Which of the following is a key characteristic of psychoactive drugs?

Psychoactive drugs are primarily characterized by their ability to alter the user's mental state, which includes changing feelings and moods. This characteristic is central to their definition and usage. These drugs can influence neurotransmitter systems in the brain that are responsible for regulating mood and emotional responses, which is why they are often used in psychological treatment contexts to manage conditions like depression, anxiety, or other mood disorders.

While some substances may cause side effects like increased hunger or be used to enhance physical performance, these effects are not defining characteristics of psychoactive drugs. Additionally, preventing illness is more associated with medications or vaccines rather than the core function of psychoactive substances. Thus, the key characteristic of altering feelings and moods is what distinguishes psychoactive drugs from other types of medications or substances.