Hallucinogens produce a wide range of effects, depending on the properties, dosage, and potency of the drug, the personality and mood of the drug taker, and the immediate environment. Visually, perception of light and space is altered, and colors and detail take on increased significance. If the eyes are closed the drug taker often sees intense visions of different kinds. Nonexistent conversations, music, odors, tastes, and other sensations are also perceived. The sensations are often either very pleasant or very distasteful and disturbing. The drugs frequently alter the sense of time and cause feelings of emptiness. For many individuals the separation between self and environment disappears, leading to a sense of oneness or holiness.
The effects, sometimes referred to as a "trip," can last from an hour to a few days. "Bad trips," full of frightening images, monsters, and paranoid thoughts are known to have resulted in accidents and suicides. Flashbacks (unexpected reappearances of the effects) can occur months later.
Physiologically, the drugs act as mild stimulants of the sympathetic nervous system, causing dilation of the pupils, constriction of some arteries, a rise in blood pressure, and increased excitability of certain spinal reflexes. Psilocybin has been shown to produce decreased blood flow and activity in the thalamus and other brain areas that connect different parts of the brains. Many hallucinogenic drugs share a basic chemical structural unit, the indole ring, which is also found in the nervous system substance serotonin. Mescaline has chemical similarities to both the indole ring and the adrenal hormone epinephrine.