Blood pressure below 90/60 mmHg is generally considered low, and a surprisingly wide range of things can cause it. Some are harmless and temporary, like standing up too fast or being pregnant. Others signal something more serious, like a heart condition or severe infection. Here’s a breakdown of the most common causes.
Dehydration and Low Fluid Volume
Your blood pressure depends partly on having enough fluid circulating through your blood vessels. When that volume drops, pressure drops with it. Dehydration is one of the most common and easily fixable causes of low blood pressure. Not drinking enough water, sweating heavily, vomiting, or having diarrhea can all reduce your blood volume enough to lower your readings. So can skipping meals or not getting enough salt in your diet.
Internal bleeding is a more dangerous version of the same problem. When blood is lost internally (from an ulcer, injury, or ruptured blood vessel), circulating volume falls and blood pressure can plummet. In severe cases, this leads to hypovolemic shock, where the heart simply can’t pump enough blood to keep organs functioning. A telltale sign of low fluid volume is getting dizzy when you stand up after sitting or lying down.
Medications
Prescription drugs are one of the most frequent causes of low blood pressure, especially in older adults. Several major drug classes can lower your pressure as a primary effect or a side effect:
- Blood pressure medications: This might seem obvious, but drugs prescribed for high blood pressure sometimes overshoot. Alpha-blockers commonly cause severe drops when standing. Beta-blockers can slow the heart rate and lower pressure. Calcium channel blockers, ACE inhibitors, and ARBs all lower pressure by design, and sometimes they lower it too much.
- Diuretics: Both thiazide and loop diuretics reduce fluid volume, which directly lowers blood pressure. Thiazide diuretics can also cause weakness from potassium loss.
- Heart medications: Nitroglycerin (commonly used for chest pain) is a well-known cause of sudden blood pressure drops and fainting. Other anti-anginal drugs work similarly.
- Antidepressants: Tricyclic antidepressants and MAOIs are particularly likely to cause blood pressure drops when standing. SNRIs commonly cause the same problem. SSRIs do so only rarely.
- Parkinson’s disease drugs: Dopamine agonists and MAO-B inhibitors both lower blood pressure as a side effect.
- Antipsychotics: Medications used for psychosis and severe agitation have blood-pressure-lowering properties.
If you’ve recently started or changed a medication and notice dizziness, lightheadedness, or feel faint when standing, the medication is a likely culprit.
Heart Problems
Your heart is the pump that generates blood pressure, so anything that weakens it or disrupts its rhythm can cause low readings. Heart failure means the heart can’t pump enough oxygen-rich blood to meet your body’s needs. In left-sided heart failure, the heart is either too weak to push blood out or too stiff to fill properly between beats. In right-sided heart failure, blood can’t reach the lungs efficiently to pick up oxygen.
Heart valve problems interfere with the smooth flow of blood through the heart’s chambers, reducing the volume pushed out with each beat. A heart attack damages heart muscle, sometimes permanently weakening the heart’s pumping ability. An abnormally slow heart rate (bradycardia) can also result in low blood pressure because fewer beats per minute means less blood flow overall.
Pregnancy
Low blood pressure is common and expected during pregnancy, particularly in the first and second trimesters. Your blood vessels expand during pregnancy, and your heart has to work harder to supply blood to both you and your growing baby. Hormonal shifts also affect your circulatory system in ways that temporarily lower pressure from its pre-pregnancy level. This usually isn’t dangerous, though it can cause lightheadedness and fatigue. Blood pressure typically returns to normal in the third trimester or after delivery.
Endocrine and Hormonal Causes
Addison’s disease, a condition where the adrenal glands don’t produce enough hormones, is a classic endocrine cause of low blood pressure. One of the hormones affected is aldosterone, which helps balance sodium and potassium levels to keep blood pressure in a healthy range. Without enough of it, blood pressure falls. People with Addison’s disease often notice dizziness or faintness when standing. In an adrenal crisis, blood pressure can drop dangerously low alongside low blood sugar and high potassium.
Thyroid disorders, particularly an underactive thyroid, can also contribute to lower blood pressure readings. Low blood sugar from any cause can trigger a drop as well, since the body redirects blood flow and heart rate may become irregular.
Severe Infections and Allergic Reactions
Some of the most dangerous causes of low blood pressure are acute emergencies. Sepsis, a life-threatening response to infection, causes blood vessels throughout the body to relax and widen, which makes pressure plummet.
Anaphylaxis, a severe allergic reaction, works through a similar but even faster mechanism. During anaphylactic shock, veins lose their tone and fluid leaks out of blood vessels into surrounding tissue. Up to 35% of circulating blood volume can shift out of the bloodstream within 10 minutes. On top of that, the heart itself becomes suppressed and may beat more slowly than expected. This combination of fluid loss, vessel relaxation, and weakened heart function makes anaphylaxis one of the most rapid and dangerous causes of blood pressure collapse.
Positional Changes and Nervous System Issues
Orthostatic hypotension is a blood pressure drop that happens specifically when you stand up. Your nervous system normally compensates for gravity by tightening blood vessels in your legs and briefly increasing your heart rate. When that reflex doesn’t work properly, blood pools in your lower body and pressure falls. This can happen from dehydration, medications, prolonged bed rest, or conditions that damage the autonomic nervous system (such as diabetes or Parkinson’s disease).
A similar reflex problem can occur after eating, when blood flow shifts toward the digestive system. This is called postprandial hypotension and is most common in older adults. Standing for long periods can also trigger a drop in some people, particularly younger adults, through a miscommunication between the heart and the brain that causes blood pressure and heart rate to fall suddenly.
Nutritional Deficiencies
Not getting enough vitamin B12, folate, or iron can lead to anemia, a condition where your blood doesn’t carry enough oxygen. Anemia reduces the blood’s ability to deliver oxygen efficiently, and the body may not be able to maintain normal blood pressure as a result. This type of low blood pressure tends to develop gradually and often comes with fatigue, pale skin, and shortness of breath. Correcting the deficiency usually brings blood pressure back to normal.