Blood pressure below 90/60 mmHg is generally considered low, and it happens when your heart, blood vessels, or the volume of fluid in your system can’t maintain enough force to push blood where it needs to go. For many people, especially those who are physically active, low blood pressure is completely normal and causes no symptoms. It becomes a problem when the drop is large enough to reduce blood flow to your brain and organs, causing dizziness, fainting, or fatigue.
The causes range from everyday and harmless to urgent and life-threatening. Understanding which category yours falls into depends on how low your pressure drops, how quickly it happens, and what other symptoms come with it.
Dehydration and Low Blood Volume
Your blood pressure depends heavily on how much fluid is circulating through your vessels. When you’re dehydrated, whether from not drinking enough, sweating heavily, vomiting, or diarrhea, total blood volume drops. With less fluid pushing against vessel walls, pressure falls. Research shows that losing roughly 5% of your body weight through fluid loss measurably reduces cardiac output, the amount of blood your heart pumps per minute.
Your body has built-in defenses against this. When fluid levels drop, your kidneys release hormones that signal the body to hold onto sodium and water. Your heart rate increases and blood vessels tighten to compensate. But these backup systems have limits. If fluid loss outpaces what the body can retain, blood pressure drops despite the compensation. This is why severe dehydration can quickly become dangerous, particularly in older adults whose compensatory reflexes are already slower.
Blood loss works through the same mechanism. Whether from an injury, surgery, or internal bleeding, losing blood directly reduces the volume your heart has to pump.
Standing Up Too Quickly
Orthostatic hypotension is the medical term for a blood pressure drop that happens when you move from lying down or sitting to standing. It’s defined as a drop of at least 20 mmHg in systolic pressure (the top number) or 10 mmHg in diastolic pressure (the bottom number) within three minutes of standing.
When you stand, gravity pulls roughly 500 to 800 milliliters of blood into the veins below your heart. Normally, sensors called baroreceptors detect this shift instantly and trigger your nervous system to tighten blood vessels and speed up your heart rate, restoring pressure within seconds. When that reflex is sluggish or absent, blood pools in your legs, less returns to your heart, and your brain briefly loses adequate blood flow. The result is that familiar head rush, or in worse cases, blacking out entirely.
Orthostatic hypotension is especially common in older adults, people with diabetes, and those taking blood pressure medications. It also happens after prolonged bed rest, when the body’s reflexes have gone unchallenged and become less responsive.
Medications That Lower Blood Pressure
Medications are one of the most common and overlooked causes of low blood pressure. Some do it intentionally (blood pressure drugs working a little too well), while others cause it as an unintended side effect. The major categories include:
- Diuretics (water pills): These increase urine output, which can deplete fluid volume enough to lower pressure.
- Alpha-blockers and beta-blockers: These relax blood vessels or slow the heart rate, both of which reduce pressure. Mixed blockers that do both carry the highest risk.
- Antidepressants: Older tricyclic antidepressants are well known for this, but newer types like SSRIs can also contribute.
- Antipsychotics and sedatives: These frequently lower blood pressure, particularly in older adults, by relaxing blood vessels.
- Opioid painkillers: Chronic use triggers the release of compounds that widen blood vessels and reduce vascular resistance.
- Nitrates (for chest pain): These dilate veins, which reduces how much blood returns to the heart.
- Parkinson’s medications: Drugs like levodopa activate receptors that widen blood vessels in the skin, gut, and kidneys.
If you’ve recently started a new medication and notice dizziness when standing, fatigue, or lightheadedness, the drug is a likely contributor. The risk increases when multiple medications from this list are combined.
Blood Pressure Drops After Eating
Postprandial hypotension is a blood pressure drop that occurs within one to two hours after a meal. When you eat, your body redirects blood flow toward your digestive system. Normally, your heart speeds up slightly and blood vessels elsewhere tighten to keep overall pressure stable. In some people, this compensation fails: the heart doesn’t speed up enough, and blood vessels don’t constrict as they should.
