Fr Matthew P. Schneider, LC has joined the discussion.
So first I read Fr. Z's post. Then I read Fr. Schneider's post.
They were both informative.
I haven't smoked in years and years and years. I liked it though. Do I miss it? Not really. But I must admit I like the drugs administered whenever I've had surgery and invasive exams, and in that state I can be reminded of weed - marijuana - and I might reminisce, thinking, it might be nice if it wasn't a sin. That's so far from perfection BTW - to be willing to do something if it wasn't a sin.
Very seriously, I don't miss it. Would I take a hit if offered? I don't think so.
So anyway...
The conversation about marijuana use is important and needful - especially since it is fast becoming legal for medical use as well as recreational use. The thing about dope today is that it is so much more potent than it was way back when it was always and everywhere illegal - and therefore immoral for recreational use.
Today it is very much more potent - or so I'm told. One hit (puff, drag, toke on a joint) just might equal a half a joint of the old stuff. Like I said, I don't know that for sure. The last time someone gave me a joint I hid it away for the Zombie apocalypse and never found it again. Now that's wasted. Back on topic: one hit might have a lot more bang for the buck - so maybe even one hit would be excessive? There is a lot we who do not use it do not know. (Medical use is not the same as recreational use of course. I would think it is morally acceptable, just as sedatives and pain killers when prescribed and used accordingly.)
Waiting to exhale?
I appreciate Fr. Schneider's take better than how it was discussed at Fr. Z's - not that I disagree with Fr. Z - but I'll quote Fr. Schneider instead, because his answer is more direct.
So, it seems that if it’s legal, very small amounts of marijuana would probably be morally acceptable – the side effects are similar to two beers and three cigarettes. That isn’t something I’d recommend, but it isn’t something I’d denounce someone for. However, there are two caveats. I wonder how many pot-smokers only smoke one joint. And we always need to seek what is best and not as bare minimum morality. - Fr. Schneider
Quite seriously, I don't think legalization is a good idea, except for prescribed medical use, but I don't think recreational use is an appropriate 'buzz'. I used to mix - drink and smoke and then chain smoked regular cigarettes - and that's intense. Fr. Schneider is correct, smokers usually smoke more than just one joint - or, as I used to do, mix substances. The potential for more serious abuse is always there. Likewise, habitual smoking of weed becomes another issue. I've known people who smoked every day and were buzzed all of the time - even at work. Most would regard that as chemical dependency or addiction.
Remember, Teresa of Avila always said, "Prayer and self-indulgence do not mix." I say, living a devout life and dope do not mix. You will either despise one and love the other - you can't serve God and marijuana. You can maybe have a rock star type of holiness - you know how they always thank God at the Grammy's, but you know they're buzzed, while their private lives are not all that edifying? I just don't think true devotion includes getting stoned.
True devotion - a holy, prayerful life - is the Christian life. Getting 'wrecked' wrecks it. If John of the Cross discourages seeking visions, consolations, and other mystical experiences, he would surely discourage getting high, hallucinating, and arriving at some sort of stoned-soul prayer of quiet. It's not exactly the narrow way.
But who am I to judge?

