Low blood pressure, clinically called hypotension, is a reading below 90/60 mmHg. Some people naturally run low without any symptoms, but when blood pressure drops enough to reduce oxygen delivery to your organs, it causes dizziness, fainting, fatigue, and blurred vision. The causes range from everyday factors like dehydration to serious emergencies like severe infection.
Dehydration and Fluid Loss
One of the most common and straightforward causes of low blood pressure is not having enough fluid in your body. 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 cardiologist at the Cleveland Clinic put it, “you’re just not filling up the pipes enough for what your vascular system needs.”
This is why blood pressure often dips during hot weather, after intense exercise, or during a stomach illness. Heavy bleeding from an injury or surgery creates the same problem on a more dangerous scale. Significant blood or fluid loss can trigger hypovolemic shock, where organs stop receiving the oxygen they need to function.
Medications That Lower Blood Pressure
Several types of medication can push blood pressure too low, especially when you first start taking them or when doses change. The most common culprits include:
- Diuretics (water pills): These reduce blood volume by making you urinate more, which can overshoot and drop pressure too far.
- Alpha blockers: Used for high blood pressure and prostate issues, these relax blood vessel walls. When you first start taking one, low blood pressure and dizziness are common enough that fainting after standing up is a known risk.
- Beta blockers: These slow the heart rate, which can reduce cardiac output and lower pressure.
- Antidepressants and anti-anxiety medications: Certain classes of these drugs affect blood vessel tone or heart rate as a side effect.
If you’re taking any blood pressure medication and feeling lightheaded regularly, that’s worth bringing up at your next appointment. The dose may simply need adjusting.
Heart Problems
Your heart is the pump driving the entire system, so anything that weakens its pumping ability can lower blood pressure. Bradycardia, an abnormally slow heart rate, is one of the more common cardiac causes. When the heart beats too slowly, it can’t push enough oxygen-rich blood out to the body. Bradycardia can result from aging-related damage to heart tissue, prior heart attacks, inflammation of the heart muscle, congenital heart defects, or complications from heart surgery.
Heart valve problems also play a role. If a valve doesn’t open or close properly, the heart moves blood less efficiently with each beat. Heart failure, where the heart muscle has weakened over time, produces a similar effect. In all these cases, the heart simply can’t generate enough force to maintain normal pressure in the arteries.
Orthostatic Hypotension: Drops When You Stand
If your blood pressure drops specifically when you go from sitting or lying down to standing, that’s orthostatic hypotension. It’s defined as a systolic drop of at least 20 mmHg or a diastolic drop of at least 10 mmHg within three minutes of standing. The sudden dizzy feeling or brief blackout you get when you stand up too fast is the hallmark symptom.
Here’s what happens physically: when you stand, gravity pulls roughly 500 to 1,000 mL of blood into your legs and abdomen. Normally, your nervous system detects this shift instantly and compensates by tightening blood vessels and speeding up your heart rate. Orthostatic hypotension occurs when that compensation fails, either because there isn’t enough blood volume to work with or because the nervous system response is too sluggish.
This type of low blood pressure becomes more common with age and is especially frequent in people with diabetes, Parkinson’s disease, or other conditions that damage the nerves controlling blood vessel tone. Prolonged bed rest can also trigger it because your body loses its ability to adjust quickly to position changes.
Postprandial Hypotension: Drops After Eating
Some people experience a noticeable blood pressure drop after meals, particularly large ones. After you eat, your body redirects a significant amount of blood to your digestive system to help absorb nutrients. Normally, your heart compensates by pumping harder and tightening blood vessels elsewhere. When that compensation doesn’t happen effectively, blood pressure falls. This is most common in older adults and people who already have nervous system dysfunction affecting blood pressure regulation.
Hormonal and Endocrine Causes
Your adrenal glands, which sit on top of your kidneys, produce hormones that are essential for blood pressure regulation. Cortisol helps control blood pressure directly. Aldosterone maintains the balance of sodium and potassium in your blood, which in turn controls salt and water balance and keeps blood pressure stable. When the adrenal glands don’t produce enough of these hormones, a condition called adrenal insufficiency (or Addison’s disease in its primary form), blood pressure can drop significantly.
During times of physical stress like illness or injury, the lack of cortisol becomes especially dangerous. It can cause life-threatening low blood pressure along with dangerously low blood sugar and low sodium levels. Thyroid disorders can also contribute to hypotension. An underactive thyroid slows metabolism and heart rate, while an overactive thyroid can cause blood vessels to dilate excessively.
Pregnancy
Blood pressure normally drops during the first half of pregnancy. Your body produces extra blood to support the growing fetus, and your circulatory system expands to accommodate it. Blood vessels dilate, and this expansion tends to outpace the increase in blood volume early on, leading to lower pressure. For most women, this is a temporary and harmless dip that resolves in the third trimester. Feeling lightheaded or needing to sit down occasionally during the first and second trimesters is common and typically not a sign of a problem.
Nutritional Deficiencies
A shortage of vitamin B12 or folate can lead to a specific type of anemia where your body produces red blood cells that are too large and don’t function properly. These oversized cells are less able to carry oxygen throughout the body. When your tissues aren’t getting adequate oxygen, your cardiovascular system struggles to maintain normal pressure. Iron deficiency anemia works through a similar pathway: fewer functional red blood cells means less oxygen delivery, which can contribute to lower blood pressure, fatigue, and dizziness.
Severe Infections and Allergic Reactions
Some causes of low blood pressure are medical emergencies. Sepsis, a dangerous complication of bacterial infection, causes blood vessels to relax and dilate dramatically. This extreme dilation drops blood pressure so far that the heart, brain, and kidneys can’t get enough blood. Anaphylaxis, a severe allergic reaction to foods, medications, or insect stings, triggers the same type of widespread vessel dilation.
Both conditions fall under a category called distributive shock, where blood is still in the body but the vessels have opened so wide that pressure collapses. Other triggers include severe burns, spinal cord injuries, pancreatitis, and toxic shock syndrome. These situations require immediate emergency treatment because organs begin to fail rapidly without adequate blood flow.
Nervous System Disorders
Conditions that damage the autonomic nervous system, the part of your nervous system that controls blood pressure, heart rate, and digestion without conscious effort, are a significant cause of chronic low blood pressure. Parkinson’s disease, multiple system atrophy, and diabetic neuropathy all fall into this category. The damage prevents your body from making the rapid adjustments to heart rate and blood vessel tone that keep pressure stable throughout the day. People with these conditions often experience the most pronounced drops when standing, after meals, or during any physical stress.
Long-standing, poorly controlled diabetes deserves special mention because it damages small nerve fibers throughout the body over years. By the time blood pressure regulation is noticeably affected, the nerve damage is usually well established.