A contact high is the experience of sensing or perceiving drug effects when in the company of someone who has actually taken drugs, without having taken the drug yourself. For example, an experienced acid (LSD) user who is spending recreational time with someone who is "tripping" on LSD may start to feel as if they, too, are under the influence of LSD. They may start to have bizarre thoughts, to experience mild pseudo-hallucinations, and to find everyday situations hilariously funny. Equally, they may share feelings of fear, anxiety, or panic that are being expressed by the other person.
A contact high can also happen when someone has taken a placebo, or a substance containing no psychoactive ingredients -- a common money-making drug dealers' trick -- believing that they have taken a real drug, when they are around others who really have taken the drug. It may also relate to set and setting, which has a profound impact on users' experience of drugs, and even the intensity of the drug effect.
Unlike the related phenomenon of expectancy effect, in which people experience drug effects when expecting them -- either through taking a placebo or "dummy" drug, or through taking a real drug -- contact highs are not well researched in the drug abuse field. The difference between an expectancy effect and a contact high is that with a contact high, the person experiencing it need not think that they have actually taken the drug to experience its effects.
In contrast, the phenomenon of the contact high is well known within the drug user community -- and many drug users avoid or seek out other users specifically to avoid or to induce a contact high. Within the rave culture, for example, a sub-set of ex-drug users and non-users choose not to use drugs, but enjoy the feelings of stimulation and excitement they feel around others who are on stimulating drugs such as ecstasy and meth.
A contact high doesn't even require direct contact with the person who has taken drugs. In his review of the book, Tripping: An Anthology of True-Life Psychedelic Adventures, Spalding Gray wrote: "I couldn't stop dipping in and out of this juicy book, flying from New York to Detroit. At times, it felt like just reading it was keeping the plane up. Wow! What a contact high."
Hayes, C. & McKenna, T. Tripping: An Anthology of True-Life Psychedelic Adventures. New York: Penguin. 2000.
Grof, S. When the Impossible Happens: Adventures in Non-Ordinary Reality. Louisville, CO: Sounds True, Inc. 2005.
Zinberg, N. Drug, Set, and Setting: The Basis for Controlled Intoxicant Use. Yale University Press. 1986.