There are a lot of the bad trip effects and paranoia that can be eliminated through environment. If you are doing an illegal substance and the worry gets into you that you are going to get caught it can very easily work it's way into your subconscious and manifest itself in the trip. Legalizing it may help with some of that, but it is also the mind of the person doing it. If a person worries excessively I would never recommend a trip. There comes a point in a lot of heavier drugs where you start noticing your body functions. Once people start wondering if something is abnormal that can lead down a bad path. Even for some people who know what they are doing. You don't want a person to start getting worried about their heart rhythm because it is a horrible spiral. Also many drugs can cause shadow movement and seeing things. Auditory hallucinations are also something that can occur and they seem to fool you more than a visual effect. I have known a couple of people to get auditory effects from Cocaine. From what they say it sounds like someone or something bumping around outside or in the walls.