Blood pressure drops when the volume of blood in your vessels decreases, when your blood vessels widen, or when your heart can’t pump with enough force. Sometimes two or three of these happen at once. A reading below 90/60 mmHg is generally considered low blood pressure, but what matters most is whether the drop causes symptoms like dizziness, lightheadedness, or fainting.
Standing Up Too Quickly
One of the most common reasons blood pressure drops is simply standing up. When you go from lying down or sitting to standing, gravity pulls blood toward your legs. Normally your body compensates within seconds: blood vessels tighten, your heart rate ticks up, and pressure stays stable. When that reflex is too slow or too weak, blood pressure falls and you feel dizzy or see spots.
This is called orthostatic hypotension, defined as a drop of at least 20 mmHg in the upper (systolic) number or 10 mmHg in the lower (diastolic) number within three minutes of standing. It’s especially common in older adults, people who are dehydrated, and anyone taking blood pressure medications. Even a sit-to-stand drop of 15 mmHg systolic can be clinically meaningful.
Dehydration and Blood Volume Loss
Your blood is mostly water. When you lose fluid through sweating, vomiting, diarrhea, or simply not drinking enough, blood volume shrinks. Less blood returns to the heart, so each heartbeat pushes out a smaller volume. The result is lower pressure throughout the system.
Research on exercise-induced dehydration shows exactly how this works: as plasma volume drops, less blood fills the heart between beats, reducing the amount pumped out with each contraction. The body tries to compensate by tightening blood vessels in the arms and legs, but this only partially offsets the loss. Significant blood loss from an injury triggers the same cascade, just faster and more dramatically.
Medications That Lower Pressure
Medications are one of the most overlooked causes of blood pressure drops, and not just blood pressure drugs. The highest-risk categories include alpha-blockers (often prescribed for prostate problems), nitrates used for chest pain, and diuretics, which lower pressure by reducing fluid volume. Beta-blockers can also contribute because they blunt the heart rate increase your body relies on when you stand.
Outside of heart medications, several psychiatric drugs carry significant risk. Antipsychotics, tricyclic antidepressants, trazodone, and benzodiazepines all reduce vascular resistance or dampen the nervous system’s ability to keep pressure steady. Levodopa, used for Parkinson’s disease, is among the highest-risk non-cardiac drugs. If you’ve recently started a new medication and notice dizziness when standing, the timing is probably not a coincidence.
Eating a Large Meal
Your intestines need a surge of blood to digest food. After a meal, blood flow shifts toward your gut, and your body is supposed to compensate by speeding up your heart rate and tightening blood vessels elsewhere. In some people, particularly older adults, these adjustments fall short. Blood pressure drops within 30 to 75 minutes of eating, causing lightheadedness or even fainting. This is called postprandial hypotension, and it tends to be worse after large, carbohydrate-heavy meals.
Vasovagal Episodes
If you’ve ever felt faint at the sight of blood, during a blood draw, or while standing in a hot room for too long, you’ve likely experienced a vasovagal episode. It’s the most common cause of fainting in otherwise healthy people.
What happens is a communication misfire in the nervous system. A trigger (pain, emotional stress, prolonged standing, heat) causes the branch of your nervous system that controls heart rate and vessel tone to overreact. Your heart rate slows abruptly, and blood vessels in your legs widen at the same time. Blood pools in your lower body, pressure plummets, and blood flow to your brain drops enough to make you pass out. Most people recover quickly once they’re lying flat, because gravity is no longer working against them.
Heart Conditions
Blood pressure depends on how much blood the heart pushes out with each beat. When the heart is weakened or structurally damaged, it can’t maintain that output. Heart failure, damaged heart valves, and cardiomyopathy (a disease of the heart muscle itself) all reduce pumping efficiency.
A slow heart rate, or bradycardia, can also be the culprit. If the heart’s internal electrical system malfunctions, signals that trigger each beat may be delayed or blocked. The result is a heart rate too slow to circulate enough blood, leading to low pressure, fatigue, and dizziness. Heart attacks can damage the electrical system or the muscle directly, sometimes causing a sudden pressure drop.
Hormonal and Adrenal Problems
Your adrenal glands produce a hormone called aldosterone that tells your kidneys to hold on to sodium and water. More sodium retention means more fluid in your bloodstream and higher pressure. When the adrenal glands are underactive (a condition called adrenal insufficiency, or Addison’s disease), aldosterone and cortisol production both fall. Without enough of these hormones, your kidneys let too much sodium and water escape into urine, blood volume drops, and pressure falls. Thyroid disorders and low blood sugar can also interfere with the body’s ability to maintain stable pressure.
Severe Allergic Reactions
Anaphylaxis is the most dangerous cause of a sudden blood pressure drop. During a severe allergic reaction, your immune system floods the bloodstream with histamine. Histamine forces blood vessels to widen dramatically and makes vessel walls more permeable, meaning fluid leaks out of the bloodstream into surrounding tissue. On top of that, histamine triggers the release of nitric oxide, a powerful chemical that relaxes the smooth muscle in vessel walls even further. The combined effect is a rapid, life-threatening collapse in blood pressure that requires emergency treatment with epinephrine.
Vitamin Deficiencies and Anemia
When your body can’t produce enough healthy red blood cells, less oxygen reaches your tissues, and blood pressure can fall. Vitamin B12 deficiency is a well-documented cause. Beyond the anemia itself, B12 deficiency appears to disrupt the autonomic nervous system, the same system responsible for adjusting heart rate and vessel tone when you stand up. Case reports describe patients with dangerously low blood pressure (readings as low as 79/40 mmHg) whose pressure returned to normal after B12 supplementation alone. Orthostatic hypotension linked to B12 deficiency has been reported even in people who don’t yet show obvious signs of anemia or nerve damage. Folate and iron deficiencies cause similar, though typically less dramatic, effects.
Infections and Sepsis
A serious infection can cause blood pressure to drop when the immune response spirals out of control. In sepsis, the body releases chemicals to fight the infection that end up widening blood vessels throughout the body. At the same time, fluid leaks from the bloodstream into tissues, reducing the volume of blood the heart has to work with. The combination of widespread vasodilation and volume loss can cause pressure to crash. Sepsis is a medical emergency, and a sudden drop in blood pressure during an illness with fever is one of its hallmark warning signs.
Why Some People Run Low All the Time
Not every case of low blood pressure has a clear medical cause. Some people, particularly young, thin, physically active individuals, simply have a resting blood pressure that sits below 90/60 without any symptoms. This is generally harmless and may even be associated with better long-term cardiovascular health. The concern starts when low pressure is new, worsening, or accompanied by symptoms like persistent dizziness, blurry vision, nausea, or fainting. A pattern of symptoms, rather than a single number on a cuff, is what separates normal variation from a problem worth investigating.