Low blood pressure, or hypotension, is generally defined as a reading below 90/60 mmHg. It has dozens of possible causes, ranging from everyday triggers like dehydration and standing up too fast to serious conditions like severe infection or heart disease. For many people, naturally low blood pressure produces no symptoms and needs no treatment. But when it causes dizziness, fainting, or fatigue, identifying the underlying cause is the key to fixing it.
How Blood Pressure Is Maintained
Your blood pressure depends on three things working together: how forcefully your heart pumps, how much blood is circulating in your vessels, and how tightly your blood vessels squeeze. A problem with any one of these can drop your pressure. Your body constantly fine-tunes all three using pressure-sensing cells called baroreceptors near your heart and neck arteries. These cells detect changes in pressure and send signals to your brain, which responds by adjusting your heart rate and vessel tightness within seconds. When something disrupts this feedback loop, blood pressure falls.
Medications Are the Most Common Culprit
Drugs prescribed for other conditions are one of the leading causes of low blood pressure, especially in older adults. The main offenders fall into a few categories.
Diuretics (water pills) increase sodium loss through urine, which reduces your blood volume. Loop diuretics also widen veins, meaning less blood returns to the heart with each beat. Blood pressure medications like alpha blockers, beta blockers, calcium channel blockers, and ACE inhibitors can all lower pressure beyond the intended target, particularly when combined.
Antidepressants are a surprisingly common cause. Older tricyclic antidepressants cause blood pressure drops in 10 to 50 percent of patients, largely because they block receptors that help keep blood vessels constricted. Newer antidepressants like SSRIs carry roughly double the risk of drops upon standing compared to non-users. SNRIs have been linked to a fivefold increase in that risk among older adults prone to falls. Even trazodone, often prescribed as a sleep aid, causes pressure drops in up to 7 percent of patients.
Medications for Parkinson’s disease, antipsychotics, muscle relaxants, erectile dysfunction drugs, and opioid painkillers round out the list. If you started a new medication and began feeling lightheaded, the timing is worth mentioning to your prescriber.
Dehydration and Low Blood Volume
Your circulatory system needs a minimum volume of fluid to maintain pressure. Dehydration from vomiting, diarrhea, heavy sweating, or simply not drinking enough water shrinks that volume. Blood loss from an injury, surgery, or internal bleeding does the same thing more dramatically. Even mild dehydration on a hot day can cause enough of a volume drop to make you dizzy when you stand.
Positional Drops: Standing Up and Eating
Orthostatic hypotension is the technical name for a blood pressure drop that happens when you stand up from sitting or lying down. Gravity pulls blood into your legs and abdomen, temporarily reducing the amount flowing back to your heart. Normally, your baroreceptors catch this within a second or two and speed up your heart rate to compensate. When that reflex is too slow or too weak, you feel dizzy or see spots.
This is especially common in people over 65. The baroreceptor cells slow down with age, and an aging heart takes longer to ramp up its rate. Combine that with one or two blood pressure medications, and the drop upon standing can be significant.
Postprandial hypotension is a related phenomenon where blood pressure falls after eating a meal. Digestion redirects blood flow to your gut, and in some people, particularly older adults, the body doesn’t compensate fast enough. Large, carbohydrate-heavy meals tend to make it worse.
Heart Conditions That Reduce Output
Your heart is the pump that generates blood pressure, so anything that weakens the pump lowers the pressure. A very slow heart rate (bradycardia) means fewer beats per minute pushing blood forward. The electrical system that controls heart rhythm can deteriorate with normal aging, and coronary artery disease accelerates that process by reducing blood supply to the heart’s pacemaker cells.
Heart valve problems, heart failure, and heart attacks all reduce the volume of blood the heart ejects with each beat. When the heart can’t keep up with the body’s demand, blood pressure drops, often along with fatigue and shortness of breath.
Nervous System Misfires
Sometimes the brain and heart miscommunicate in a way that actively tanks blood pressure. This is called neurally mediated hypotension, and it’s the mechanism behind common fainting spells.
