Low blood pressure, generally defined as a reading below 90/60 mmHg, has a wide range of causes, from everyday factors like dehydration to serious conditions like severe infection. For many people, naturally low blood pressure produces no symptoms and isn’t a problem. It only becomes a concern when it causes dizziness, fainting, or fatigue, or when it drops suddenly. A change of just 20 mmHg in systolic pressure (the top number) can be enough to make you feel lightheaded or pass out.
Dehydration and Fluid Loss
One of the most common and straightforward causes of low blood pressure is not having enough fluid in your body. Your blood is mostly water, so when you lose fluid through sweating, vomiting, diarrhea, or simply not drinking enough, your blood volume drops. With less blood filling your blood vessels, pressure falls. Think of it like a garden hose that’s only half full: there’s not enough inside to maintain strong flow.
This is why low blood pressure often shows up during hot weather, after intense exercise, or during a stomach illness. Severe bleeding from an injury or surgery causes the same effect, just more dramatically and dangerously.
Medications
Prescription drugs are one of the most frequent culprits behind low blood pressure, especially in older adults taking multiple medications. The categories most likely to cause a drop include:
- Blood pressure medications: These are designed to lower pressure, so overshooting the target is a known risk. This includes diuretics (water pills), calcium channel blockers, and ACE inhibitors or ARBs.
- Diuretics: Both thiazide and loop diuretics push fluid out through urine, which can lead to dehydration and low potassium, both of which lower blood pressure.
- Alpha blockers: Often prescribed for prostate problems, these commonly cause significant drops in blood pressure, especially when standing.
- Antidepressants: Tricyclics and SNRIs frequently lower blood pressure. Older antidepressants called MAOIs can cause severe drops, though they’re rarely prescribed today.
- Heart medications: Nitroglycerin and similar drugs used for chest pain are a common cause of sudden blood pressure drops and fainting.
- Parkinson’s disease drugs: Dopamine-based medications used for Parkinson’s often cause low blood pressure as a side effect.
If you’ve recently started or changed a medication and notice dizziness when you stand up, the drug is a likely explanation.
Standing Up Too Fast
Orthostatic hypotension is the medical term for a blood pressure drop that happens when you move from sitting or lying down to standing. Normally, your body compensates almost instantly. Sensors in your neck and chest called baroreceptors detect the pressure change and signal your heart to beat faster while your blood vessels tighten to push blood upward. When that reflex doesn’t work properly, blood pools in your legs and your brain briefly loses adequate blood flow.
This type of low blood pressure affects 10 to 30% of older adults. Aging itself weakens the baroreceptor reflex, and conditions like diabetes and Parkinson’s disease damage the nerves involved even further. Medications, dehydration, and prolonged bed rest all make it worse. The result is that classic wave of dizziness or near-blackout when you stand, which is also a significant cause of falls in older people.
Drops After Eating
Some people experience a noticeable blood pressure drop within one to two hours after a meal, a condition called postprandial hypotension. Digestion requires a large increase in blood flow to the gut. Normally, your heart speeds up slightly and blood vessels elsewhere in the body constrict to compensate. When that compensation fails, pressure falls.
You’re at higher risk if you’re over 65, have high blood pressure, diabetes, Parkinson’s disease, or heart failure. Large meals and hot weather make episodes more likely, as does not drinking enough water. Eating smaller, more frequent meals and staying hydrated can reduce the severity.
Heart Conditions
Your heart is the pump that generates blood pressure, so any condition that weakens its pumping ability can lower pressure. Heart failure, heart valve problems, and very slow heart rates (bradycardia) all reduce the amount of blood pushed out with each beat. A heart attack can also cause a sudden dangerous drop. These are among the more serious causes and typically come with other symptoms like chest pain, shortness of breath, or extreme fatigue.
Hormonal and Endocrine Problems
Several glands in your body help regulate blood pressure, and when they malfunction, pressure can drop. The most notable examples:
Addison’s disease occurs when the adrenal glands (which sit on top of your kidneys) don’t produce enough of the hormones that help your body retain salt and water. Without those hormones, blood volume drops and pressure falls. An underactive thyroid slows many body processes, including heart rate and the tone of blood vessel walls, which can contribute to lower readings. Low blood sugar, whether from diabetes treatment or other causes, also triggers drops in blood pressure because the body diverts resources in response to the energy crisis.
Pregnancy
Blood pressure typically drops during pregnancy, and the decline starts earlier than most people expect. A measurable decrease begins around 6 to 8 weeks of gestation. Pressure continues falling until it reaches its lowest point during the second trimester, typically dropping 5 to 10 mmHg below pre-pregnancy levels. This happens because pregnancy hormones cause blood vessels to relax and widen, reducing resistance to blood flow.
During the third trimester, blood pressure gradually rises back toward pre-pregnancy levels and usually returns to normal within about two weeks after delivery. The dip is considered a normal part of pregnancy, though it can cause dizziness, especially when standing quickly or in warm environments.
Nutrient Deficiencies
Your body needs vitamin B12 and folate to produce healthy red blood cells. When you’re deficient in either nutrient, you develop a type of anemia where the red blood cells your body makes are too large and don’t carry oxygen efficiently. Fewer functional red blood cells means less oxygen delivery to tissues, and the resulting anemia can lower blood pressure. Adults need about 2.4 micrograms of B12 and 400 micrograms of folate daily. Vegetarians, vegans, and older adults with reduced absorption are most at risk for B12 deficiency.
Severe and Life-Threatening Causes
Some causes of low blood pressure are medical emergencies. In sepsis, a bodywide response to infection, bacterial toxins trigger massive inflammation. White blood cells release chemicals that cause blood vessels to dilate widely, and the walls of small blood vessels become leaky, allowing fluid to seep out of the bloodstream and into surrounding tissue. The combination of extreme vasodilation and fluid loss causes blood pressure to plummet.
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 and leak simultaneously, dropping pressure within minutes. Both conditions involve the same dangerous combination: blood vessels that are too relaxed and not enough fluid inside them to maintain circulation.
Severe internal or external bleeding causes a more direct version of the same problem. With rapid blood loss, there simply isn’t enough volume left to maintain adequate pressure, and organs begin to lose their blood supply.
Why Some People Run Low Naturally
Not every case of low blood pressure has a worrying explanation. Young, physically active people often have resting blood pressure readings below 90/60 with no symptoms whatsoever. This is generally a sign of cardiovascular fitness, not a problem. What counts as “too low” varies from person to person. A reading that would cause fainting in one person may be completely normal for another. The key distinction is whether the low reading comes with symptoms like dizziness, blurred vision, nausea, or unusual fatigue. Without symptoms, a low number on its own is rarely a concern.