Low blood pressure, clinically called hypotension, is generally defined as a reading below 90/60 mmHg. It has dozens of possible causes, ranging from something as simple as not drinking enough water to serious conditions like severe infection or heart failure. For many people, naturally low blood pressure causes no symptoms and needs no treatment. But when it drops suddenly or produces dizziness, fainting, or fatigue, understanding the cause matters.
Dehydration and Blood Volume Loss
Your blood pressure depends heavily on how much fluid is circulating through your vessels. When you lose fluid through sweating, vomiting, diarrhea, or simply not drinking enough, your blood volume drops. The body can compensate for losses up to about 10% of total blood volume without any noticeable change in blood pressure. Beyond that threshold, your heart can’t push enough blood forward with each beat, and pressure starts to fall. Losing more than 20% of blood volume causes a significant drop in arterial pressure.
Your body does fight back. When it senses falling volume, it releases stress hormones that tighten blood vessels and speed up your heart rate. This is why you might feel your heart pounding when you’re dehydrated. That compensation can mask the problem for a while, keeping your blood pressure looking normal on a reading even as your fluid levels continue to decline. Eventually, though, the system can’t keep up.
Blood loss from injury, surgery, or internal bleeding works through the same mechanism. Any significant reduction in circulating volume, whether from fluid loss or actual blood loss, reduces the amount of blood returning to the heart and lowers pressure.
Medications That Lower Blood Pressure
Medications are one of the most common causes of low blood pressure, especially in older adults taking multiple prescriptions. The obvious culprits are drugs designed to lower blood pressure: diuretics (water pills), beta-blockers, and calcium channel blockers. But many medications that aren’t prescribed for blood pressure can cause it to drop as a side effect.
- Antidepressants: Older tricyclic antidepressants are well-known causes, but newer types like SSRIs and SNRIs can also impair your body’s ability to maintain pressure when you stand up.
- Antipsychotics: Some carry a high risk of blood pressure drops, particularly when standing.
- Prostate medications: Alpha-blockers prescribed for enlarged prostate reduce vascular resistance throughout the body, not just in the prostate.
- Parkinson’s medications: Levodopa causes blood vessel dilation that can lead to low readings.
- Opioid painkillers: Morphine and related drugs can lower blood pressure.
- Sedatives: Benzodiazepines may reduce blood pressure by dampening the nervous system’s ability to keep vessels tightened.
If you started a new medication and began feeling lightheaded or dizzy, the timing is worth mentioning to your prescriber. Drug-related drops in blood pressure are often most noticeable when you stand up quickly.
Orthostatic Hypotension: The Standing Problem
When you go from sitting or lying down to standing, gravity pulls about 500 to 700 milliliters of blood into your legs and abdomen. A healthy body compensates within seconds: sensors in your neck arteries and aorta detect the pressure change and signal your nervous system to tighten blood vessels and increase heart rate. This all happens automatically, without you thinking about it.
Orthostatic hypotension occurs when this reflex fails or responds too slowly. Your blood pressure drops within a few minutes of standing, and you feel dizzy, lightheaded, or faint. It’s especially common in older adults, people with diabetes, and those on the medications listed above. Prolonged bed rest can also weaken this reflex because the body becomes deconditioned to the challenge of gravity.
Blood Pressure Drops After Eating
Digesting a meal requires a large increase in blood flow to your stomach and intestines. This diverts blood away from the rest of the body, and in a healthy person, the nervous system compensates by tightening blood vessels elsewhere and slightly increasing heart rate. In some people, particularly older adults, this compensation fails. Blood pressure drops within one to two hours of eating, causing dizziness, lightheadedness, or even fainting.
Larger, carbohydrate-heavy meals tend to produce the biggest drops. Eating smaller, more frequent meals and avoiding large amounts of refined carbohydrates in one sitting can reduce the effect.
