For a person in the middle of a serious drug trip, hallucinations, confusion and raging emotions can make it feel as if he or she is teetering on the edge of insanity. Depending on the drug and the person, it very well might be true. In an issue of Marie Claire, singer Stacey "Fergie" Ferguson of the Black Eyed Peas describes a harrowing drug trip from crystal meth -- a drug known for its distressing psychological aftermath. "I had about 20 different conspiracy theories. I painted the windows in my apartment black so they couldn't see in," Fergie told Marie Claire, explaining that she thought the FBI was after her during her brief addiction to methamphetamines around 2001. "One day, when I was about 90 pounds, a guy comes up to me. ... I'm searching in the bushes for clues about whatever they're after me for. I'm in a cowboy hat and red lips. He hands me a muffin. I'm thinking, he's in on it," Fergie said. But for all of the horror in Fergie's FBI scare, she might have been at risk for something worse. The paranoid delusions of a drug user and the experience of a person with mental health issues barely differ. "They're the same thing," said Dr. William Compton, director of the division of epidemiology services and prevention research at the National Institute on Drug Abuse. And unlike other drugs, methamphetamines can directly cause a permanent psychosis -- the technical term for a delusion or a hallucination. A Cause, or Just a Trigger? More commonly referred to as a "bad trip," the psychotic symptoms from drugs can arise for a number of reasons. Marijuana, ecstasy and alcohol withdrawal can all indirectly trigger symptoms of a psychosis. But for these drugs, a secondary culprit also contributes to the hallucinations or delusions, and the mechanism isn't always clear, says Dr. Karen Miotto, director of the University of California-Los Angeles' Alcoholism and Addiction Medicine Service. "Is it a sleep deprivation? Is it an underlying anxiety disorder that's being exacerbated?" Miotto said.