My two cents: Been an off and on smoker since 15 (32 now) and I've tried it all from South Carolina dirt weed with sand in the bag to Amsterdam cafes. I've had the pass out highs, throwing up, the whole nine yards. I quit smoking for a while in my twenties because that was the first panic attack I'd had. It was different than the tunnel vision / white blurry vision stuff I'd had before. I'd never had a panic attack so I freaked out which made it worse. I took a lot of time to chill. Since I've gone back to smoking I've had much better quality weed in general and I've experienced mostly good but also the bad. Three times in particular I might have smoked weed that was tainted (the smell and taste were not great) or I react differently to Sativa or Indica or various levels of THC...who knows. Experiences for different people with different strains range from euphoria to dread apparently. My bad experiences have all included panic attacks but without much of a marijuana high that I expected to have. It seemed much more dissociative and, well, chemical. I also end up with strange sensations on my left side and my left pupil gets larger than my right. I don't quite feel like myself and I feel doomed for some reason. I've heard that certain kinds of weed can do this as well and it can be a panic attack thing in itself. I've had a full body work up and there are no heart / lung problems, it has to do with the nervous system apparently. Just to mention this: PCP can create the same physical effects as a panic attack (sweating and chills, anxiety and paranoia...and of course much worse sometimes) so it's not impossible that some of us may have gotten 'wet' a time or two before (or smoked Raid or something like that). I've recently had a 'panic attack' if that is what it was after trying a different strain (I've been smoking Trainwreck for about a month now and just switched) and it's apparent that certain kinds of weed will not agree with me (Trainwreck did not adversely affect me at all but this stuff did). In my opinion this has nothing to do with tolerance, it has everything to do with biological make-up, what age you start smoking (the younger you are the more likely you'll develop a panic / mood disorder), and whether or not you're smoking just weed. Unfortunately weed does something for me that I love and I have a hard time moderating so I'm better off taking a break completely. I've realized that it's not worth it for me to keep smoking and risk these attacks as they seem to start to occur more frequently. I seriously doubt I was unfortunate enough to find laced / treated weed three times in the past 5 years so I assume it's either the type of weed or other circumstances. This past experience makes me think I'll never smoke again unless it comes from a certified organic medical vendor (hopefully NY will legalize in a couple of years) and if I have a panic attack I'll know it wasn't meant to be. Moral of the story: marijuana is a psychoactive drug and can screw with your mind in ways you may not like, some of us are better suited to smoke it than others and it is an illegal substance in most places so you might get more than you bargained for from time to time. But as the saying goes: Everything in Life is a Wager.