Low blood pressure, defined as a reading of 90/60 mmHg or lower, happens when your body can’t keep enough force behind the blood moving through your arteries. The causes range from everyday and harmless (not drinking enough water, standing up too fast) to serious and urgent (severe infection, major blood loss). Understanding what drives blood pressure down helps you figure out whether your situation needs attention or is simply how your body works.
Dehydration and Fluid Loss
Your blood is mostly water. When you lose fluid through sweating, vomiting, diarrhea, or simply not drinking enough, the total volume of blood circulating in your body drops. Less blood volume means less pressure pushing against artery walls, and your blood pressure falls. The effect compounds: as dehydration worsens, your body pulls water from tissues to try to keep the bloodstream full, but eventually it can’t compensate. Severe dehydration can progress to shock, where organs stop getting enough blood to function.
Even mild dehydration from skipping water on a hot day or during exercise can produce noticeable lightheadedness. People who are older, very young, or taking water pills (diuretics) are especially vulnerable because their fluid reserves are thinner to begin with.
Medications That Lower Blood Pressure
Several common drug classes can push blood pressure too low, especially when you change positions. The ones most strongly linked to drops upon standing include alpha blockers, beta blockers, tricyclic antidepressants, nitrates, certain medications for erectile dysfunction, and antipsychotics. These medications either relax blood vessels or slow the heart’s ability to speed up when you stand, which normally keeps blood flowing to your brain.
Diuretics, particularly the stronger “loop” type, also contribute by reducing fluid volume. Blood pressure medications are the most obvious culprits, but the drop can also come from drugs prescribed for completely unrelated conditions, like depression or psychosis, catching people off guard. If you’ve recently started a new medication and notice dizziness when standing, the timing is probably not a coincidence.
Standing Up Too Fast
When you go from sitting or lying down to standing, gravity pulls blood toward your legs. Normally, your nervous system detects this shift instantly and tightens blood vessels in the lower body while nudging your heart rate up. When that reflex is too slow or too weak, blood pressure drops and you feel dizzy or see spots. This is called orthostatic hypotension, and it’s one of the most common reasons people notice low blood pressure.
It becomes more frequent with age, after prolonged bed rest, and during dehydration. Medications that blunt the nervous system’s reflexes make it worse.
Eating a Large Meal
After you eat, your body redirects extra blood to the digestive tract to break down food. To compensate, your heart rate normally picks up and blood vessels elsewhere tighten. In some people, that compensation falls short: the heart doesn’t speed up enough and blood vessels don’t constrict the way they should. The result is a blood pressure drop within one to two hours of eating, sometimes enough to cause lightheadedness or fainting. This postprandial hypotension is most common in older adults and people with conditions like Parkinson’s disease or diabetes that affect nerve signaling.
Heart Conditions
Blood pressure depends on how much blood the heart pumps with each beat and how much resistance the arteries provide. Any problem that reduces the heart’s pumping ability can lower blood pressure. Heart failure weakens the muscle so it can’t push enough blood out. A very slow heart rate (bradycardia) means fewer beats per minute, which reduces overall output. Damaged or leaky heart valves let blood flow backward instead of forward, cutting the effective volume that reaches your arteries.
A heart attack can also cause a sudden pressure drop if it damages enough heart muscle to impair pumping. Unlike the other causes on this list, heart-related low blood pressure often comes with additional symptoms like chest pain, shortness of breath, or swelling in the legs.
Hormonal and Adrenal Problems
Your adrenal glands produce two hormones that directly control blood pressure. Cortisol helps maintain the tone of blood vessel walls. Aldosterone balances sodium and potassium in the blood, which regulates how much water your body retains and, by extension, your blood volume. In adrenal insufficiency (sometimes called Addison’s disease), the adrenal glands don’t make enough of either hormone. The result is chronically low blood pressure, low blood sodium, and fatigue that worsens over time.
During periods of physical stress, like illness or surgery, the adrenal glands normally ramp up cortisol production. People with adrenal insufficiency can’t do this, which can trigger a crisis with dangerously low blood pressure, low blood sugar, and confusion that requires emergency treatment.
Pregnancy
Blood pressure drops early in pregnancy, sometimes noticeably. The decline starts as early as five weeks of gestation, well before the placenta is fully formed. By the second trimester, blood pressure typically sits 5 to 10 mmHg below its pre-pregnancy baseline, though the biggest drop actually happens between weeks 6 and 8.
The reason is widespread relaxation of blood vessels. Resistance in the blood vessel walls falls by 35% to 40% compared to pre-pregnancy levels, driven by rising estrogen, progesterone, and a hormone called relaxin that loosens arterial walls. The body responds by retaining more salt and water to fill the expanded vascular space, but it can’t always keep up, leaving blood pressure lower than usual. By the third trimester, pressure generally climbs back toward pre-pregnancy levels.
Nervous System Misfires
Sometimes the brain and heart miscommunicate in a way that causes a sudden, dramatic pressure drop. The classic example is a vasovagal episode, the most common cause of fainting. A trigger activates the vagus nerve, which overreacts by slowing the heart and widening blood vessels in the legs. Blood pools in the lower body, pressure plummets, and the brain loses blood flow fast enough to make you pass out.
Common triggers include standing for long periods, heat exposure, seeing blood, having blood drawn, fear of injury, and straining on the toilet. Most people recover quickly once they’re horizontal again, because gravity redistributes blood back toward the head. These episodes are usually harmless on their own, but they can lead to injuries from falling.
Nutritional Deficiencies
Your body needs vitamin B12 and folate to build normal red blood cells. When either nutrient is too low, the bone marrow produces red blood cells that are oversized and inefficient at carrying oxygen. With fewer functioning red blood cells in circulation, the blood’s capacity to deliver oxygen drops. The body may compensate by widening blood vessels to increase flow, which lowers overall pressure. Vegans, people with digestive absorption problems, and older adults with reduced stomach acid are at the highest risk for B12 deficiency.
Severe Infections and Allergic Reactions
Sepsis, a body-wide response to infection, causes one of the most dangerous forms of low blood pressure. The infection triggers massive inflammation that damages the lining of blood vessels throughout the body. Vessels dilate uncontrollably, fluid leaks out of the bloodstream into surrounding tissue, and blood can no longer circulate effectively. The result is a type of shock where blood pressure collapses even though the heart may still be pumping hard.
Anaphylaxis, a severe allergic reaction, produces a similar pattern. The immune system floods the body with chemicals that widen blood vessels and cause them to leak, dropping blood pressure within minutes. Both conditions are medical emergencies where the speed of treatment determines survival.
When Low Blood Pressure Is Just Your Normal
Not every low reading signals a problem. Some people, particularly younger adults and those who exercise regularly, naturally run blood pressures below 90/60 without any symptoms at all. If you feel fine, a low number on the cuff is generally not a concern. Blood pressure only becomes a medical issue when it’s low enough to reduce blood flow to your organs, which you’d notice as dizziness, blurred vision, nausea, fatigue, or fainting. The reading matters less than how you feel.