What characteristic symptom is often associated with someone on a hallucinogenic drug?

The characteristic symptom often associated with someone using hallucinogenic drugs is excessive sweating. Hallucinogens affect the brain's perception, leading to altered sensory experiences, and they can also stimulate the autonomic nervous system. This stimulation can result in physiological changes, such as increased body temperature and excessive sweating.

Other options may correspond to different classes of substances or conditions. Increased appetite is more typically associated with stimulants or certain depressants rather than hallucinogens. Rapid heartbeat is often associated with stimulants or anxiety responses rather than the typical effects of hallucinogens. Slurred speech is commonly linked to alcohol or certain depressants, where motor function is impaired, which isn't a primary effect of hallucinogens. Therefore, excessive sweating stands out as a notable response during the use of hallucinogenic drugs.