John Lennon took over a thousand trips and said he never had a flashback or a bad experience and he never knew anyone who had any problems from LSD either.
not according to him in the interview mentioned above, here is the link again,
http://taz4158.tripod.com/johnint.html/
Here's what lennon had to say when asked about bad trips.
interviewer: Did you have many bad trips?
lennon: I had many. Jesus Christ, I stopped taking it because of that. I just couldn't stand it.
interviewer: You got too afraid to take it?
lennon: It got like that, but then I stopped it for I don't know how long, and then I started taking it again just before I met Yoko. I got the message that I should destroy my ego, and I did, you know. I was slowly putting myself together round about Maharishi time. Bit by bit over a two-year period, I had destroyed me ego. I didn't believe I could do anything. I just was nothing. I was shit. Then Derek [Taylor, Apple press officer] tripped me out at his house after he got back from L.A. He sort of said, ``You're all right,'' and pointed out which songs I had written: ``You wrote this,'' and ``You said this,'' and ``You are intelligent, don't be frightened.'' The next week I went to Derek's with Yoko, and we tripped again, and she made me realize that I was me and that it's all right. That was it; I started fighting again, being a loudmouth again and saying, ``I can do this. Fuck it. This is what I want,'' you know. ``I want it, and don't put me down.'' I did this, so that's where I am now. At some point, right between `Help!' and `Hard Day's Night,' you got into drugs and got into doing drug songs. A Hard Day's Night, I was on pills. That's drugs, that's bigger drugs than pot. I started on pills when I was fifteen, no, since I was seventeen, since I became a musician. The only way to survive in Hamburg to play eight hours a night, was to take pills. The waiters gave you them - the pills and drink. I was a fucking dropped-down drunk in art school. Help! was where we turned on to pot, and we dropped drink, simple as that. I've always needed a drug to survive. The others, too, but I always had more, more pills, more of everything because I'm more crazy probably.
Interviewer: How do you think LSD affected your conception of the music? In general?
lennon: It was only another mirror. It wasn't a miracle. It was more of a visual thing and a therapy, looking at yourself a bit. It did all that. You know, I don't quite remember. But it didn't write the music. I write the music in the circumstances in which I'm in, whether it's on acid or in the water.