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Psychoactive drugs are substances that influence the mind, mood, and behavior by affecting the nervous system. The inherent characteristic of these substances is their capacity to alter brain function and thereby change consciousness, perception, cognition, and emotional state.

When someone consumes a psychoactive drug, it interacts with neurotransmitters and receptors in the brain. This interaction can lead to various effects, such as increased relaxation, heightened alertness, changes in thought processes, and mood alterations. Thus, stating that they act on the nervous system to alter consciousness highlights the primary function of these substances in providing alterations to how a person experiences reality and responds to stimuli.

Other options introduce concepts that do not accurately describe the nature of psychoactive drugs. For example, stating that they act on the immune system or only affect physical health does not recognize the primary impact of these drugs on mental processes. Likewise, suggesting that they alter mood without affecting consciousness overlooks the interconnectedness of mood and conscious experience that psychoactive drugs fundamentally change.