This is most common in people over 65, partly because aging stiffens arteries and makes them less able to adjust quickly. It’s also more frequent in people with Parkinson’s disease, diabetes, heart failure, and kidney disease. Large, carbohydrate-heavy meals tend to trigger larger drops.
Nerve Signaling Problems
A type called neurally mediated hypotension involves a miscommunication between your heart and brain. During prolonged standing, blood naturally pools in the legs. Your heart, sensing the reduced filling, contracts harder on a partially empty chamber. The brain misreads this strong contraction as a signal that pressure is too high and responds by slowing the heart and widening blood vessels, the exact opposite of what’s needed. The result is a sudden drop in blood pressure, often leading to fainting.
This form tends to affect younger, otherwise healthy people and is a common cause of unexplained fainting episodes. Triggers include standing for long periods, hot environments, and emotional stress. Unlike orthostatic hypotension, which happens within seconds of standing, neurally mediated drops can develop after several minutes or longer on your feet.
Hormonal and Nutritional Causes
Your endocrine system plays a direct role in maintaining blood pressure. Adrenal glands produce hormones that help regulate how tightly your blood vessels constrict and how much salt and water your kidneys retain. When the adrenal glands fail, a condition called Addison’s disease, blood pressure can drop chronically because the body loses its ability to hold onto sodium and maintain blood volume. Low blood sugar can also cause pressure to fall, as the body’s stress response falters without adequate fuel.
Vitamin B12 deficiency offers a less obvious but well-documented pathway. Beyond causing anemia (which itself reduces the blood’s oxygen-carrying capacity), B12 deficiency can damage the nerves that control blood vessel tightening. This creates a form of autonomic neuropathy where the body can’t properly increase vascular resistance when it needs to. Blood pools in the veins, less returns to the heart, and pressure drops. The good news is that this type of nerve dysfunction is often reversible once B12 levels are restored.
Pregnancy
Low blood pressure during pregnancy is extremely common. As the body builds new blood vessels to supply the placenta, peripheral vascular resistance drops, which naturally lowers both systolic and diastolic pressure. This effect is most pronounced in the second trimester but continues into the third.
In later pregnancy, the growing uterus can compress the large vein that returns blood from the lower body to the heart, particularly when lying on your back. This compression reduces how much blood the heart receives and can cause a sudden pressure drop with dizziness or faintness. Lying on your left side relieves the compression and restores normal flow.
Severe and Emergency Causes
In some situations, dangerously low blood pressure signals a medical emergency. Two of the most serious are sepsis and anaphylaxis, both of which cause a form of shock called distributive shock.
In sepsis, an overwhelming infection triggers a flood of inflammatory signals throughout the body. These cause blood vessels to dilate widely and become leaky, allowing fluid to seep out of the bloodstream into surrounding tissues. The combination of widespread vessel dilation and fluid loss can cause blood pressure to plummet. The heart itself can also weaken under the inflammatory burden.
Anaphylaxis works through a similar mechanism but with a different trigger. Exposure to an allergen causes a massive release of histamine from immune cells, which rapidly dilates blood vessels and makes capillaries leak. The result is the same: blood pressure drops suddenly and severely, starving the brain and organs of blood flow. Both conditions require immediate emergency treatment because the body cannot compensate on its own once shock sets in.
When Low Blood Pressure Is Normal
Not all low blood pressure is a problem. Physically active people, particularly endurance athletes, often have resting blood pressures well below 90/60 without any symptoms. Their cardiovascular systems are simply more efficient, pumping more blood per beat and requiring less force to circulate it. Some people are genetically predisposed to lower blood pressure throughout their lives, and this is associated with better long-term cardiovascular outcomes, not worse ones.
The key distinction is symptoms. A reading of 85/55 in someone who feels fine is very different from the same reading in someone who is dizzy, confused, or fainting. Blood pressure becomes clinically concerning when it drops low enough to compromise oxygen delivery to the brain, kidneys, and heart, and the threshold where that happens varies from person to person.