Low blood pressure might seem desirable, and for some people, it causes no problems. However, for many people, abnormally low blood pressure (hypotension) can cause dizziness and fainting. In severe cases, low blood pressure can be life-threatening.

A blood pressure reading lower than 90 millimeters of mercury (mm Hg) for the top number (systolic) or 60 mm Hg for the bottom number (diastolic) is generally considered low blood pressure.

The causes of low blood pressure can range from dehydration to serious medical disorders. It's important to find out what's causing your low blood pressure so that it can be treated.

For some people, low blood pressure signals an underlying problem, especially when it drops suddenly or is accompanied by signs and symptoms such as:

• Dizziness or lightheadedness
• Fainting
• Blurred or fading vision
• Nausea
• Fatigue
• Lack of concentration

Shock

Extreme hypotension can result in this life-threatening condition. Signs and symptoms include:

• Confusion, especially in older people
• Cold, clammy, pale skin
• Rapid, shallow breathing
• Weak and rapid pulse

When to see a doctor

If you have signs or symptoms of shock, seek emergency medical help.

If you have consistently low blood pressure readings but feel fine, your doctor will likely just monitor you during routine exams.

Even occasional dizziness or lightheadedness may be a relatively minor problem — the result of mild dehydration from too much time in the sun or a hot tub, for example. Still, it's important to see your doctor if you have signs or symptoms of low blood pressure because they can point to more-serious problems. It can be helpful to keep a record of your symptoms, when they occur and what you're doing at the time.

Blood pressure is a measurement of the pressure in your arteries during the active and resting phases of each heartbeat.

• Systolic pressure. The top number in a blood pressure reading is the amount of pressure your heart produces when pumping blood through your arteries to the rest of your body.
• Diastolic pressure. The bottom number in a blood pressure reading refers to the amount of pressure in your arteries when your heart is at rest between beats.

Current guidelines identify normal blood pressure as lower than 120/80 mm Hg.

Blood pressure varies throughout the day, depending on:

• Body position
• Breathing rhythm
• Stress level
• Physical condition
• Medications you take
• What you eat and drink
• Time of day

Blood pressure is usually lowest at night and rises sharply on waking.

Blood pressure: How low can you go?

What's considered low blood pressure for you may be normal for someone else. Most doctors consider blood pressure too low only if it causes symptoms.

Some experts define low blood pressure as readings lower than 90 mm Hg systolic or 60 mm Hg diastolic. If either number is below that, your pressure is lower than normal.

A sudden fall in blood pressure can be dangerous. A change of just 20 mm Hg — a drop from 110 systolic to 90 mm Hg systolic, for example — can cause dizziness and fainting when the brain fails to receive enough blood. And big drops, such as those caused by uncontrolled bleeding, severe infections or allergic reactions, can be life-threatening.

Conditions that can cause low blood pressure

Medical conditions that can cause low blood pressure include:

• Pregnancy. Because the circulatory system expands rapidly during pregnancy, blood pressure is likely to drop. This is normal, and blood pressure usually returns to your pre-pregnancy level after you've given birth.
• Heart problems. Some heart conditions that can lead to low blood pressure include extremely low heart rate (bradycardia), heart valve problems, heart attack and heart failure.
• Endocrine problems. Parathyroid disease, adrenal insufficiency (Addison's disease), low blood sugar (hypoglycemia) and, in some cases, diabetes can trigger low blood pressure.
• Dehydration. When your body loses more water than it takes in, it can cause weakness, dizziness and fatigue. Fever, vomiting, severe diarrhea, overuse of diuretics and strenuous exercise can lead to dehydration.
• Blood loss. Losing a lot of blood, such as from a major injury or internal bleeding, reduces the amount of blood in your body, leading to a severe drop in blood pressure.
• Severe infection (septicemia). When an infection in the body enters the bloodstream, it can lead to a life-threatening drop in blood pressure called septic shock.
• Severe allergic reaction (anaphylaxis). Common triggers of this severe and potentially life-threatening reaction include foods, certain medications, insect venoms and latex. Anaphylaxis can cause breathing problems, hives, itching, a swollen throat and a dangerous drop in blood pressure.
• Lack of nutrients in your diet. A lack of the vitamin B-12, folate and iron can keep your body from producing enough red blood cells (anemia), causing low blood pressure.

