Episodes of vasovagal syncope are typically recurrent and usually occur when the predisposed person is exposed to a specific trigger. Before losing consciousness, the individual frequently experiences early signs or symptoms such as lightheadedness, nausea, the feeling of being extremely hot or cold (accompanied by sweating), ringing in the ears, an uncomfortable feeling in the heart, fuzzy thoughts, confusion, a slight inability to speak or form words (sometimes combined with mild stuttering), weakness and visual disturbances such as lights seeming too bright, fuzzy or tunnel vision, black cloud-like spots in vision, and a feeling of nervousness can occur as well. The symptoms may become more intense over several seconds to several minutes before the loss of consciousness (if it is lost). Onset usually occurs when a person is sitting up or standing.

When people lose consciousness, they fall down (unless prevented from doing so) and, when in this position, effective blood flow to the brain is immediately restored, allowing the person to regain consciousness. If the person does not fall into a fully flat, supine position, and the head remains elevated above the trunk, a state similar to a seizure may result from the blood's inability to return quickly to the brain, and the neurons in the body will fire off and generally cause muscles to twitch very slightly but mostly remain very tense. Fainting occurs with a loss of oxygen to the brain.

The autonomic nervous system's physiological state (see below) leading to loss of consciousness may persist for several minutes, so

- If sufferers try to sit or stand when they wake up, they may pass out again

- The person may be nauseated, pale, and sweaty for several minutes or hours

Reflex syncope occurs in response to a trigger due to dysfunction of the heart rate and blood pressure regulating mechanism. When heart rate slows, blood pressure drops, and the resulting lack of blood to the brain causes fainting.

The primary symptoms of hypotension are lightheadedness or dizziness.

If the blood pressure is sufficiently low, fainting may occur.

Low blood pressure is sometimes associated with certain symptoms, many of which are related to causes rather than effects of hypotension:

- chest pain

- shortness of breath

- irregular heartbeat

- fever higher than 38.3 °C (101 °F)

- headache

- stiff neck

- severe upper back pain

- cough with sputum

- Prolonged diarrhea or vomiting

- dyspepsia (indigestion)

- dysuria (painful urination)

- adverse effect of medications

- acute, life-threatening allergic reaction

- seizures

- loss of consciousness

- profound fatigue

- temporary blurring or loss of vision

- Black tarry stools

Hypotension is low blood pressure, especially in the arteries of the systemic circulation. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. A systolic blood pressure of less than 90 millimeters of mercury (mm Hg) or diastolic of less than 60 mm Hg is generally considered to be hypotension. However, in practice, blood pressure is considered too low only if noticeable symptoms are present.

Hypotension is the opposite of hypertension, which is high blood pressure. It is best understood as a physiological state, rather than a disease. Severely low blood pressure can deprive the brain and other vital organs of oxygen and nutrients, leading to a life-threatening condition called shock.

For some people who exercise and are in top physical condition, low blood pressure is a sign of good health and fitness.

For many people, excessively low blood pressure can cause dizziness and fainting or indicate serious heart, endocrine or neurological disorders.

Treatment of hypotension may include the use of intravenous fluids or vasopressors. When using vasopressors, trying to achieve a mean arterial pressure (MAP) of greater than 70 mmHg does not appear to result in better outcomes than trying to achieve a MAP of greater than 65 mm Hg in adults.

The hallmark sign of POTS is a measured increase in heart rate by at least 30 beats per minute within 10 minutes of assuming an upright position. For people aged between 12 and 19, the minimum increase for diagnosis is 40 beats per minute. This symptom is known as orthostatic (upright) tachycardia (fast heart rate). It occurs without any coinciding drop in blood pressure, as that would indicate orthostatic hypotension. It should be noted, however, that certain medications to treat POTS may cause orthostatic hypotension. It is accompanied by other features of orthostatic intolerance—symptoms which develop in an upright position and are relieved by reclining. These orthostatic symptoms include palpitations, light-headedness, chest discomfort, shortness of breath, nausea, weakness or "heaviness" in the lower legs, blurred vision and cognitive difficulties. Symptoms may be exacerbated with prolonged sitting, prolonged standing, alcohol, heat, exercise, or eating a large meal.

In up to one third of people with POTS, fainting occurs in response to postural changes or exercise. Migraine-like headaches are common, sometimes with symptoms worsening in an upright position (orthostatic headache). Some people with POTS develop acrocyanosis, or blotchy, red/blue skin upon standing, especially over the feet (indicative of blood pooling). 48% of people with POTS report chronic fatigue and 32% report sleep disturbances. Others exhibit only the cardinal symptom of orthostatic tachycardia.

