consumption. That is a big deal.
Reason helps us to distinguish the true from the false. Once you have trouble doing that, it’s time to call it a day… If sober people around you start laughing at everything you say, it’s probably not because you’re witty and charming, it’s because you’re diminishing your material brain’s capability to interact with your immaterial intellect (AKA you’re becoming drunk). And so on.
If you’re starting to forget stuff that you shouldn’t forget, then that’s another sign your faculties are slipping. So put it down.
When you feel like doing something really dumb that you normally wouldn’t, STOP and don’t do that much again. Once you know that tequila makes your clothes fall off, then kiss it goodbye. It’s better to enter into life without a bottle of Patrón than into Gehenna with all you could ever drink. (And Tequila burns even without being on fire…)
If you can’t walk right and are slurring words, then your brain is shutting down. Same story.
So no hard answers here today, but basically the faster and stronger the drug, the less morally safe it is to use. AND, if one uses any substance for the pleasure of changing his mental state in a way that diminishes its capacity to execute its proper function, as distinct from some some other effect, this too is a red flag… We should not delight in an unnatural state!
All this would make the average consumption of pot pretty bad.