A panic attack is a discrete period of sudden onset of intense fear or terror. During these attacks there are symptoms such as shortness of breath or smothering sensations; palpitations, pounding heart, or accelerated heart rate; chest pain or discomfort; choking, and fear of going crazy or losing control. Panic attacks may be unexpected, in which the onset of the attack is not associated with an obvious trigger and instead occurs "out of the blue," or expected, in which the panic attack is associated with an obvious trigger, either internal or external. 
Panic attacks are usually triggered in moments of severe anxiety, such as that caused by a bad trip. They are so subjectively overwhelming both physically and mentally that the user may believe they are dying, or that some great calamity is imminent, and are commonly mistaken for heart attacks. The subjective sensations can overwhelm rational thought even when the user recognizes that they are having a panic attack, especially in those who have not experienced them before.
Panic attacks are often accompanied by uncomfortable physical symptoms that may further aggravate a person’s anxiety as they may be mistaken for a serious health problem. The strongest mental effect of panic attacks is a crushing sense of impending doom,  accompanied by despair, panic, and dread. These usually begin abruptly and may reach their peak within 10 to 20 minutes, but may also continue for hours in extreme cases before subsiding on their own. Although this experience is incredibly stressful it is important to note that it is not physically dangerous or harmful.
The various cognitive and physical symptoms of a panic attack are described and listed below:
- Hyperventilation - Hyperventilation occurs when one breathes deeper and more rapidly than usual. When hyperventilating, one may feel as though they are struggling to get enough air. As this causes a decrease of carbon dioxide in the blood, it may result in lightheadedness, a rapid heartbeat, chest pain, or a tingling sensation in a person's limbs.
- Abnormal heart rate and palpitations - Due to the release of stress hormones, one may experience heart symptoms including missed beats, palpitations, chest pain, and an accelerated heart rate.
- Tactile suppression - This can be described as a loss of sensation as well as numbness and tingling sensations throughout the body. It may feel as if one's skin or body parts are numb to the touch, and this can occur in a small area or become all-encompassing throughout multiple body parts or the entire body. Numbness most frequently occurs within the hands, legs, arms, feet, and face. This effect is often accompanied by a pins and needle sensation and it generally increases alongside of - hyperventilation.
- Shortness of breath
- Trembling or shaking
- Feelings of choking
- Chest pain or discomfort
- Bodily pressures
- Nausea or abdominal distress
- Feeling dizzy, unsteady, lightheaded, or faint
- Fear of losing control or going insane
- Feelings of impending
- Chills or hot flashes
- American Psychiatric Association. (2013). Diagnostic and statistical manual of mental disorders (5th ed.), 826. Arlington, VA: American Psychiatric Publishing. | https://doi.org/10.1176/appi.books.9780890425596.GlossaryofTechnicalTerms
- Hyperventilation | http://www.webmd.com/a-to-z-guides/hyperventilation-topic-overview