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What is Low Blood Pressure and What Causes It

Updated on April 24, 2013

How Low is Too Low for Blood Pressure?

Normal blood pressure is equal to or less than 120/80. There is no defined blood pressure that is too low. Blood pressure is too low when symptoms of low blood pressure are occurring. The most common symptoms of low blood pressure are dizziness, lightheadedness and fainting.

What Is Low Blood Pressure?

Blood pressure is the measurement of the force of blood against the arteries. It is measured in two ways: systolic blood pressure and diastolic blood pressure. Systolic blood pressure (the top number in a blood pressure reading) measures this force during the heartbeat; diastolic blood pressure (the bottom number in a blood pressure reading) measures this force while between heartbeats or while the heart is at rest. Blood pressure is measured in millimeters of Mercury (mm Hg).

According to the Mayo Clinic, a normal blood pressure reading is 120/80 or lower. High blood pressure is any blood pressure reading above this. There is no designated threshold for low blood pressure that is too low. This means that blood pressure may be lower than 120/80 and still not considered too low as long as no symptoms of low blood pressure are occurring. Blood pressure can be very different from one person to another. This is why there is no predetermined blood pressure level that is considered too low.

What Causes Low Blood Pressure?

Low blood pressure is linked to several different causes. Some common causes include:

  • Dehydration - When the body loses more fluid than it consumes dehydration is the result. Illness resulting in nausea, vomiting, diarrhea can lead to dehydration. Medications like diuretics (water pills) are designed to eliminate excess fluid from the body and can cause dehydration. Mild dehydration causes dizziness and fainting. More severe dehydration can be life threatening as it results in a marked decrease in blood volume leading to hypovolemic shock.
  • Pregnancy - Because a woman's circulatory system expands during pregnancy, she will most likely experience a drop of 5-10 points from her systolic blood pressure (top number) and 10-15mm Hg from her diastolic blood pressure (bottom number). This is expected, and her pre-pregnancy blood pressure should return to normal after delivery.
  • Heart Disease - Low blood pressure occurs when the heart is unable to pump enough blood to the rest of the body. Those with weakened or failing heart valves, slow heart rate, history of heart attack, or congestive heart failure may suffer from low blood pressure.
  • Thyroid Problems - Too much (hyperthyroid) or too little (hypothyroid) thyroid hormone can cause low blood pressure.
  • Low Blood Sugar - This can sometimes cause low blood pressure.
  • Blood Loss - Because any type of severe blood loss automatically decreases the amount of blood flowing through the body system, dangerously low blood pressures can occur. This can be life threatening.
  • Septic Shock - This is caused by an infection within the blood itself and can result in life threatening low blood pressure.
  • Anaphylactic Shock - This is caused by an extreme allergy to medicines, bee stings, foods, and latex. In this type of shock the airway becomes blocked and breathing is very difficult. Low blood pressure can also occur.
  • Anemia - Too few red blood cells within the blood stream can result in low blood pressure. Anemia can be caused by poor nutrition and lack of B12 and folate in the diet.

Medications That Cause Low Blood Pressure

Drug Type
Used For
Common Drug Names
Elimination of excess body fluid
Lasix, Demadex, Zaroxolyn, Dyazide
Alpha Blockers
High Blood Pressure
Flomax, Cardura
Beta Blockers
High Blood Pressure, Prevention of Heart Attack, Heart Problems
Metoprolol, Atenolol, Carvedilol
Parkinson's Disease
Sinemet, Parlodel, Requip
Tricyclic Antidepressants
Elavil, Pamelor, Sinequan
Phosphodiesterase Inhibitor*
Erectile Dysfunction
Viagra, Cialis
* Use of nitroglycerin for chest pain combined with this medication can cause a lower blood pressure. Ask your doctor if you should take these medications together.

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What are Low Blood Pressure Symptoms?

The most common symptoms of low blood pressure are dizziness and lightheadedness. Other symptoms include:

  • fainting;
  • lack of concentration;
  • blurry vision;
  • nausea;
  • cold, clammy skin;
  • fatigue;
  • depression;
  • thirst; and,
  • quick, shallow breathing.

What are the Health Risks from Low Blood Pressure?

Because low blood pressure can cause dizziness and lightheadedness, falls can occur. Falls can be minor incidents or can lead to serious injuries. Additionally, because low blood pressure can cause inadequate amounts of oxygen to get to the heart and brain, damage to these organs can result.

When to see a Doctor for Low Blood Pressure

If you normally have a blood pressure equal to or less than 120/80 but have no symptoms, you do not need to call your doctor.

If you have high blood pressure or normally low blood pressure and are experiencing any of the symptoms noted above, call your doctor for an appointment. You blood pressure could be lower than usual. You may need medication changes or other treatments depending on what your doctor finds.


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    • kansasyarn profile image

      Teresa Sanderson 5 years ago from Rural Midwest

      Thanks for taking the time to read. Glad you found it useful. :-)

    • Learning in Life profile image

      Megan Sisko 5 years ago from SW Florida

      I have always had low blood pressure and my doctors have always said it's a good thing. But, on the other hand it has never been dangerously low. Great hub. Voted up.