Medications that can cause low blood pressure

Some medications can cause low blood pressure, including:

• Water pills (diuretics), such as furosemide (Lasix) and hydrochlorothiazide (Microzide, others)
• Alpha blockers, such as prazosin (Minipress)
• Beta blockers, such as atenolol (Tenormin) and propranolol (Inderal, Innopran XL, others)
• Drugs for Parkinson's disease, such as pramipexole (Mirapex) or those containing levodopa
• Certain types of antidepressants (tricyclic antidepressants), including doxepin (Silenor) and imipramine (Tofranil)
• Drugs for erectile dysfunction, including sildenafil (Revatio, Viagra) or tadalafil (Adcirca, Alyq, Cialis), particularly when taken with the heart medication nitroglycerin (Nitrostat, others)

Types of low blood pressure

Doctors often break down low blood pressure (hypotension) into categories, depending on the causes and other factors. Some types of low blood pressure include:

• Low blood pressure on standing up (orthostatic or postural) hypotension). This is a sudden drop in blood pressure when you stand up from a sitting position or after lying down.

  Gravity causes blood to pool in your legs when you stand. Ordinarily, your body compensates by increasing your heart rate and constricting blood vessels, thereby ensuring that enough blood returns to your brain.

  But in people with orthostatic hypotension, this compensating mechanism fails and blood pressure falls, leading to dizziness, lightheadedness, blurred vision and even fainting.

  Orthostatic hypotension can occur for various reasons, including dehydration, prolonged bed rest, pregnancy, diabetes, heart problems, burns, excessive heat, large varicose veins and certain neurological disorders.

  A number of medications also can cause orthostatic hypotension, particularly drugs used to treat high blood pressure — diuretics, beta blockers, calcium channel blockers and angiotensin-converting enzyme (ACE) inhibitors — as well as antidepressants and drugs used to treat Parkinson's disease and erectile dysfunction.

  Orthostatic hypotension is especially common in older adults, but it also affects young, otherwise healthy people who stand up suddenly after sitting with their legs crossed for long periods or after squatting for a time.

• Low blood pressure after eating (postprandial hypotension). This drop in blood pressure occurs one to two hours after eating and affects mostly older adults.

  Blood flows to your digestive tract after you eat. Ordinarily, your body increases your heart rate and constricts certain blood vessels to help maintain normal blood pressure. But in some people these mechanisms fail, leading to dizziness, faintness and falls.

  Postprandial hypotension is more likely to affect people with high blood pressure or autonomic nervous system disorders such as Parkinson's disease.

  Eating small, low-carbohydrate meals; drinking more water; and avoiding alcohol might help reduce symptoms.

• Low blood pressure from faulty brain signals (neurally mediated hypotension). This disorder, which causes a blood pressure drop after standing for long periods, mostly affects young adults and children. It seems to occur because of a miscommunication between the heart and the brain.
• Low blood pressure due to nervous system damage (multiple system atrophy with orthostatic hypotension). Also called Shy-Drager syndrome, this rare disorder has many Parkinson disease-like symptoms. It causes progressive damage to the autonomic nervous system, which controls involuntary functions such as blood pressure, heart rate, breathing and digestion. It's associated with having very high blood pressure while lying down.

Low blood pressure (hypotension) can occur in anyone, though certain types of low blood pressure are more common depending on your age or other factors:

• Age. Drops in blood pressure on standing or after eating occur primarily in adults older than 65. Neurally mediated hypotension primarily affects children and younger adults.
• Medications. People who take certain medications, for example, high blood pressure medications such as alpha blockers, have a greater risk of low blood pressure.
• Certain diseases. Parkinson's disease, diabetes and some heart conditions put you at a greater risk of developing low blood pressure.

Even moderate forms of low blood pressure can cause dizziness, weakness, fainting and a risk of injury from falls.

And severely low blood pressure can deprive your body of enough oxygen to carry out its functions, leading to damage to your heart and brain.

The goal in testing for low blood pressure is to find the cause. Besides taking your medical history, doing a physical exam and measuring your blood pressure, your doctor might recommend the following:

• Blood tests. These can provide information about your overall health as well as whether you have low blood sugar (hypoglycemia), high blood sugar (hyperglycemia or diabetes) or a low red blood cell count (anemia), all of which can cause lower than normal blood pressure.

• Electrocardiogram (ECG). During this painless, noninvasive test, soft, sticky patches (electrodes) are attached to the skin of your chest, arms and legs. The patches detect your heart's electrical signals while a machine records them on graph paper or displays them on a screen.

