Let me ask you this, can you enter samadhi when you're high? How easy is it to maintain mindfulness when you've smoked a bowl? Is your awareness as sharp? Can you establish atapi or sampajanna? I know when I used to smoke, getting rid of anything that could be construed as ardent and clearly comprehending was kinda the point.
One thing that I think gets lost in regards to the fifth precept is this blunting of perception. When we reduce this training rule to only those conditions that cause heedlessness, we ignore the larger way this precepts figures into our training. I mean, think about it. Are all of those fermented substances listed by the precept really the only things that cause intoxication? Of course not. There are plenty of other psychoactive compounds and not all of them are chemically derived. Porn, biased news coverage, our phones, and countless other vices remove us from full engagement with experience. They cut us off from the moment. There is only so much mental real estate we're afforded. When we crap it up with these mental intoxicants, there's no room left for the path.
Of course, this doesn't mean we should beat ourselves up. If a life saving medicine makes you a little woozy (e.g. chemotherapy), by all means go for it. You aren't much use as a Buddha if you're dead. But if the choice is between popping a couple of Benadryl and going all balloon head or having a running nose, sit down on the cushion and let your nose drip down your chin. I think you'll find that samadhi is a rather impressive panacea. More importantly, our egos hate it when we suffer. It's an affront to everything it stands for. Embrace these minor irritations and club it over the head.