A bad trip (also known as challenging experiences, acute intoxication from hallucinogens, psychedelic crisis, or emergence phenomenon) is an acute adverse psychological reaction to classic hallucinogens. With proper screening, preparation, and support in a regulated setting these are usually benign. A bad trip on psilocybin, for instance, often features intense anxiety, confusion, and agitation, or even psychotic episodes. They manifest as a range of feelings, such as anxiety, paranoia, the unshakeable sense of one's inevitable and imminent personal demise or states of unrelieved terror that they believe will persist after the substance's effects have worn off. As of 2011, exact data on the frequency of bad trips are not available.
Bad trips can be exacerbated by the inexperience or irresponsibility of the user or the lack of proper preparation and environment for the trip, and are often reflective of unresolved psychological tensions triggered during the course of the experience. In clinical research settings, precautions including the screening and preparation of participants, the training of the session monitors who will be present during the experience, and the selection of appropriate physical setting can minimize the likelihood of psychological distress. Researchers have suggested that the presence of professional "trip sitters" (i.e., session monitors) may significantly reduce the negative experiences associated with a bad trip. In most cases in which anxiety arises during a supervised psychedelic experience, reassurance from the session monitor is adequate to resolve it; however, if distress becomes intense it can be treated pharmacologically, for example with the benzodiazepine diazepam.
The psychiatrist Stanislav Grof wrote that unpleasant psychedelic experiences are not necessarily unhealthy or undesirable, arguing that they may have the potential for psychological healing and lead to breakthrough and resolution of unresolved psychic issues.