POTS can co-occur in all types of Ehlers–Danlos syndrome (EDS), a hereditary connective tissue disorder marked by loose hypermobile joints prone to subluxations and dislocations, skin that exhibits moderate or greater laxity, easy bruising, and many other symptoms. A trifecta of POTS, EDS, and Mast Cell Activation Syndrome (MCAS) is becoming increasingly more common, with a genetic marker common among all three conditions. POTS is also often accompanied by vasovagal syncope, with a 25% overlap being reported. There is significant overlap between POTS and chronic fatigue syndrome, with evidence of POTS in 25–50% of CFS cases. Fatigue and reduced exercise tolerance are prominent symptoms of both conditions, and dysautonomia may underlie both conditions.

Postural orthostatic tachycardia syndrome (POTS) is a condition in which a change from lying to standing causes an abnormally large increase in heart rate. This occurs with symptoms that may include lightheadedness, trouble thinking, blurry vision, or weakness. Other commonly associated conditions include irritable bowel disease, insomnia, chronic headaches, Ehlers-Danlos syndrome, fibromyalgia.

The cause of POTS is poorly understood. Often it begins after a viral infection, surgery, or pregnancy. Risk factors include a family history of the condition. Diagnosis in adults is based on an increase in heart rate of more than 30 beats per minute within ten minutes of standing up which is accompanied by symptoms. Low blood pressure with standing, however, does not occur. Other conditions which can cause similar symptoms, such as prolonged bedrest, dehydration, hyperthyroidism, anemia, and certain medications, must not be present.

Treatment may include avoiding factors that bring on symptoms, increasing dietary salt and water, compression stockings, exercise, cognitive behavioral therapy, and medications. Medications used may include beta blockers, pyridostigmine, midodrine, or fludrocortisone. More than 50% of people whose condition was triggered by a viral infection get better within five years. About 90% get better with treatment. It is estimated that 0.5 to 3 million people are affected in the United States. The average age of onset is 20 years old and it occurs more often in females.

Postpartum chills is a physiological response that occurs within two hours of childbirth. It appears as uncontrollable shivering that is not under voluntary control. It is seen in many women after delivery and can be unpleasant. It lasts for a short time. It is thought to be a result of a nervous system response. It may also be related to fluid shifts and the actual strenuous work of labor. It is considered a normal response and there is no accompanying fever. If a fever does develop further assessments may reveal the presence of an infection. Treatment consists of an explanation from clinicians that the shivering is a normal response and that it only lasts for a short time. Warm blankets are given to the women and fluid replacement is encouraged. It has been described as a fairly common and normal occurrence.

After discharge to home with the baby, chills that accompany uncontrolled bleeding, shortness of breath, cold clammy skin, dizziness, heart pain, and racing heart can be a sign of shock that needs immediate medical attention. Mastitis can also cause shivering.

Septic shock is a serious medical condition that occurs when sepsis, which is organ injury or damage in response to infection, leads to dangerously low blood pressure and abnormalities in cellular metabolism.

The primary infection is most commonly caused by bacteria, but also may be by fungi, viruses or parasites. It may be located in any part of the body, but most commonly in the lungs, brain, urinary tract, skin or abdominal organs. It can cause multiple organ dysfunction syndrome (formerly known as multiple organ failure) and death.

Frequently, people with septic shock are cared for in intensive care units. It most commonly affects children, immunocompromised individuals, and the elderly, as their immune systems cannot deal with infection so effectively as those of healthy adults. The mortality rate from septic shock is approximately 25–50%.

Hyperalgesic fear of needles is another form that does not have as much to do with fear of the actual needle. Patients with this form have an inherited hypersensitivity to pain, or hyperalgesia. To them, the pain of an injection is unbearably great and many cannot understand how anyone can tolerate such procedures.

This form of fear of needles affects around 10% of needle phobes. The symptoms include extreme explained anxiety, and elevated blood pressure and heart rate at the immediate point of needle penetration or seconds before. The recommended forms of treatment include some form of anesthesia, either topical or general.

Exercise hypertension is an excessive rise in blood pressure during exercise. Many of those with exercise hypertension have spikes in systolic pressure to 250 mmHg or greater.

A rise in systolic blood pressure to over 200 mmHg when exercising at 100 W is pathological and a rise in pressure over 220 mmHg needs to be controlled by the appropriate drugs.

Similarly, in healthy individuals the response of the diastolic pressure to 'dynamic' exercise (e.g. walking, running or jogging) of moderate intensity is to remain constant or to fall slightly (due to the improved blood flow), but in some individuals a rise of 10 mmHg or greater is found.

