Some people have low blood pressure because of how their body regulates blood volume, heart rate, or blood vessel tone. A reading below 90/60 mmHg is generally considered low. For many people, naturally low blood pressure causes no symptoms and needs no treatment. But when it does cause dizziness, fainting, or fatigue, the underlying reason usually falls into one of several categories: dehydration, medications, heart or hormonal conditions, pregnancy, or a nervous system quirk that causes pressure to drop at predictable moments.
Not Enough Fluid in the System
Blood pressure depends partly on how much fluid is circulating through your blood vessels. When that volume drops, there’s less force pushing blood through your arteries, and pressure falls. Dehydration is the most common version of this. Vomiting, diarrhea, heavy sweating, and not drinking enough water can all shrink your blood volume enough to lower your pressure noticeably. In extreme cases, losing more than 15 to 20 percent of your blood volume can cause a dangerous condition called hypovolemic shock, where the heart simply can’t pump enough blood to supply your organs.
Nutritional deficiencies can also reduce blood volume in a less obvious way. When your body lacks vitamin B12 or folate, it produces red blood cells that are too large and don’t function properly. Those cells carry less oxygen, and the resulting anemia can contribute to low blood pressure along with fatigue, weakness, and shortness of breath.
Medications That Lower Blood Pressure
A surprisingly long list of medications can cause low blood pressure as a side effect, and not just the ones designed to lower it. Blood pressure drugs like diuretics (both the milder thiazide type and stronger loop diuretics), beta blockers, calcium channel blockers, and ACE inhibitors or ARBs all reduce pressure through different mechanisms. Diuretics do it by pulling fluid out of your system, which can lead to dehydration. Beta blockers slow the heart rate. Calcium channel blockers and other vasodilators relax blood vessel walls.
What catches many people off guard is that psychiatric and neurological medications do it too. Older antidepressants called tricyclics commonly cause blood pressure to drop when you stand up. So do medications for Parkinson’s disease, antipsychotics, and even some newer antidepressants that affect norepinephrine. Nitrate medications used for chest pain are another well-known culprit, sometimes causing sudden drops in pressure that lead to fainting. If you started a new medication and noticed dizziness or lightheadedness, the drug is a likely suspect.
Blood Pressure That Drops After Standing or Eating
Some people have blood pressure that’s fine most of the time but drops sharply in specific situations. Orthostatic hypotension is the version that hits when you stand up. Gravity pulls blood toward your legs, and normally your blood vessels tighten quickly to compensate. When that reflex is slow or weak, pressure drops and you feel dizzy or see spots. This becomes more common with age because arteries stiffen and lose their ability to adjust quickly.
Postprandial hypotension is a similar problem triggered by eating. After a meal, your body diverts extra blood to the digestive system. To keep pressure stable, your heart rate should increase and blood vessels elsewhere should tighten. When those compensations don’t happen fast enough, pressure drops. This tends to affect older adults more, and larger, carbohydrate-heavy meals make it worse.
Heart and Hormonal Conditions
Your heart sets the pace for blood pressure. If it beats too slowly (a condition called bradycardia), less blood gets pumped per minute, and pressure can fall. Heart valve problems and heart failure can have the same effect, reducing the volume of blood pushed out with each beat.
Hormonal conditions play a role too. The adrenal glands produce cortisol and aldosterone, hormones that help regulate blood pressure by controlling salt balance and blood vessel tone. In adrenal insufficiency (sometimes called Addison’s disease), the adrenal glands don’t produce enough of these hormones. The result is often persistent low blood pressure that doesn’t respond to the usual fixes like fluids and rest. Cortisol also supports the sympathetic nervous system, the part that keeps your heart rate up. Without enough of it, heart rate can drop alongside blood pressure. Thyroid disorders can cause similar issues when hormone levels fall too low.
Pregnancy
Blood pressure naturally decreases during pregnancy. It drops steadily through the first half of pregnancy, reaching its lowest point around mid-gestation, then gradually climbs back up toward delivery. The final readings typically return close to where they started early in pregnancy. This dip happens because pregnancy hormones relax blood vessel walls, and the circulatory system expands rapidly to support the growing fetus. Most of the time, this is completely normal, though it can cause lightheadedness, especially when standing quickly or in hot environments.
Nervous System Misfires
In some people, the nervous system sends the wrong signals at the wrong time, causing blood pressure to plummet. This is called neurally mediated hypotension, and it’s the mechanism behind many common fainting episodes. The brain and heart miscommunicate: instead of tightening blood vessels to maintain pressure, the nervous system triggers the opposite response, dilating vessels and sometimes slowing the heart at the same time. The result is a sudden drop in blood flow to the brain.
Common triggers include prolonged standing, emotional stress (especially in warm or crowded spaces), severe pain, and nausea. If you’ve ever felt woozy getting blood drawn or standing in a long line on a hot day, this is likely what happened. Some people are simply more prone to these episodes than others, and the tendency often shows up in adolescence or early adulthood.
Severe, Life-Threatening Causes
In emergency situations, blood pressure can drop dangerously low because blood vessels throughout the body dilate all at once. This is called distributive shock, and it happens during severe infections (sepsis) and serious allergic reactions (anaphylaxis). The blood vessels become so relaxed and leaky that pressure collapses, and organs stop receiving enough blood to function. Unlike the gradual or situational causes above, this kind of low blood pressure develops rapidly and requires immediate treatment.
Why Some People Run Low Without a Clear Cause
For a significant number of people, especially younger women and people with slim builds, low blood pressure is simply their baseline. Their readings consistently fall below 90/60, yet they feel fine. This is sometimes called constitutional hypotension, and it’s not a disease. It may reflect efficient cardiovascular function, genetics, or a naturally lower resting heart output. As long as there are no symptoms like chronic dizziness, fatigue, or fainting, a low reading on its own isn’t a problem. In fact, it’s associated with lower cardiovascular risk over the long term.
When low blood pressure does cause symptoms, the pattern of when it happens, what triggers it, and what other symptoms come with it usually points toward the cause. Pressure that drops only when standing suggests an orthostatic issue. Pressure that’s always low alongside fatigue and salt cravings may point toward an adrenal problem. And pressure that dropped after starting a new medication is often the simplest puzzle to solve.