What is the side effect of using LSD commonly observed in users?

The side effect commonly observed in users of LSD is piloerection, which refers to the phenomenon of hair standing on end, often described as "goosebumps." This effect occurs as part of the body’s autonomic response triggered by the psychedelic properties of LSD that can lead to heightened sensory perceptions and emotional responses. Users may experience changes in mood, body temperature, and sensory perceptions, which can manifest in physical reactions such as piloerection.

Other options like xerostomia (dry mouth), hypersomnia (excessive sleeping), and dysphoria (a state of unease or generalized dissatisfaction) may occur in conjunction with various substances or conditions, but they are not typically identified as primary or characteristic effects of LSD use. Piloerection, on the other hand, is more closely associated with the visceral and altered physical sensations that LSD can induce, making it the most relevant answer in the context of this question.