Understanding Fainting: The Basics

Medically Reviewed by Christopher Melinosky, MD on October 24, 2023
6 min read

Fainting, also called syncope (pronounced SIN-ko-pee), is a sudden, brief loss of consciousness and posture caused by decreased blood flow to the brain.

Many different conditions can cause fainting. These include heart problems such as irregular heartbeats,  low blood sugar (hypoglycemia), anemia (a deficiency in healthy oxygen-carrying cells), and problems with how the nervous system regulates blood pressure. Some types of fainting seem to run in families.

While fainting may happen because of a particular medical condition, sometimes it may occur in an otherwise healthy person. Fainting is a particular problem for the elderly, who may suffer serious injuries from falls when they faint. Most episodes are very brief. In most cases, the individual who has fainted regains complete consciousness within just a few minutes.

Fainting is a common problem, accounting for 3% of emergency room visits and 6% of hospital admissions. A person may feel faint and lightheaded (presyncope) or lose consciousness (syncope).

Fainting may have a variety of causes. A simple episode, also called a vasovagal attack or neurally mediated syncope, is the most common type of fainting spell. It is most common in children and young adults. A vasovagal attack happens because blood pressure drops, reducing circulation to the brain and causing loss of consciousness. Typically an attack occurs while standing and is frequently preceded by a sensation of warmth, nausea, lightheadedness, and visual "grayout." If the syncope is prolonged, it can trigger something called a convulsive syncope which is similar to a seizure.

You may have a simple fainting spell due to anxiety, fear, pain, intense emotional stress, hunger, or use of alcohol or drugs. Most people who have a simple fainting spell have no underlying heart or neurological (nerve or brain) problem.

Some people have a problem with the way their body regulates their blood pressure, particularly when they move too quickly from a lying or sitting position to a standing position. This condition is called postural hypotension and may be severe enough to cause fainting. This type of fainting is more common in the elderly, people who recently had a lengthy illness that kept them in bed, and people who have poor muscle tone.

The following can cause fainting, too:

Diseases of the autonomic nervous system. Your autonomic nervous system is the part of the nervous system that controls involuntary vital functions, such as the beating of your heart, the degree to which your blood vessels are constricted, and breathing. Autonomic nervous system problems include acute or subacute dysautonomia, chronic post-ganglionic autonomic insufficiency, and chronic pre-ganglionic autonomic insufficiency. If you have one of these disorders, you are likely to have other symptoms, such as erectile dysfunction (inability to have or maintain an erection), loss of bladder and bowel control, loss of the normal reflexes of your pupils, or decreased sweating, tearing, and salivation.

Conditions that interfere with the parts of the nervous system that regulate blood pressure and heart rate. These conditions include diabetes, alcoholism, malnutrition, and amyloidosis (in which waxy protein builds up in the tissues and organs). If you take certain high blood pressure drugs, which act on your blood vessels, you may be more likely to faint. If you are dehydrated, which may affect the amount of blood in your body and, thus, your blood pressure, you may be more likely to faint.

Heart or blood vessel problems that interfere with blood flow to the brain. These may include heart block (a problem with the electrical impulses that control your heart muscle), problems with the sinus node (a specialized area of your heart that helps it beat), heart arrhythmia (irregular heart rhythm), a blood clot in the lungs, an abnormally narrowed aortic heart valve, or certain other problems with the structure of your heart.

Conditions that may cause unusual patterns of stimulation to particular nerves. These include micturition syncope (fainting during or after urination), glossopharyngeal neuralgia (fainting due to inflammation and pain in a particular nerve to the mouth); cough syncope (fainting after intense coughing), and stretch syncope (fainting that occurs when stretching the neck and arms).

Hyperventilation. If you become intensely anxious or panicked and breathe too quickly, you may feel faint from hyperventilation (taking in too much oxygen and getting rid of too much carbon dioxide too quickly).

Vagus nerve stimulation or irritation. This is also called vasovagal syncope. When your vagus nerve gets stimulated, your heart rate and blood pressure can lower, leading to fainting. This can happen when going to the bathroom, having blood taken, receiving a shot, or experiencing extreme emotions.

Some people use the terms blackout and fainting interchangeably, but they are different things. A blackout is a loss of memory. Fainting, also called passing out, is a loss of consciousness. Both of these can have several different causes.

Intoxication. One of the main causes of temporary memory loss is intoxication, often with alcohol. According to one survey, over half of undergraduate college students have blacked out from drinking at least once in their lives.

When your blood alcohol level reaches 0.15%, you are likely to black out. This is nearly double the legal limit for driving in most states.

A blackout from intoxication is due to a brain malfunction. Your brain stops saving the things you do as memories. You may act normally and do things like socialize, eat, drive, and drink. But your brain is impaired and does not record your memories during this time.

Some people are more prone to blacking out than others. Blacking out is not always a sign that you abuse alcohol, but it can be.

Some people who drink heavily can also experience a brownout. This is when you remember some of the events of the time you were drinking. Sometimes, the events can come back to you with a cue that sparks your memory.

‌Head injury. Even mild head injuries can lead to a concussion. This can cause you to have memory issues and confusion. Most of the time, if you have a concussion, you can't remember the events that led to the injury.‌

Epilepsy. Seizures can also cause memory problems. Sometimes, directly after a seizure, you can enter a state of post-ictal confusion. This means you may be confused and not remember what happened directly before the seizure or what you did after the seizure happened. Generally, your memory of those events will come back within 5-30 minutes, once the post-ictal state is over.


If you faint a lot, you should visit your doctor to make sure it is not from an underlying health condition. You should also visit your doctor if you faint and have a history of heart disease.

Fainting while exercising is not common and may be a cause for concern. Additionally, if you fall and hit your head, causing you to lose consciousness, you should always get evaluated by a doctor to rule out a head injury.

Frequent blackouts may be a sign of an underlying medical condition. It can also be a sign that you have an alcohol use disorder. Drinking to the point of blacking out frequently can lead to other problems, like issues with long-term memory. It can also make you more likely to injure yourself accidentally while intoxicated. If you would like to stop drinking, there are resources that can help.

The Substance Abuse and Mental Health Services Administration has a 24-hour hotline you can call whenever you need support with mental health or substance use disorders. The number is 800-662-HELP (4357). If you use a TTY device, you can call 800-487-4889.