Low blood pressure, or hypotension, happens when blood moves through your arteries with less force than your body needs to deliver oxygen to your organs. A reading below 90/60 mmHg is generally considered low, though some people naturally run below that number without any problems. The real concern starts when low blood pressure causes symptoms like dizziness, fainting, or fatigue, which signals that something specific is driving it down.
The causes range from everyday factors like dehydration to serious conditions involving your heart, hormones, or nervous system. Understanding which category your low blood pressure falls into is the first step toward addressing it.
Dehydration and Low Blood Volume
The most common and most fixable cause of low blood pressure is not having enough fluid in your system. Your blood is mostly water, and when you lose fluid through sweating, vomiting, diarrhea, or simply not drinking enough, your blood volume drops. Less blood volume means less pressure pushing against your artery walls. As one Cleveland Clinic cardiologist put it, “you’re just not filling up the pipes enough for what your vascular system needs.”
This is why low blood pressure often shows up during heat waves, after intense exercise, or during a stomach illness. When blood volume falls far enough, your organs stop getting the oxygen they need to work properly. The fix is straightforward: replace the lost fluid. But severe dehydration, especially from prolonged illness, can drop blood pressure to dangerous levels quickly.
Heart Problems That Reduce Pressure
Your heart is the pump that generates blood pressure, so anything that weakens or disrupts the pump can lower the pressure it creates. An unusually slow heart rate means fewer beats pushing blood forward each minute. Damaged heart valves that don’t open or close properly allow blood to leak backward instead of flowing efficiently through your body. Heart failure, where the muscle itself has weakened, reduces the volume of blood your heart can move with each contraction.
A heart attack can also cause a sudden drop in blood pressure because damaged heart tissue can no longer contract with full force. These cardiac causes tend to produce low blood pressure alongside other symptoms like chest pain, shortness of breath, or swelling in the legs, which helps distinguish them from simpler causes like dehydration.
Medications Are a Leading Cause
Several categories of prescription drugs lower blood pressure as either their intended effect or a side effect. Diuretics (water pills) reduce blood volume by making your kidneys excrete more fluid. Blood pressure medications themselves can sometimes overcorrect, especially if your dose is too high or you’ve become dehydrated. Alpha blockers, commonly prescribed for prostate problems, relax blood vessel walls and can cause pressure to drop.
Antidepressants, particularly older types like tricyclics, are known to lower blood pressure. So are drugs used for Parkinson’s disease and erectile dysfunction. If your low blood pressure started around the same time you began a new medication, or had a dose adjustment, that connection is worth flagging to whoever prescribed it.
Hormonal and Endocrine Conditions
Your endocrine system produces hormones that help regulate blood pressure behind the scenes. When certain glands underperform, blood pressure can fall as a result. Addison’s disease, where the adrenal glands don’t produce enough cortisol and aldosterone, is one of the more recognized hormonal causes. Aldosterone specifically tells your kidneys to retain salt and water, so without enough of it, blood volume drops.
An underactive thyroid can also contribute to low blood pressure by slowing heart rate and reducing the force of each heartbeat. Low blood sugar, whether from diabetes medication or skipping meals, triggers a similar effect because the body’s stress response falters without adequate fuel.
Nervous System Misfires
Sometimes the brain sends the wrong signal to the heart and blood vessels, causing a sudden and dramatic pressure drop. This is called vasovagal syncope, and it’s the most common reason otherwise healthy young people faint. The vagus nerve, which runs from the brain to the abdomen and controls heart rate and blood vessel tone, becomes overstimulated. Your heart rate slows and your blood vessels widen at the same time, pulling blood pressure down and reducing blood flow to the brain.
Common triggers include prolonged standing, having blood drawn, the sight of blood, intense pain, sudden emotional stress, and dehydration. Less obvious triggers include coughing hard, straining during a bowel movement, or urinating while standing (in men). These episodes are usually brief and not dangerous on their own, but the fainting itself can cause injury from a fall.
Standing Up Too Fast
Orthostatic hypotension is what happens when your blood pressure drops specifically upon standing. Gravity pulls blood toward your legs when you rise, and normally your nervous system compensates within a second or two by tightening blood vessels and slightly increasing heart rate. When that reflex is sluggish or impaired, pressure drops before your body can catch up. The CDC defines it as a drop of 20 mmHg or more in the top number, or 10 mmHg or more in the bottom number, within a few minutes of standing.
This type of low blood pressure becomes more common with age, and it’s frequently worsened by dehydration, blood pressure medications, or conditions that damage the autonomic nervous system like diabetes and Parkinson’s disease. If you regularly feel lightheaded when getting out of bed or standing from a chair, rising slowly and pausing at the edge of the seat for a few seconds before standing can help.
Blood Pressure Drops After Eating
After a meal, your body directs extra blood flow to the digestive system. Normally, your heart rate ticks up and blood vessels elsewhere in your body constrict to keep overall pressure stable. Postprandial hypotension occurs when that compensation doesn’t happen fast enough or strongly enough, and pressure falls.
This is surprisingly common in older adults. Research reviews have found that roughly 40% of people between ages 65 and 86 experience it. Your risk is higher if you have high blood pressure (paradoxically), diabetes, Parkinson’s disease, heart failure, or end-stage kidney disease. Eating smaller, more frequent meals and limiting refined carbohydrates can reduce the blood flow spike to the gut and keep pressure more stable.
Nutritional Deficiencies
Your body needs vitamin B12 and folate to produce properly functioning red blood cells. Without enough of these nutrients, it makes red blood cells that are too large and inefficient at carrying oxygen. The resulting anemia reduces the oxygen-carrying capacity of your blood, which can contribute to low blood pressure along with dizziness, fatigue, and pale skin. Iron deficiency causes a similar problem through a different mechanism, producing red blood cells that are too small and too few.
Severe Infections and Allergic Reactions
The most dangerous causes of low blood pressure involve the body’s own inflammatory response spiraling out of control. In sepsis, a widespread infection triggers a cascade of inflammatory signals that cause blood vessels throughout the body to relax and become leaky. Fluid seeps out of the bloodstream and into surrounding tissues, blood vessels stop responding to the body’s normal tightening signals, and the heart muscle itself can be directly weakened by inflammatory chemicals. The result is a rapid, life-threatening drop in blood pressure.
Anaphylaxis, a severe allergic reaction, works through a similar mechanism. The immune system floods the body with chemicals that cause widespread blood vessel dilation and swelling, pulling pressure down within minutes. Both of these are medical emergencies where the low blood pressure itself becomes the immediate threat to survival, because organs starved of blood flow begin to fail.
When Low Blood Pressure Is Just Normal
Not all low blood pressure points to a problem. Some people, particularly those who are physically fit, naturally maintain readings below 90/60 mmHg and feel perfectly fine. Regular exercise strengthens the heart so it pumps more efficiently with less effort, which can lower resting blood pressure. In this context, low numbers are a sign of cardiovascular fitness rather than a medical concern. The distinction that matters is whether low blood pressure comes with symptoms. A reading of 85/55 in someone who feels energetic and alert is very different from the same reading in someone who feels faint every time they stand up.