If you are greatly afraid, however, such as being terrified that there is a burglar in your home that is about to harm you, the body produces a high degree stress response. We generally experience high degree stress responses as being anxiety attacks: where the changes are so profound they get our full attention. The greater the degree of anxiety and stress response, the more changes the body experiences.
Panic attacks can occur unexpectedly during a calm state or in an anxious state. Although panic attacks are a defining characteristic of panic disorder, it is not uncommon for individuals to experience panic attacks in the context of other psychological disorders. For example, someone with social anxiety disorder might have a panic attack before giving a talk at a conference and someone with obsessive-compulsive disorder might have a panic attack when prevented from engaging in a ritual or compulsion.  
To receive a diagnosis of panic disorder, the panic attacks must be unexpected and during the attack four or more of the above symptoms must occur. For panic attacks that are expected, meaning they might not be expected by the individual but are expected in relation to any phobia, anxiety or other mental health disorder, four or more symptoms must also occur.
Several drugs can cause or worsen anxiety, whether in intoxication, withdrawal or from chronic use. These include alcohol, tobacco, cannabis, sedatives (including prescription benzodiazepines), opioids (including prescription pain killers and illicit drugs like heroin), stimulants (such as caffeine, cocaine and amphetamines), hallucinogens, and inhalants.[86] While many often report self-medicating anxiety with these substances, improvements in anxiety from drugs are usually short-lived (with worsening of anxiety in the long term, sometimes with acute anxiety as soon as the drug effects wear off) and tend to be exaggerated. Acute exposure to toxic levels of benzene may cause euphoria, anxiety, and irritability lasting up to 2 weeks after the exposure.[87]
Although the exact causes of panic attacks and panic disorder are unclear, the tendency to have panic attacks runs in families. There also appears to be a connection with major life transitions such as graduating from college and entering the workplace, getting married, or having a baby. Severe stress, such as the death of a loved one, divorce, or job loss can also trigger panic attacks.

When we experience an involuntary high degree stress response, the sensations can be so profound that we think we are having a medical emergency, which anxious personalities can react to with more fear. And when we become more afraid, the body is going to produce another stress response, which causes more changes, which we can react to with more fear, and so on.
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