迷幻剂是药理学上对迷幻药物、游离药物和谵妄药物的统称。这类精神药物产生的作用主要是改变人的知觉、思维、情绪以及意识。最根本的致幻剂是迷幻药物，游离药品让人产生无痛麻醉感、记忆缺失乃至昏厥感受，致谵妄药则是使人陷入癫狂状态。

由 L. E. Hollister 提出致幻剂的标准：

并非所有的致幻剂都可以产生同样的效果，同样的药物在不同情境下对同一个体也会产生不同的效果。

Hallucinogens are a class of drugs that cause hallucinations—profound distortions in a person’s pe****tions of reality. Hallucinogens can be found in some plants and mushrooms (or their extracts) or can be man-made, and they are commonly divided into two broad categories: classic hallucinogens (such as LSD) and dissociative drugs (such as PCP).