Which of the following side effects can occur during hallucinogen use?

During hallucinogen use, one of the notable side effects can indeed be dehydration. Hallucinogens, such as LSD or psilocybin, can lead to physical symptoms that affect the body's hydration levels. The intense sensory experiences and altered perceptions may result in a reduced awareness of bodily needs, including the need for fluids. Additionally, individuals might experience increased energy or activity levels, which can further contribute to dehydration, especially if they are in an environment that encourages physical exertion or if they neglect to drink water.

This effect contrasts with other potential side effects of hallucinogens. For instance, drowsiness is less common because many hallucinogens tend to induce heightened alertness or an 'activated' state rather than sedation. Cold, clammy skin may indicate other medical issues or different types of substance use rather than hallucinogen intoxication. Rapid heartbeat can occur in response to various stimuli and might be more accurately associated with anxiety or panic, frequently reported phenomena during hallucinogen use, but it isn’t a direct effect of the drug itself like dehydration is. Thus, dehydration stands out as a clear possibility for someone using hallucinogens.