Acid is not like pot, or alcohol, or any other "mind-altering" drug. You don't know what mind altering means unless you've been on a hard core trip.13 It's like you suddenly realize you've been seeing the world in two dimensions and now you can see four. The rules change. People around you become perfectly comprehensible and beautiful creatures, instead of the average dicks they are most of the time. I reject completely that acid doesn't bring some insight about the universe; yes, it's all in your head, but so is the everyday, non-tripping, non-psychotic universe. You have one tool with which to experience the totality of your conscious life; if you make any deductions about the world around you with it, you have to reevaluate your deductions when you realize you can interpret the world in a completely alien way by rewiring your perceptions with half a gram of chemicals. Acid puts the whole meaning and being versus reality play you leave on the TV as background noise directly in front of you on an IMAX screen with surround sound and popcorn. Whatever experience of the universe there is to be unveiled by the modern human brain, a investment worthy percentage of it can be seen during a really good acid trip.
My personal belief is that, moreso than any religious belief, it's important to manage the karma we experience-- to learn from the bad, and not to turn the good into bad by becoming hubristic. Negative karma will ripen, and the important thing to do is to let it do so in a way that doesn't turn into recursive negativity that generates more bad karma (either for oneself, or in negative actions toward others). What bothers me about these drugs is that they seem to take away some of that ability to manage the karma that comes through. You experience a lot of sudden karmic ripening, which is interesting, but the insight that needs to develop over time isn't there for a lot of people, and those I've met who I think could use those drugs effectively (rather than haphazardly) often have no interest. I've been very unimpressed by most of the drug mystics I've met, because most tend to be sophomoric and unbalanced. I'd rather talk to a zen master any day of the week.
Personally, I'm not interested in using these drugs at this point in my life, but I don't think they should be illegal or as stigmatized (in the larger society) as they are. They're probably less dangerous than alcohol. The problem is that, if you choose to use these drugs, you're trying out a technology that's still in the very early experimental stages, mostly because the research has been (wrongly, I would argue) outlawed.
Your brain gets data, and has models of how all the data fits together. For example, if you look at someone you don't know, your brain will get the data "appearance" and plug it into the model "assorted stereotypes" and get out some expectations about who that stranger is.
What LSD does is perturb those models of how data fits together. If your models are in line with reality, you don't have much to worry about. After your trip, everything will settle back to about where it was before. If, however, your models are out of whack (e.g. you blame others for failures that are your own fault) then you run the risk of having some really really unpleasant realizations while you're looking at the data without your old model mucking things up.
If a person's already on a bad course, then psychedelics (under typical recreational use patterns) are going to throw this person farther out into space. The negative experience, in this case, isn't that person being ripped back into reality, but going even further away from it.
That might play a role (assuming there is such a thing as karma). However, I think there's a lot more to it.
Psychedelics can be used constructively or destructively, therapeutically, religiously, or recreationally. They can be used with positive intention, negative intention, or no particular intention. They can be used in a safe, pleasant setting, in indifferent settings, or in a scary, unpleasant setting. You could use them alone or with friends, strangers, with wise, knowledgable, trusted guides, or in a mix of any of the above. The person using them may not know what to expect, or might be well educated on the effects of psychedelics. The person might be very experienced or this might be their first trip. The dose of the substance might be small or large. The substance may be pure or impure. The person might fight against the effects of the substance or give in. They may have great respect for the substance or have a very casual or cavalier attitude towards it.
All of these factors and more can greatly affect what happens during the experience. To reduce it all to karma or any other single factor is way too simplistic.
I'd like to see a show of hands of past psychedelic use at an American dharma teachers' conference.
And I really wonder why one of these teachers hasn't written a little how-to guide for how to deal with the effects. I wouldn't really mind if it was really just a pamphlet for meditation. Just something from some kind of wise -- and nonjudgmental -- perspective. Maybe it's been done.