Low blood pressure, generally defined as a reading below 90/60 mmHg, happens when your heart, blood vessels, or nervous system can’t maintain enough force to push blood where it needs to go. Some people run naturally low without any problems. But when blood pressure drops enough to reduce oxygen flow to the brain, it causes dizziness, lightheadedness, blurred vision, fatigue, and sometimes fainting. The causes range from everyday triggers like dehydration to serious medical conditions.
Standing Up Too Fast
One of the most common causes of a sudden blood pressure drop is simply changing position. When you stand up from sitting or lying down, gravity pulls blood into your legs and abdomen. Normally, pressure-sensing cells near your heart and neck arteries detect this shift within seconds. They signal your brain to speed up your heart rate, pump harder, and tighten blood vessels to compensate.
Orthostatic hypotension occurs when that compensation system fails or responds too slowly. The result is a brief but sometimes dramatic drop in pressure that makes you feel dizzy or faint. Older adults are especially prone to this because the pressure-sensing cells become less responsive with age. Dehydration, prolonged bed rest, and several medications can also impair the reflex.
Medications That Lower Blood Pressure
Prescription drugs are one of the most frequent culprits behind low blood pressure, sometimes intentionally (as with blood pressure medications) and sometimes as an unwanted side effect. The list is long:
- Diuretics (water pills): Both thiazide and loop diuretics reduce fluid volume in your bloodstream. Loop diuretics can cause dehydration significant enough to trigger hypotension. Thiazides also deplete potassium, adding muscle weakness to the problem.
- Blood pressure medications: Alpha blockers commonly cause severe drops when standing. Beta blockers can slow the heart rate and lower pressure. Calcium channel blockers and ACE inhibitors or ARBs may also push pressure too low, particularly when doses are adjusted.
- Antidepressants: Older tricyclic antidepressants and SNRIs commonly cause blood pressure drops on standing. SSRIs do so rarely, but it happens.
- Antipsychotics: These medications block receptors that help blood vessels constrict, which can lower pressure.
- Parkinson’s disease drugs: Dopamine agonists and certain enzyme inhibitors used for Parkinson’s frequently cause orthostatic hypotension.
- Heart medications: Nitrates and other antianginal drugs are a common cause of sudden blood pressure drops and fainting.
If you’re on any of these and feeling dizzy or lightheaded, the medication is a likely suspect. Dosage adjustments or switching to a different drug often resolves it.
Dehydration and Blood Loss
Your blood pressure depends partly on having enough fluid in your bloodstream. When you lose fluid through vomiting, diarrhea, heavy sweating, or simply not drinking enough water, your blood volume drops and pressure follows. Even mild dehydration can be enough to cause dizziness when standing.
Severe blood loss from an injury, surgery, or internal bleeding causes a more dramatic version of the same problem. The body doesn’t have enough circulating blood to maintain pressure, and organs begin to receive insufficient oxygen. This is hypovolemic shock, and it’s a medical emergency.
Heart Conditions
Your heart is the pump that generates blood pressure, so any condition that weakens its output can lower your readings. Bradycardia, an abnormally slow heart rate, means the heart doesn’t pump blood frequently enough to maintain adequate pressure. This often happens when the heart’s electrical signaling system malfunctions. Electrical signals can slow down or get blocked between the upper and lower chambers of the heart, a condition called heart block.
Heart valve problems also play a role. If a valve doesn’t open fully or leaks backward, less blood moves forward with each beat. Heart failure, heart attacks, and weakened heart muscle all reduce the volume of blood the heart pushes out per contraction, directly lowering blood pressure.
Hormonal and Endocrine Problems
Several glands in your body produce hormones that directly regulate blood pressure. When those glands malfunction, pressure can drop significantly.
Adrenal insufficiency (Addison’s disease) is a classic example. Your adrenal glands produce cortisol, which helps control blood pressure, and aldosterone, which manages the balance of sodium and potassium in your blood. Sodium and potassium work together to control salt and water balance, keeping blood pressure stable. When the adrenal glands fail, the severe lack of cortisol can cause life-threatening low blood pressure, low blood sugar, and dangerous shifts in sodium and potassium levels.
Thyroid disorders, particularly an underactive thyroid, can also contribute to low readings. Low blood sugar from any cause, whether related to diabetes treatment or skipping meals, may temporarily lower blood pressure as well.
Nervous System Triggers
Your nervous system can actively cause blood pressure to plummet in response to certain triggers. This is called neurally mediated hypotension, and the most familiar version is a vasovagal episode: that feeling of getting lightheaded or fainting during a blood draw, in a hot crowded room, or after seeing something disturbing.
Common triggers include prolonged standing, emotional stress (especially in warm or crowded environments), severe pain, and episodes of nausea or vomiting. What happens is a miscommunication between the heart and brain. Instead of tightening blood vessels and speeding up the heart, the nervous system does the opposite, dilating vessels and slowing the heart. The result is a rapid blood pressure drop.
Blood Pressure Drops After Eating
Postprandial hypotension is a drop in blood pressure that occurs within one to two hours after a meal, particularly a large one. Digestion requires extra blood flow to the stomach and intestines. Normally, your heart rate increases and blood vessels elsewhere in the body constrict to compensate. When that response is sluggish or incomplete, your overall blood pressure falls.
This tends to affect older adults more than younger people, and it’s more common in those who already have high blood pressure or conditions affecting the autonomic nervous system like Parkinson’s disease. Eating smaller, more frequent meals and limiting high-carbohydrate foods can reduce the severity.
Nutritional Deficiencies
Your body needs vitamin B12 and folate to produce properly functioning red blood cells. Without enough of these nutrients, the body makes red blood cells that are too large and poor at carrying oxygen. The resulting anemia, with fewer effective oxygen-carrying cells in circulation, can contribute to low blood pressure along with fatigue, shortness of breath, and dizziness. Folate deficiency can produce symptoms within weeks. Iron deficiency anemia works through a similar mechanism.
Pregnancy
Blood pressure commonly drops during pregnancy, particularly in the second trimester. A large study tracking blood pressure patterns found that the majority of pregnant women maintain consistently low blood pressure throughout pregnancy, while others experience a noticeable dip in mid-pregnancy before pressure climbs again in the third trimester. This happens because the circulatory system expands rapidly to supply blood to the placenta, and blood vessels relax under the influence of pregnancy hormones. The drop is usually harmless but can worsen dizziness, especially when standing quickly.
Severe Infections and Allergic Reactions
Two emergency conditions can cause blood pressure to collapse rapidly. In sepsis, a severe infection triggers widespread inflammation that forces blood vessels to dilate dramatically. The vessels become so relaxed and leaky that blood pressure plummets, and organs can’t get the blood flow they need. Anaphylaxis, a severe allergic reaction, works through a similar mechanism. The body releases a flood of chemicals that cause blood vessels to widen and capillaries to leak fluid, dropping pressure within minutes. Both conditions require immediate emergency treatment.