http://www.nida.nih.gov/Infofacts/LSD.html
Quote Originally Posted by TFA
Flashbacks usually occur in people who use hallucinogens chronically or have an underlying personality problem;
There are many informative links here:
http://www.erowid.org/chemicals/lsd/lsd.shtml
If you really want to feel informed read everything here.

If you use it recreationally you should be fine. As the article says, it's not physically addicting. I've seen studies say it's not mentally addicting, but I had a couple friends that had a mental addiction.

Galapagos's friend probably had a hit laced with something else, especially if it was a gel or tablet. LSD does not permanently alter the brain. The only drug I'm aware of that does that is DXM which is in cough medicine. It is very powerful, with the effective dosage being ~100ug, where Psilocybin takes ~100mg (1,000 times more).

The actual medical affects are really sketchy because the government banned all study on the drug in the 70s when they made it illegal (they were afraid the communists could use it for mind control).