Doctors Explain 10 Common Causes of Fainting and Lightheadedness

Syncope (the medical term for fainting) is generally caused by the brain not getting enough blood for a short time.

“The cause can stem from many different parts of the body, including a person’s blood pressure going down, heart rate going down, and from neurologic reasons independent of the heart,” explains Lawrence Phillips, M.D., a cardiologist at NYU Langone Cardiology Associates and an associate professor in the Department of Medicine at NYU Grossman School of Medicine. “We try to find out why the blood pressure or heart rate would go down. Some of these reasons are common and not worrisome, but others need more evaluation.”

Below, experts explain 10 common causes of fainting or lightheadedness.

Additional expert sources include Aline M. Holmes, D.N.P., R.N., treasurer of the American College of Medical Quality; Hannah Goldberg, M.D., a primary care physician with Mercy Medical Center, and Christopher Palazzo, M.D., a family medicine practitioner at Spectrum Health.

Dehydration

Some people are simply predisposed to feeling lightheaded or even fainting when they get hot and sweaty and lose too much fluid. “It’s common in a hot room, like standing in church in the summer,” Thiruganasambandamoorthy says. “Heat triggers a pathway in the nervous system that causes blood pressure to drop.”

When you feel lightheaded because of dehydration—which includes symptoms like dry mouth and skin, dark-colored urine, and headaches—lying down resupplies the heart and the brain with blood, helping you to feel better pretty quickly, he says.

Surprises

A similar reaction can be triggered when your college roommate jumps out from behind the couch at your surprise birthday party. Your nervous system essentially goes into overdrive in these scenarios, Thiruganasambandamoorthy says, and your blood pressure drops suddenly, leading to lightheadedness. Usually, you do get a little bit of a warning if you’re really going to faint: You might turn a little green and feel nauseated, he says.

Standing up too quickly

Feeling lightheaded or even seeing black spots in your vision when you hop up quickly from a seated position actually has a name: orthostatic hypotension, which describes a sudden drop in blood pressure upon standing. It’s usually no big deal, but if it happens a lot or if it gets worse instead of better after a few minutes have passed, it’s worth bringing up with your doctor.

Abnormal heart rhythm

Compared to the relatively slow onset of symptoms caused by “Surprise!”-related fainting, heart-related fainting comes on fast, so you might not even notice any lightheadedness. An irregular heartbeat, called an arrhythmia, means your heart beats either too slow or too fast, which can in turn affect the blood supply that reaches your brain, Dr. Phillips says.

This kind of sudden fainting, often without any warning, is most concerning, says Melissa Burroughs, M.D., a cardiologist with Wellstar Medical Group.

“Someone might be in the middle of talking and all of a sudden pass out and wake up on the floor without remembering feeling anything beforehand,” she explains. That kind of experience immediately makes emergency docs think of abnormal heart rhythms, which are the most common cause of sudden cardiac death, according to the Mayo Clinic.

Heart valve problems

Your heart relies on four valves to evenly distribute blood throughout your entire body. However, some people develop problems with their heart valves at birth, meaning their hearts have to work harder to function properly. These congenital issues are likely to occur in younger people, Dr. Burroughs says, whereas people 60 and up are at a higher risk of an arrhythmia. Valve problems can restrict blood flow and may cause lightheadedness or dizziness, especially during exercise, she says.

Medication side effects

Certain meds, like painkillers and some anti-anxiety pills, can produce dizziness or lightheadedness. This happens because they affect your brain directly or they slow your heart rate or lower your blood pressure in a way that can provoke those symptoms, Dr. Phillips says.

“Sometimes when a patient has recurrent lightheadedness and I can’t explain why, I’ll be surprised to find it listed among less common side effects in pharmacy reports,” adds Dr. Burroughs, so your doctor may need to double-check your medication list.

There’s also a small chance you could be allergic to a medication you’re taking, she adds. In rare instances when people have a life-threatening allergic reaction to a drug, they might become lightheaded or even pass out. “It’s a very dramatic immune system reaction,” she says, which results in the blood vessels dilating and blood pressure dropping. “It’s still a blood pressure change that causes the lightheadedness, but it’s an immune reaction that causes it.”

Low blood sugar

If you’ve skipped a meal, you probably know what it’s like to feel seriously hangry. That’s because not eating enough can seriously mess with your blood sugar levels—and when your blood sugar dips too low, you may feel lightheaded, since your brain isn’t getting the fuel—aka glucose (a type of sugar)—that it needs for energy.

This is more of a metabolic concern, Dr. Burroughs says. Most of the time, grabbing a bite to eat will resolve your symptoms. Pro tip: Stocking up on satiating, non-perishable snacks in your desk drawer (like protein bars) can help you stay fueled on crazy busy days.

But if you have diabetes and take medication to lower blood sugar, lightheadedness should be taken seriously, as it might be a sign your blood sugar is dipping dangerously low, she adds, which can lead to seizures and unconsciousness.

Flu

The flu, a respiratory infection caused by a virus, can cause some nasty symptoms, like fever, muscle aches, chills, sore throat, headaches, and a stuffy nose. But if you’re feeling lightheaded? Blame dehydration and low blood sugar: You probably don’t feel much like eating or drinking, but both can keep lightheadedness and other awful flu symptoms at bay, Dr. Burroughs says. Not sure what you can stomach? Here’s exactly what to eat when you have a cold or flu.

Stroke

If you feel lightheaded (or dizzy) in conjunction with muscle weakness, difficulty speaking, or numbness and tingling, a stroke (when blood flow gets cut off from an area in the brain) may be behind the symptoms, Dr. Phillips says, and you should seek emergency medical attention immediately.

The decrease in blood flow that leads to feeling lightheaded could be caused by a blood clot in the brain, Dr. Burroughs says, which can cause what’s called an ischemic stroke.

Postural Orthostatic Tachycardia Syndrome (POTS)

POTS is a condition in which your heart starts beating “much faster” than usual when you change positions, like going from sitting to standing, says Aline M. Holmes, D.N.P., R.N., treasurer of the American College of Medical Quality. “Normally, when you change position like that, your body can adjust the slight drop in blood pressure caused by the change to a standing position and pooling of some blood in your legs, with a slight increase in heart rate,” she says. “But with POTS, your body is not able to make those changes as it should, resulting in dizziness and lightheadedness.”

What to do if you feel lightheaded

If you find yourself feeling lightheaded, “it is important to focus on adequate hydration, as dehydration is a very common cause of lightheadedness,” says Christopher Palazzo, M.D., a family medicine practitioner at Spectrum Health. It’s also a good idea to make sure you’re eating regular meals and snacks, he says, adding that making sure you’re getting enough sleep and minimizing alcohol and caffeine can also help.

Eating enough is an important element, says Hannah Goldberg, M.D., a primary care physician with Mercy Medical Center. “Some people need to eat at regular intervals or they’ll end up with low blood sugar, which can make them feel lightheaded,” she says.

When to see a doctor for lightheadedness

If you feel lightheaded once or twice, you shouldn’t panic, Dr. Goldberg says. But, if this is a regular or semi-regular thing for you, she says it’s a good idea to get it checked out.

“If your lightheadedness is persistent for longer than a few days or associated with headache or vision changes, numbness or weakness, diarrhea or vomiting, chest pain or shortness of breath, or if you feel like you are going to faint, you should be evaluated by your doctor,” Dr. Palazzo says.