Recent work at Johns Hopkins involving a group of athletes aged 55 to 75 with mild hypertension has found a correlation of those with exercise hypertension to a reduced ability of the major blood vessels to change in size in response to increased blood flow (probably due to impaired function of the endothelial cells in the vessel walls). This is to be differentiated from stiffness of the blood-vessel walls, which was not found to be correlated with the effect.

Whilst witnessing procedures involving needles it is possible for the phobic present to suffer the symptoms of a needle phobic attack without actually being injected. Prompted by the sight of the injection the phobic may exhibit the normal symptoms of vasovagal syncope and fainting or collapse is common. While the cause of this is not known, it may be due to the phobic imagining the procedure being performed on themselves. Recent neuroscience research shows that feeling a pin prick sensation and watching someone else's hand get pricked by a pin activate the same part of the brain.

Septic shock is a subclass of distributive shock, a condition in which abnormal distribution of blood flow in the smallest blood vessels results in inadequate blood supply to the body tissues, resulting in ischemia and organ dysfunction. Septic shock refers specifically to distributive shock due to sepsis as a result of infection.

Septic shock may be defined as sepsis-induced low blood pressure that persists despite treatment with intravenous fluids. Low blood pressure reduces tissue perfusion pressure, causing the tissue hypoxia that is characteristic of shock. Cytokines released in a large scale inflammatory response result in massive vasodilation, increased capillary permeability, decreased systemic vascular resistance, and low blood pressure. Finally, in an attempt to offset decreased blood pressure, ventricular dilatation and myocardial dysfunction occur.

Septic shock may be regarded as a stage of SIRS (Systemic Inflammatory Response Syndrome), in which sepsis, severe sepsis and multiple organ dysfunction syndrome (MODS) represent different stages of a pathophysiological process. If an organism cannot cope with an infection, it may lead to a systemic response - sepsis, which may further progress to severe sepsis, septic shock, organ failure, and eventually, result in death.

Acute stress reaction (also called acute stress disorder, psychological shock, mental shock, or simply shock) is a psychological condition arising in response to a terrifying or traumatic event, or witnessing a traumatic event that induces a strong emotional response within the individual. It should not be confused with the unrelated circulatory condition of shock/hypoperfusion. Acute stress reaction (ASR) may develop into delayed stress reaction (better known as PTSD) if stress is not correctly managed. ASR is characterized by re-living and avoiding reminders of an aversive event, as well as generalized hypervigilance after initial exposure to a traumatic event. ASD is differentiated from PTSD as a disorder that precedes it, and if symptoms last for more than one month, it will develop into PTSD. It can thus be thought of as the acute phase of PTSD.

When triggered, most people with a specific phobia experience an increase in blood pressure and an extreme amount of anxiety; however, an individual who has BII phobia experiences something different: first, the person’s blood pressure increases and then swiftly decreases the next moment. During this rapid change in blood pressure, a period of a vasovagal response occurs, which causes less blood and oxygen to be sent to the brain, resulting in loss of consciousness momentarily. The heart rate then slows down, due to the activation of the parasympathetic nervous system. The parasympathetic activation is believed to be associated with the disgust response, which is a characteristic of BII phobia, making it stand out from other phobias. Other symptoms can include extreme discomfort in the chest and tunnel vision.

Moreover, FMRI studies have confirmed that the activation level of the prefrontal cortex, which is responsible for controlling and regulating emotions, is lower in people with this type of phobia when exposed to a fear-inducing stimuli. This shows that people with BII phobia have less control over their emotions because of the lessened activity in their prefrontal cortex. This lessened emotional control could contribute to the high disgust reaction as well as less control over phobic symptoms, such as fainting.

Patients affected by ADT tachyphylaxis experience a noticeably sudden progressive decrease in response to SSRIs. The reported rates of this condition vary from 9% to 33% of SSRI users, and the majority of those affected are less responsive to subsequent treatments. In most observational studies, these individuals suffer a recurrence or relapse of depression without changing the previously effective dose.

ADT tachyphylaxis incorporates drug sensitivity as a potential causal factor for the decreased response. However, tolerance provides a more accurate explanation. While the exact cause of ADT tachyphylaxis in individual cases is unknown, drug tolerance is a more comprehensive model, as it includes mechanisms of pharmacodynamic tolerance, metabolic tolerance, and others.

The "DSM-IV" specifies that ASD must be accompanied by the presence of dissociative symptoms, which largely differentiates it from PTSD.

Dissociative symptoms include a sense of numbing or detachment from emotional reactions, a sense of physical detachment, such as seeing oneself from another perspective, decreased awareness of one’s surroundings, the perception that one’s environment is unreal or dreamlike, and the inability to recall critical aspects of the traumatic event (dissociative amnesia).