Heart Conditions
Your heart is the pump that generates blood pressure. Any condition that weakens the pump or disrupts its rhythm can lower the pressure it produces. Heart failure means the heart muscle can’t contract forcefully enough to maintain normal output. Extremely slow heart rates (bradycardia) reduce the volume of blood pushed forward per minute. Heart valve problems can allow blood to leak backward instead of moving efficiently through the circulatory system.
Heart-related low blood pressure tends to develop gradually and comes with other symptoms: shortness of breath, fatigue, swelling in the legs, or an irregular heartbeat. A sudden, severe drop in blood pressure from a heart attack or dangerous arrhythmia is a medical emergency.
Hormonal and Endocrine Causes
Your adrenal glands, which sit on top of your kidneys, produce hormones that are critical for blood pressure regulation. One of these, aldosterone, controls the balance of sodium and potassium in your blood and, through that, helps regulate fluid volume and vessel tone. Another, cortisol, supports blood pressure during physical stress.
In adrenal insufficiency (Addison’s disease), production of both hormones drops. Without enough aldosterone, you lose sodium through your urine, which pulls water with it and shrinks blood volume. Without enough cortisol, blood pressure can fall dangerously low during illness, injury, or surgery. Thyroid disorders can also contribute: an underactive thyroid slows heart rate, while severe thyroid hormone deficiency can lower blood pressure.
Nutritional Deficiencies
Vitamin B12 and folate deficiency can cause a type of anemia where the body produces fewer, abnormally large red blood cells. Beyond the fatigue and weakness that come with any anemia, this specific deficiency appears to impair the nervous system’s ability to tighten blood vessels when you stand. Research has shown that people with B12-related anemia release less norepinephrine (the chemical that constricts blood vessels) when moving to an upright position, leading to drops in blood pressure. Correcting the deficiency with B12 and folate supplementation has been shown to steadily restore normal blood pressure.
Iron deficiency anemia can have a similar effect. When you don’t have enough red blood cells to carry oxygen efficiently, the heart has to work harder but with less volume to work with, and blood pressure may fall.
Vasovagal Syncope: Fainting Spells
Vasovagal syncope is the most common cause of fainting in people without heart disease. It happens when the nervous system overreacts to a trigger, sending a signal that widens blood vessels and slows the heart at exactly the wrong moment. Common triggers include standing for long periods, heat exposure, the sight of blood, extreme emotional distress, or straining during a bowel movement.
The mechanism is more complex than a simple “nerve misfire.” In about two-thirds of people who experience presyncope, the drop comes from a combination of blood vessel relaxation and falling heart output. In the other third, the dominant problem is a sharp drop in heart rate. In both cases, blood pools in the lower body, less returns to the heart, and pressure falls. Warning signs typically include feeling warm, nauseous, sweaty, or tunnel-visioned. Lying down and elevating the legs can often abort an episode before a full faint occurs.
Severe Infections and Allergic Reactions
Sepsis occurs when the body’s response to an infection spirals out of control, triggering widespread inflammation that causes blood vessels throughout the body to dilate. This sudden expansion of the vascular space means there isn’t enough blood volume to fill it, and pressure plummets. Septic shock is a life-threatening emergency that requires immediate treatment.
Anaphylaxis, a severe allergic reaction to foods, insect stings, or medications, works through a similar mechanism. The immune system floods the body with chemicals that cause blood vessels to widen dramatically while airways constrict. Blood pressure can drop to dangerous levels within minutes. Both conditions cause a type of shock called distributive shock, where the problem isn’t a lack of blood but a sudden, massive expansion of the space it needs to fill.
Pregnancy
Blood pressure commonly drops during the first 24 weeks of pregnancy. The circulatory system expands rapidly to supply the growing placenta, and blood volume hasn’t yet caught up with the larger network of vessels. This is normal and typically resolves on its own as pregnancy progresses into the third trimester, when blood volume peaks. Occasional dizziness when standing quickly is common during this period, but persistent symptoms or very low readings are worth monitoring.