A hallucination is a sensory experience that appears real but is created entirely by the mind, occurring in the absence of any external stimulus. This is distinct from an illusion, which involves a misinterpretation of a real external stimulus. Hallucinogenic drugs alter the normal function of the brain’s neurotransmitter systems, leading to vivid perceptual changes. Different classes of substances affect specific neurochemical pathways, resulting in experiences that vary significantly in their nature and intensity. This exploration classifies the main pharmacological categories of drugs that induce these alterations in perception and reality.
Classic Psychedelics
Classic psychedelics include substances like lysergic acid diethylamide (LSD), psilocybin (the active compound in “magic mushrooms”), and dimethyltryptamine (DMT). These compounds act as agonists on the serotonin system, specifically targeting the 5-HT2A receptor in the cerebral cortex, the area responsible for perception and cognition.
Activation of the 5-HT2A receptor disrupts the brain’s filtering mechanisms, leading to hyperconnectivity between different brain regions. This disruption produces characteristic effects, such as intense visual imagery, often involving geometric patterns, and profound changes in emotional processing. The sensory alterations induced are often described as pseudo-hallucinations, meaning the user typically retains a degree of awareness that the perceptions are not objectively real.
The experience tends to be highly visual and emotional, with a sense of sensory enhancement. The involvement of the serotonin system contributes to the emotional and introspective nature of the experience. The underlying mechanism remains centered on the serotonergic pathway.
Dissociative Hallucinogens
Dissociative hallucinogens fundamentally alter the perception of one’s body and environment, often leading to a feeling of detachment. Drugs in this category include phencyclidine (PCP), ketamine, and dextromethorphan (DXM). The primary pharmacological action of these drugs is to block the N-methyl-D-aspartate (NMDA) receptor, a subtype of glutamate receptor.
Glutamate is the main excitatory neurotransmitter in the central nervous system. By acting as non-competitive antagonists, dissociatives inhibit glutamatergic signaling. This reduction in activity leads to dissociative anesthesia, characterized by a trance-like state, pain relief, and amnesia.
The resulting experience is less focused on geometric visuals and more on psychological separation, including depersonalization and derealization. Users often report “out-of-body” experiences, a direct consequence of the disruption of the brain’s communication pathways that integrate sensory information with the sense of self. Dissociatives target the glutamate system to create disconnection, unlike the overstimulation of the serotonin system caused by classic psychedelics.
Deliriants
Deliriants induce a state of true delirium, marked by profound confusion and a total lack of insight. These substances, such as scopolamine, atropine (found in plants like deadly nightshade), and high doses of certain antihistamines like diphenhydramine, exert their effects by blocking the action of acetylcholine. They are known as anticholinergic drugs because they act as competitive antagonists at muscarinic acetylcholine receptors.
Acetylcholine is a neurotransmitter involved in memory, attention, and regulating the sleep-wake cycle. By inhibiting these receptors, deliriants severely impair cognitive processing, which leads to disorientation and confusion.
The hallucinations caused by deliriants are typically vivid and indistinguishable from reality to the person experiencing them, often involving interactions with non-existent people or objects. Because the user genuinely believes the hallucination is real, the profound cognitive impairment makes deliriants particularly dangerous due to the loss of rational thought and poor judgment.
Atypical Causes of Hallucination
Beyond the primary pharmacological classes, hallucinations can arise as a secondary or unintended consequence of other substances, often due to toxicity, high doses, or withdrawal. High-dose use of stimulants, such as methamphetamine or cocaine, can lead to stimulant-induced psychosis. This effect is attributed to excessive dopamine activity in the brain, resulting in symptoms that include intense paranoia and vivid visual or auditory hallucinations.
A common type of stimulant-induced hallucination is formication, the sensation of insects crawling on the skin. Furthermore, the sudden cessation of depressant drugs, particularly alcohol or benzodiazepines, can trigger severe withdrawal symptoms. Alcohol withdrawal can progress to delirium tremens (DTs), a medical emergency characterized by severe confusion and vivid auditory and visual hallucinations. This withdrawal-induced state is caused by the brain becoming hyperactive after the removal of the depressant effect.
Certain prescription medications can also produce hallucinations as an adverse side effect, particularly in older individuals. Examples include dopaminergic medications used for Parkinson’s disease, which increase dopamine levels, and anticholinergic medications. In these cases, the hallucinations are a manifestation of the drug disrupting the delicate balance of neurotransmitters in the brain.