In addition to the characteristic dissociative symptoms, ASD shares many of the symptoms with PTSD, including:

- the experience or witnessing of a threatening event that resulted in intense fear or horror

- the re-experiencing of the event by means of flashbacks, recurrent thoughts or dreams, and distress when reminded of the event

- the avoidance of stimuli that serve as reminders of the event, such as feelings, thoughts, places, individuals, and activities

- anxiety, including restlessness, difficulty sleeping and concentrating, and hypervigilance

- a significant disruption in normal social or work functioning

Antidepressant treatment tachyphylaxis (ADT tachyphylaxis), also known as Prozac poop-out, is a medical condition in which progressive or acute tolerance effects are seen following chronic administration of a drug. ADT tachyphylaxis specifically refers to a sudden decrease in response to selective serotonin reuptake inhibitors (SSRIs), which are the most commonly prescribed antidepressants. Although less commonly prescribed as antidepressants (having lost popularity following the introduction of SSRIs), monoamine oxidase inhibitors, or MAOIs, have also incurred a "poop-out" effect among depressed patients.

Lethargy is a state of tiredness, weariness, fatigue, or lack of energy. It can be accompanied by depression, decreased motivation, or apathy. Lethargy can be a normal response to inadequate sleep, overexertion, overworking, stress, lack of exercise, improper nutrition, boredom, or a symptom of a disorder. It may also be a side-effect of medication or caused by an interaction between medications or medication(s) and alcohol. When part of a normal response, lethargy often resolves with rest, adequate sleep, decreased stress, physical exercise and good nutrition.

Allodynia (Ancient Greek "" "állos" "other" and "" "odúnē" "pain") refers to central pain sensitization (increased response of neurons) following normally non-painful, often repetitive, stimulation. Allodynia can lead to the triggering of a pain response from stimuli which do not normally provoke pain. Temperature or physical stimuli can provoke allodynia, which may feel like a burning sensation, and it often occurs after injury to a site. Allodynia is different from hyperalgesia, an extreme, exaggerated reaction to a stimulus which is normally painful.

BII phobia tends to have an affect on people's health, since the individuals with this phobia, usually avoid needles, vaccinations, and blood tests, making them susceptible to diseases and other health-related issues. Women, in particular, suffering from BII phobia, reported to have avoided pregnancy in fear of injections, vaccinations, and the pain associated with labor. BII phobia patients suffering from diabetes or multiple sclerosis are often unable to get injections and are thus more likely to discontinue their treatment.

In individuals with eccentric hypertrophy there may be little or no indication that hypertrophy has occurred as it is generally a healthy response to increased demands on the heart. Conversely, concentric hypertrophy can make itself known in a variety of ways. Most commonly, chest pain, either with or without exertion is present, along with shortness of breath with exertion, general fatigue, syncope, and palpitations. Overt signs of heart failure, such as edema, or shortness of breath without exertion are uncommon.

There are different kinds or types of allodynia:

- Mechanical allodynia (also known as tactile allodynia)

- Static mechanical allodynia – pain in response when touched

- Dynamic mechanical allodynia – pain in response to stroking lightly

- Thermal (hot or cold) allodynia – pain from normally mild skin temperatures in the affected area

- Movement allodynia – pain triggered by normal movement of joints or muscles

Blood phobia (also AE: hemophobia or BE: haemophobia) is the extreme and irrational fear of blood, a type of specific phobia. Severe cases of this fear can cause physical reactions that are uncommon in most other fears, specifically vasovagal syncope (fainting). Similar reactions can also occur with trypanophobia and traumatophobia. For this reason, these phobias are categorized as "blood-injection-injury phobia" by the DSM-IV. Some early texts refer to this category as "blood-injury-illness phobia."

Tachyphylaxis is characterized by the rate sensitivity: the response of the system depends on the rate with which a stimulus is presented. To be specific, a high-intensity prolonged stimulus or often-repeated stimulus may bring about a diminished response also known as desensitization.

Chronic Somogyi rebound is a contested explanation of phenomena of elevated blood sugars in the morning. Also called the Somogyi effect and posthypoglycemic hyperglycemia, it is a rebounding high blood sugar that is a response to low blood sugar. When managing the blood glucose level with insulin injections, this effect is counter-intuitive to insulin users who experience high blood sugar in the morning as a result of an overabundance of insulin at night.

This theoretical phenomenon was named after Michael Somogyi, a Hungarian-born professor of biochemistry at the Washington University and Jewish Hospital of St. Louis, who prepared the first insulin treatment given to a child with diabetes in the USA in October 1922. Somogyi showed that excessive insulin makes diabetes unstable and first published his findings in 1938.

Compare with the dawn phenomenon, which is a morning rise in blood sugar in response to waning insulin and a growth hormone surge (that further antagonizes insulin).