Low blood pressure, or hypotension, is generally defined as a reading below 90/60 mmHg. While some people naturally run low without any problems, a sudden or persistent drop can signal an underlying issue worth understanding. The causes range from something as simple as not drinking enough water to more complex problems involving the heart, hormones, or nervous system.
Dehydration and Blood Loss
The most common and straightforward cause of low blood pressure is not having enough fluid in your system. When you’re dehydrated from illness, exercise, heat, or simply not drinking enough, your blood volume drops. Think of it as not filling the pipes enough for what your vascular system needs. Less fluid means less pressure pushing blood through your arteries, which means your organs may not get the oxygen they need to function properly.
Blood loss works the same way but more urgently. Heavy menstrual periods, internal bleeding from an ulcer or injury, or surgical blood loss all reduce the volume circulating through your body. Even moderate blood loss can cause noticeable drops in pressure, producing dizziness and fatigue before more serious symptoms develop.
Heart Problems That Reduce Output
Your blood pressure depends on how forcefully and frequently your heart pumps. Several cardiac conditions can reduce that output. An abnormally slow heart rate (below about 60 beats per minute in some people) means less blood gets pushed out per minute. Heart valve problems can prevent valves from opening or closing properly, reducing the amount of blood that moves forward with each beat. Heart failure, where the heart muscle weakens over time, directly lowers the force behind each contraction.
A heart attack can also cause a sudden, dangerous drop in blood pressure because damaged heart muscle can no longer pump effectively. If low blood pressure comes on suddenly with chest pain, shortness of breath, or an irregular heartbeat, that combination points to a cardiac cause.
Medications
Several categories of medication lower blood pressure as either their intended effect or a side effect. Blood pressure drugs themselves are the most obvious culprit. Diuretics (water pills) reduce fluid volume. Other blood pressure medications relax blood vessels or slow the heart rate, and if the dose is too high or interacts with another drug, the result can overshoot into hypotension.
Beyond blood pressure medications, certain antidepressants (particularly older tricyclic types), drugs for Parkinson’s disease, and erectile dysfunction medications can all lower blood pressure. This is especially common in older adults, where even a standard dose can cause pressure to drop too far when standing up. If you notice new dizziness or lightheadedness after starting or changing a medication, the timing is usually the clue.
Orthostatic Hypotension
Orthostatic hypotension is a specific pattern: your blood pressure drops when you stand up from sitting or lying down. The CDC defines it as a drop of 20 mmHg or more in the top number, or 10 mmHg or more in the bottom number, within a few minutes of standing. It affects an estimated 7% to 10% of adults with high blood pressure, and it’s especially common in older adults.
Normally, when you stand, your body quickly tightens blood vessels and slightly increases your heart rate to keep blood flowing to your brain. When that reflex is sluggish, whether from aging, medications, dehydration, or nerve damage, blood pools in your legs and pressure drops. The result is that lightheaded, “room is spinning” feeling, or in worse cases, fainting.
Blood Pressure Drops After Eating
Digestion requires a large blood supply. After a meal, your body reroutes extra blood to your stomach and small intestine. To compensate, the heart beats faster while blood vessels elsewhere in the body narrow, maintaining pressure to the brain and extremities. In some people, particularly older adults, this compensating mechanism doesn’t work well enough, and blood pressure falls.
Harvard Health identifies a few reasons this happens. High blood pressure itself, somewhat ironically, is a major culprit because it stiffens arteries over time, making them less able to narrow and relax on demand. Nerve-damaging conditions like diabetes and Parkinson’s disease can also impair the pressure sensors in arteries and stretch receptors in the stomach that normally coordinate the body’s response to eating. Postprandial hypotension (as this is called) tends to be worst after large, carbohydrate-heavy meals.
Hormonal and Endocrine Causes
Several glands in your body help regulate blood pressure, and when they malfunction, pressure can drop. An underactive thyroid slows your metabolism and heart rate, both of which can lower blood pressure. Addison’s disease, where the adrenal glands don’t produce enough of certain hormones, causes a particularly significant drop because those hormones help control the balance of salt and water in your blood and how your blood vessels respond to stress.
Low blood sugar can also trigger a temporary drop in blood pressure. Diabetes makes the list too, both because blood sugar swings affect pressure and because long-term diabetes damages the nerves that help regulate it. If low blood pressure is accompanied by unexplained weight changes, fatigue, or changes in skin pigmentation, an endocrine problem may be the underlying cause.
Nervous System Misfires
Sometimes the brain and heart simply miscommunicate. In neurally mediated hypotension, the nervous system sends the wrong signal: instead of tightening blood vessels when pressure starts to fall, it relaxes them further and slows the heart rate at the same time. This is the mechanism behind the classic faint triggered by the sight of blood, intense pain, extreme anxiety, or prolonged standing.
Research from the Vanderbilt Autonomic Dysfunction Center shows that about 94% of these fainting episodes are related to posture, occurring while standing or walking, where gravity plays a key role. Emotional triggers like fear or pain account for the remaining cases by activating brain circuits that inappropriately shut down the body’s “fight or flight” response. The result is a sudden drop in blood pressure paired with a slowed heart rate, which together can cause lightheadedness or a full loss of consciousness.
Pregnancy
Low blood pressure is expected during pregnancy, especially in the first and second trimesters. The reason is largely mechanical and hormonal: blood vessels expand significantly to accommodate increased blood flow to the uterus, and hormonal shifts relax blood vessel walls. Your heart works harder to supply both your body and your growing baby, but the expanded vascular system means pressure often drops below your pre-pregnancy baseline.
For most women, blood pressure returns to normal levels on its own during the third trimester. Mild dizziness when standing up quickly is common and usually not concerning, though persistent or severe symptoms are worth mentioning at a prenatal visit.
Nutritional Deficiencies
Your body needs certain nutrients to produce healthy red blood cells. Without enough vitamin B12 or folate, the body produces abnormally large red blood cells that can’t carry oxygen efficiently. This type of anemia reduces the oxygen delivery to tissues throughout your body, which can contribute to low blood pressure along with fatigue, weakness, and pale skin.
Iron deficiency anemia works similarly. Fewer functional red blood cells means less oxygen-carrying capacity, and the heart has to work harder to compensate. Over time, this strain can lower blood pressure, particularly in people who are also dehydrated or taking medications that affect pressure.
Severe Infections and Allergic Reactions
Sepsis, a life-threatening response to infection, causes a dramatic drop in blood pressure. When bacteria enter the bloodstream, the body’s inflammatory response causes blood vessels to widen dramatically, and fluid leaks out of the bloodstream into surrounding tissues. The combination of dilated vessels and reduced blood volume can send pressure dangerously low, starving organs of oxygen.
Anaphylaxis, a severe allergic reaction, triggers a similar cascade. Massive release of chemicals from the immune system causes blood vessels to widen suddenly while the airways narrow. Blood pressure can plummet within minutes. Both sepsis and anaphylaxis are medical emergencies where the drop in blood pressure is a warning sign that the body’s circulation is failing.