  An ECG, which can be performed in your doctor's office, detects irregularities in your heart rhythm, structural problems in your heart, and problems with the supply of blood and oxygen to your heart muscle. It can also tell if you're having a heart attack or have had one in the past.

• Tilt table test. If you have low blood pressure on standing or from faulty brain signals (neurally mediated hypotension), a tilt table test can evaluate how your body reacts to changes in position.

  During the test, you lie on a table that's tilted to raise the upper part of your body, which simulates the movement from horizontal to a standing position.

Low blood pressure that either doesn't cause signs or symptoms or causes only mild symptoms rarely requires treatment.

If you have symptoms, treatment depends on the cause. For instance, when medication causes low blood pressure, treatment usually involves changing or stopping the medication or lowering the dose.

If it's not clear what's causing low blood pressure or no treatment exists, the goal is to raise your blood pressure and reduce signs and symptoms. Depending on your age, health and the type of low blood pressure you have, you can do this in several ways:

• Use more salt. Experts usually recommend limiting salt in your diet because sodium can raise blood pressure, sometimes dramatically. For people with low blood pressure, that can be a good thing.

  But because excess sodium can lead to heart failure, especially in older adults, it's important to check with your doctor before increasing the salt in your diet.

• Drink more water. Fluids increase blood volume and help prevent dehydration, both of which are important in treating hypotension.

• Wear compression stockings. The elastic stockings commonly used to relieve the pain and swelling of varicose veins can help reduce the pooling of blood in your legs.

  Some people tolerate elastic abdominal binders better than they do compression stockings.

• Medications. Several medications can be used to treat low blood pressure that occurs when you stand up (orthostatic hypotension). For example, the drug fludrocortisone, which boosts your blood volume, is often used to treat this form of low blood pressure.

  Doctors often use the drug midodrine (Orvaten) to raise standing blood pressure levels in people with chronic orthostatic hypotension. It works by restricting the ability of your blood vessels to expand, which raises blood pressure.

Depending on the reason for your low blood pressure, you might be able to reduce or prevent symptoms.

• Drink more water, less alcohol. Alcohol is dehydrating and can lower blood pressure, even if you drink in moderation. Water, on the other hand, fights dehydration and increases blood volume.

• Pay attention to your body positions. Gently move from a prone or squatting to a standing position. Don't sit with your legs crossed.

  If you begin to get symptoms while standing, cross your thighs in a scissors fashion and squeeze, or put one foot on a ledge or chair and lean as far forward as possible. These moves encourage blood flow from your legs to your heart.

• Eat small, low-carb meals. To help prevent blood pressure from dropping sharply after meals, eat small portions several times a day and limit high-carbohydrate foods such as potatoes, rice, pasta and bread.

  Your doctor also might recommend drinking one or two strong cups of caffeinated coffee or tea with breakfast. Don't drink caffeine throughout the day because you will become less sensitive to caffeine, and caffeine can cause dehydration.

• Exercise regularly. Aim for 30 to 60 minutes a day of exercise that raises your heart rate and resistance exercises two or three days a week. Avoid exercising in hot, humid conditions.

No special preparations are necessary to have your blood pressure checked. Don't stop taking medications you think might affect your blood pressure without your doctor's advice.

Here's some information to help you get ready for your appointment.

What you can do

Make a list of:

• Your symptoms, including any that seem unrelated to low blood pressure, and when they occur
• Key personal information, including a family history of low blood pressure and major stresses or recent life changes
• All medications, vitamins or other supplements you take, including doses
• Questions to ask your doctor

For low blood pressure, basic questions to ask your doctor include:

• What is likely causing my symptoms or condition?
• What are other possible causes?
• What tests will I need?
• What's the most appropriate treatment?
• How often should I be screened for low blood pressure?
• I have other health conditions. How can I best manage them together?
• Are there restrictions I need to follow?
• Should I see a specialist?
• Are there brochures or other printed materials I can have? What websites do you recommend?

Don't hesitate to ask other questions.

What to expect from your doctor

Your doctor is likely to ask you questions, including:

• Have your symptoms been continuous or occasional?
• How severe are your symptoms?
• What, if anything, seems to improve your symptoms?
• What, if anything, appears to worsen your symptoms?
• Do you have a family history of heart disease?