Seeing as how we allow people to drive/carry while on caffeine or under the influence of nicotine, my main thing would be: Is somebody, under the influence of THC, substantially more likely to use a weapon improperly? I want to see actual studies, not suppositions.
As far as caffeine and nicotine go, the Mormons (I think?) have it right, they do affect you negatively. As an addict of both, however I think I can fairly say they affect you WAY differently than alcohol and pot. Except in very large quantities, they only make you jittery and hyper, but don't substantially impair your reflexes or judgement. I'll add sugar to that list of "drugs". Watch the reaction of a small child to eating a dozen brownies/cookies.
Having used pot several times myself, and having known a few potheads, I think the problem would be more physical than emotional in regards to using the weapon improperly. That would actually be a WORSE concern for someone who was genuinely using pot medicinally as opposed to a pothead, as the medicinal user would be less likely to have acclimated to the physical impairments that accompany use of the drug.
I actually have LESS concern for a medicinal pot user having a firearm (legally), than I do a drinker (who drinks to the point of intoxication), as I've met the drinkers who alcohol seems to disinhibit the angry mean side of their character, but never the pot user.
My concerns about medicinal pot users carrying (and not carrying) firearms are their own safety. It would suck to have a pain problem (and I do know a little something about pain, both mental and physical) that is so bad that you cannot for large time periods effectively defend yourself without a risk of shooting yourself in the leg when you attempt to draw a weapon.
I'm WAY more comfortable with a pot user (who is otherwise law abiding) carrying a gun, than a driver (who is otherwise law abiding) driving after drinking (more than a very small amount), because a pistol is just a pound or two of steel and plastic until a finger is put in the vicinity of the trigger, whereas a moving car is a ton of steel with no direction other than that given it by the driver.
If I were a law enforcement officer who came across a medicinal pot user who was carrying, my first question would be "right now, at this time, are you high?" If that person said they were (or I disbelieved them when they said they weren't) , my second concern wouldn't be that that person would want to hurt me, but that that person would be so eager to show their compliance with my authority, that they would shoot themself in the leg trying to take their weapon out of their holster to surrender it to me.
