Anxiety is medically recognized as the experience of negative feelings of apprehension, worry, and general unease.  These feelings can range from subtle and ignorable to intense and overwhelming enough to trigger panic attacks or feelings of impending doom. Anxiety is often accompanied by nervous behaviour, such as restlessness, difficulty concentrating, irritability, and muscular tension. 
Fear is the emotional response to a real or perceived imminent threat, whereas anxiety is the anticipation of a future threat. These two states often overlap, but they also differ in that fear is more often associated with surges of autonomic arousal necessary for fight or flight, thoughts of immediate danger, and escape behaviours, while anxiety is more often associated with muscle tension and vigilance in preparation for future danger and cautious or avoidant behaviours.   This focus of anticipated danger may be internally or externally derived. 
Psychoactive substance-induced anxiety can be caused as an inescapable effect of the drug itself,  by a lack of experience with the substance or its intensity, as an enhancement of a pre-existing state of mind, or by the experience of negative hallucinations.
Anxiety is often accompanied by other coinciding effects, such as depression and irritability. It is most commonly induced under the influence of moderate dosages of hallucinogenic compounds, such as cannabinoids  , psychedelics  , dissociatives, and deliriants  . However, it can also occur during the withdrawal symptoms of GABAergic depressants  and during stimulant comedowns  .
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