Low blood pressure, defined as a reading below 90/60 mmHg, happens when your body can’t maintain enough force to push blood efficiently to your organs. Some people naturally run low without any symptoms or problems. But when blood pressure drops enough to cause dizziness, fainting, or fatigue, something specific is usually driving it. The causes range from everyday factors like dehydration to serious conditions like heart disease or severe infections.
Dehydration and Fluid Loss
Your blood pressure depends partly on having enough fluid circulating through your vessels. When that volume drops, so does the pressure. Dehydration from not drinking enough water, heavy sweating, vomiting, or diarrhea is one of the most common and fixable causes of low blood pressure. Even mild dehydration can make you feel lightheaded when you stand up quickly.
Significant blood loss takes this further. Whether from an injury, surgery, or internal bleeding, losing a large amount of blood reduces the volume your heart has to pump. In its most severe form, this can lead to hypovolemic shock, where the heart simply can’t deliver enough blood to the brain, kidneys, and other vital organs. This is a medical emergency, but even smaller amounts of blood loss (heavy menstrual periods, for instance) can contribute to chronically low readings.
Heart Problems That Reduce Blood Flow
Your heart is the pump behind your blood pressure, so anything that weakens it or disrupts its rhythm can cause readings to fall. Bradycardia, an abnormally slow heart rate, means the heart may not pump enough oxygen-rich blood to meet the body’s needs. It can result from heart tissue damage related to aging, prior heart attacks, congenital heart defects, or inflammation of the heart muscle.
Heart valve problems also play a role. When valves don’t open or close properly, less blood moves forward with each beat. Heart failure, where the heart muscle is too weak to pump effectively, has the same result. Electrical signaling problems, such as heart block (where signals between the upper and lower chambers get disrupted), can cause the heart to beat too slowly or irregularly to maintain adequate pressure.
Medications
Several common drug classes lower blood pressure as either their intended effect or a side effect. Blood pressure medications are the most obvious culprit. They’re designed to bring readings down, but sometimes they overshoot, especially when you stand up from sitting or lying down. This is called orthostatic hypotension, and it’s particularly common in older adults taking multiple medications.
Diuretics (water pills) reduce fluid volume, which directly lowers pressure. Some antidepressants, drugs for Parkinson’s disease, and medications for erectile dysfunction can also cause drops. If you’ve recently started a new medication or changed your dose and notice dizziness or lightheadedness, the timing is worth noting.
Hormonal and Endocrine Conditions
Your endocrine system helps regulate blood pressure through hormones, and when that system malfunctions, pressure can fall. Addison’s disease is one of the clearest examples. In this condition, the adrenal glands produce too little cortisol and often too little aldosterone, a hormone that balances sodium and potassium to keep blood pressure stable. People with Addison’s disease frequently experience dizziness or fainting when standing, and in a crisis, blood pressure can drop dangerously low alongside falling blood sugar.
An underactive thyroid can also contribute to low blood pressure by slowing the heart rate and reducing the force of each heartbeat. Low blood sugar, whether from diabetes management or skipping meals, triggers a similar effect because the body can’t fuel the cardiovascular system properly.
Nutritional Deficiencies and Anemia
When your body lacks vitamin B12 or folate, it produces red blood cells that are too large and don’t carry oxygen efficiently. This type of anemia forces the heart to work harder to deliver oxygen to tissues, and blood pressure can drop as a result. Symptoms overlap with low blood pressure itself: fatigue, shortness of breath, and dizziness. Iron deficiency anemia works through a similar mechanism, reducing the blood’s oxygen-carrying capacity and making it harder for the cardiovascular system to maintain normal pressure.
Pregnancy
Blood pressure naturally falls during pregnancy, particularly in the first and second trimesters. Research published in the American Heart Association’s journal Hypertension found that in healthy pregnancies, blood pressure steadily decreases until roughly the midpoint of gestation, then gradually rises again toward delivery, typically returning to early-pregnancy levels. The second trimester shows a statistically significant drop compared to the first for both the upper and lower numbers. This happens because the circulatory system expands rapidly to supply blood to the placenta, and the blood vessels relax to accommodate the increased volume. For most women, this is completely normal and resolves after delivery.
Severe Infections and Allergic Reactions
Some of the most dangerous causes of low blood pressure involve the body’s own response to infection or allergens. In septic shock, a bacterial infection triggers widespread inflammation that causes blood vessels to relax and become “flaccid,” drastically reducing the pressure driving blood to organs. The small blood vessels also become leaky, allowing fluid to escape the circulation entirely. This creates a double hit: vessels that are too relaxed to maintain pressure, and less fluid inside them.
Anaphylaxis, a severe allergic reaction to things like peanuts, bee stings, or certain medications, works through a similar mechanism. The immune system floods the body with chemicals that cause blood vessels to dilate suddenly and massively. Blood pressure can plummet within minutes. Both septic shock and anaphylactic shock are life-threatening emergencies that require immediate treatment.
Nervous System Triggers
Your brain and heart communicate constantly to regulate blood pressure, and sometimes that communication misfires. In neurally mediated syncope, emotional triggers like pain, fear, the sight of blood, or extreme anxiety activate areas of the brain that simultaneously slow the heart and relax blood vessels. The result is a sudden drop in blood pressure and, often, fainting. This is the mechanism behind the classic “passing out at the sight of blood” response.
Standing for long periods can trigger a similar response. When you’re upright, gravity pulls blood toward your legs. Normally, your nervous system compensates by tightening blood vessels and slightly increasing heart rate. In some people, this compensation fails. The veins in the legs dilate instead of constricting, reducing the amount of blood returning to the heart and causing pressure to fall.
Postprandial hypotension, a drop in blood pressure after eating, is another variant. Digestion redirects blood flow to the stomach and intestines, and in people with autonomic nervous system problems, the body can’t compensate adequately. This is most common in older adults and people with conditions like Parkinson’s disease or diabetes that affect nerve function.
Who’s More Likely To Have Low Blood Pressure
Age is a significant factor, though it works in two directions. Young, otherwise healthy people (especially women) sometimes have naturally low blood pressure that causes no symptoms and needs no treatment. On the other end, older adults are more vulnerable to orthostatic and postprandial drops because the reflexes that stabilize blood pressure slow with age. Prolonged bed rest weakens these reflexes too, which is why blood pressure often drops when someone gets up after days or weeks of limited movement.
People with certain chronic conditions, including diabetes, Parkinson’s disease, and heart disease, face a higher risk because these conditions damage the nerves or organs responsible for maintaining stable pressure. And anyone who is significantly dehydrated, malnourished, or recovering from blood loss is at increased risk until the underlying problem is corrected.