Here’s what happens: when blood volume in the heart drops (say, from standing in a hot room for too long), the heart contracts harder to compensate. Pressure-sensing receptors inside the heart interpret that forceful squeezing of an under-filled chamber as a false signal that pressure is too high. They tell the brain to hit the brakes. The brain responds by slowing the heart and widening blood vessels, which is exactly the opposite of what’s needed. Blood pressure plummets, and you faint.
Triggers vary widely: prolonged standing, emotional stress, coughing, swallowing, urination, and even physical exercise can set it off in susceptible people. Nervous system diseases like Parkinson’s disease, Lewy body dementia, and a condition called multiple system atrophy can permanently impair the body’s ability to regulate blood pressure, causing chronic drops.
Hormonal and Endocrine Causes
Your adrenal glands produce hormones that help regulate blood vessel tightness, salt balance, and fluid retention. When those glands fail, a condition called adrenal insufficiency, blood pressure can drop chronically. This can stem from a problem in the adrenal glands themselves or from the pituitary gland in the brain, which sends the signal telling the adrenals to work. Either way, the result is too little of the hormones needed to keep vessels constricted and blood volume adequate.
Thyroid disorders and low blood sugar (hypoglycemia) can also contribute to drops in blood pressure, though they usually come with other recognizable symptoms like fatigue, weight changes, or shakiness.
Nutritional Deficiencies
Vitamin B12 deficiency can cause low blood pressure through a mechanism most people wouldn’t expect. Beyond its well-known role in making red blood cells, B12 is essential for nerve function. When levels run low, the nerves responsible for tightening blood vessels and adjusting heart rate stop working properly. This impairs the body’s reflex response to changes in posture, leading to drops when standing. Notably, this can happen even before anemia or other neurological symptoms appear, making it easy to overlook.
Folate deficiency works through a similar pathway. When you lack the nutrients to produce enough red blood cells, the reduced oxygen-carrying capacity of your blood forces the heart to work harder, and pressure regulation suffers.
Pregnancy
Blood pressure naturally falls during the first half of pregnancy. The body rapidly expands its blood volume to supply the developing placenta and fetus, but the blood vessels widen faster than the volume can keep up. Pressure typically reaches its lowest point around 22 to 24 weeks, then gradually climbs back toward pre-pregnancy levels by 36 weeks. This dip is normal and expected, though it can cause dizziness or lightheadedness, particularly when standing quickly.
Severe Infections and Allergic Reactions
Two emergency situations can cause blood pressure to crash rapidly. In sepsis, an overwhelming infection triggers a system-wide inflammatory response. The body releases signaling molecules that force blood vessels wide open and cause fluid to leak out of capillaries into surrounding tissue. The combination of dilated vessels and lost fluid volume can drop blood pressure to dangerous levels. The inflammatory molecules can also weaken the heart muscle itself, compounding the problem.
Anaphylaxis, a severe allergic reaction, works through a different pathway but with a similar result. Exposure to a trigger (foods, insect stings, medications) causes immune cells to dump histamine into the bloodstream. Histamine widens blood vessels throughout the body and makes capillaries leak, causing a sudden, steep drop in pressure. Both conditions are medical emergencies where the blood pressure drop is a symptom of a larger crisis, not a standalone problem.
Who Is Most at Risk
Certain groups are more likely to experience problematic low blood pressure. Adults over 65 face higher risk because of age-related slowing of the baroreceptor reflex and the likelihood of taking multiple medications. People with Parkinson’s disease or other neurological conditions have impaired autonomic regulation. Anyone on combinations of blood pressure drugs, antidepressants, or diuretics faces compounding effects. And people with diabetes may develop nerve damage over time that blunts the body’s pressure-regulating reflexes.
If your blood pressure runs low but you feel fine, it’s generally not a concern. The causes worth investigating are the ones that produce symptoms: persistent dizziness, fainting, blurred vision, nausea, or unusual fatigue. Those symptoms point to a body that’s struggling to deliver enough blood where it needs to go.