Low blood pressure has a wide range of causes, from something as simple as not drinking enough water to serious conditions like heart failure or severe infections. A reading below 90/60 mmHg is generally considered low, though some people naturally run lower without any symptoms or problems. When low blood pressure does cause dizziness, fainting, or fatigue, identifying the underlying cause is the key to fixing it.
Dehydration and Fluid Loss
One of the most common and straightforward causes is dehydration. Your body holds roughly 5 quarts of blood at any given time, and that volume depends on staying well-hydrated. When you lose fluid through sweating, vomiting, diarrhea, or simply not drinking enough, your blood volume drops. Less blood in the system means less pressure pushing against artery walls. As one cardiologist at the Cleveland Clinic put it, “you’re just not filling up the pipes enough for what your vascular system needs.”
Heavy bleeding, whether from injury, surgery, or internal sources, creates the same problem on a more dangerous scale. Losing enough blood volume can trigger hypovolemic shock, where organs stop receiving adequate oxygen.
Heart Conditions
Your blood pressure depends on two things: how much blood your heart pumps and how much resistance your arteries provide. Any heart condition that weakens the pump side of that equation can lower blood pressure. Heart failure, heart valve disease, a heart attack, and an abnormally slow heart rate (bradycardia) all reduce the amount of blood leaving the heart with each beat. If the heart can’t push enough blood into the arteries, pressure falls.
Medications That Lower Blood Pressure
Several classes of medication can cause low blood pressure, sometimes intentionally and sometimes as an unwanted side effect. The most common culprits include:
- Diuretics (both thiazide and loop types), which lower blood volume by increasing urine output
- Alpha blockers, often prescribed for prostate problems or high blood pressure, which can cause severe drops in pressure
- Beta blockers, which slow the heart rate and can trigger low blood pressure
- Nitrate medications for chest pain, which relax blood vessels and can cause sudden pressure drops
- Calcium channel blockers, another class of blood pressure and heart medication
- Certain antidepressants, including tricyclics and SNRIs, which interfere with how blood vessels constrict
If you started a new medication and noticed dizziness or lightheadedness, the timing is worth mentioning to your prescriber. Blood pressure drops from medications are especially common in older adults taking multiple prescriptions.
Hormonal and Endocrine Problems
Your adrenal glands, which sit on top of your kidneys, produce hormones that directly regulate blood pressure. One of these, aldosterone, balances sodium and potassium levels to keep pressure in a healthy range. In Addison’s disease, the adrenal glands don’t produce enough cortisol or aldosterone, and blood pressure can drop significantly. People with Addison’s disease often experience dizziness or fainting when they stand up, and a sudden worsening called an adrenal crisis can cause dangerously low blood pressure along with low blood sugar.
Thyroid disorders and other hormonal imbalances can also contribute, though the adrenal connection is the most direct.
Drops When Standing or After Eating
Orthostatic hypotension is a specific type of low blood pressure 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, pressure sensors near your heart and neck arteries detect this change within seconds and signal your heart to beat faster, compensating almost instantly. Orthostatic hypotension happens when that correction system doesn’t work properly.
This is especially common in people over 65. The pressure sensors slow down with age, and the heart may not speed up quickly enough to compensate. Dehydration, medications, and prolonged bed rest all make it worse.
A related pattern, called postprandial hypotension, causes blood pressure to drop after meals. Digestion draws a large share of blood flow to the gut, and in some people, particularly older adults, the body doesn’t compensate well enough to maintain normal pressure elsewhere.
Severe Allergic Reactions and Infections
Two life-threatening conditions can cause blood pressure to plummet rapidly. In anaphylaxis (a severe allergic reaction), blood vessels dilate dramatically and become leaky, causing blood pressure to crash within minutes. In sepsis, a body-wide response to infection, the same kind of massive blood vessel relaxation occurs. In both cases, the vessels open so wide that even a normal amount of blood can’t maintain adequate pressure, and organs begin losing their blood supply. These are medical emergencies, not causes of chronic low blood pressure.
Nutritional Deficiencies
Vitamin B12 and folate deficiencies can contribute to low blood pressure through a few pathways. Both vitamins are essential for producing healthy red blood cells. When levels fall too low, anemia develops, meaning your blood carries less oxygen per unit of volume. Research published in the European Heart Journal found that people with recurrent fainting episodes had significantly lower B12 levels compared to those who fainted less often, suggesting a connection between B12 status and the body’s ability to regulate blood pressure during sudden changes.
Iron deficiency anemia works through the same basic mechanism: fewer functional red blood cells means less oxygen delivery, and the cardiovascular system may struggle to maintain normal pressure.
Pregnancy
Blood pressure commonly drops during the first 24 weeks of pregnancy. The circulatory system expands rapidly to supply the growing placenta, and blood volume hasn’t yet caught up. This usually causes a modest drop in pressure that resolves on its own in the third trimester as blood volume increases. Mild dizziness when standing is common during this phase and is rarely a sign of anything serious.
When Low Blood Pressure Is Just Normal
Some people consistently run below 90/60 and feel perfectly fine. This is common in young, fit adults and people who exercise regularly. A well-conditioned heart pumps blood more efficiently, so it doesn’t need to generate as much pressure to deliver oxygen throughout the body. In this case, low blood pressure isn’t a problem to solve. It only becomes a medical concern when it causes symptoms like persistent dizziness, blurred vision, nausea, fatigue, or fainting.