Late addition to this post, but I trained in rebirthing which is pretty much the same as holotropic breathwork (I'm not affiliated, so not a plug to the organization), and I'm surprised the consensus seems to be it doesn't work. I didn't have overly trippy experiences the first time I tried, but I commented to my instructor afterwards that if felt like acid, not knowing anything about holotropic breathwork at the time and he smiled knowingly. I've since had many mind blowing experiences on it but not as confusing as trips. Like others pointed out, it isn't as quick and guaranteed as a psych (it's hard work), but can be just as profound. I also take issue with the statement that it involves hyperventilation. I think this is the wrong word entirely. If you are hyperventilating you are doing it wrong. This is consciously-connected breathing. Your in breath is even with your out breath, it's deeper breathing than usual (you're running a marathon here so breathing too deeply can be unsustainable) and there is no pause in the transition between exhale and inhale. You keep it up for about an hour. I've gone as long as 2 and a half or so. I've was a true psychonaut at one point in my life and can attest to the effects being real.