Blood pressure drops when your heart pumps less forcefully, when your blood vessels relax and widen, or when you don’t have enough fluid in your bloodstream. A reading below 90/60 mmHg is generally considered low. For some people, naturally low blood pressure causes no problems at all. But when it drops suddenly or produces symptoms like dizziness, fainting, or fatigue, something specific is usually driving it.
How Blood Pressure Works
Your blood pressure depends on two things: how much blood your heart pushes out with each beat (cardiac output) and how much resistance your blood vessels create as that blood flows through them. If either one drops significantly, your blood pressure falls. Your body has a built-in correction system for this. Specialized cells near your heart and neck arteries detect changes in pressure and send signals to your brain, which tells your heart to beat faster and your blood vessels to tighten. Low blood pressure happens when something disrupts this process or overwhelms it.
Dehydration and Low Blood Volume
The simplest and most common cause of low blood pressure is not having enough fluid in your system. When you’re dehydrated from illness, heat, heavy exercise, or simply not drinking enough water, the total volume of blood circulating through your body decreases. Less blood returning to the heart means less blood pumped out, and pressure drops. Vomiting, diarrhea, heavy sweating, and significant blood loss all reduce blood volume in the same way.
Nutritional Deficiencies
Low levels of vitamin B12 and folate can lead to a type of anemia where your body produces red blood cells that are too large and don’t function properly. These oversized cells carry oxygen less effectively, and the resulting anemia can lower blood pressure. B12 deficiency often happens because the body can’t absorb the vitamin, not just because you aren’t eating enough of it. Conditions like Crohn’s disease and celiac disease interfere with absorption in the intestines. An autoimmune condition called pernicious anemia destroys the stomach cells that produce a substance needed to absorb B12. Gastric surgery can have the same effect by removing the tissue responsible for absorption.
Folate deficiency follows a similar pattern. It can result from a diet low in leafy greens and legumes, from intestinal conditions that block absorption, from heavy alcohol use, or from certain antiseizure medications.
Medications That Lower Blood Pressure
Several classes of medication can push blood pressure too low, sometimes intentionally and sometimes as a side effect:
- Blood pressure medications. All of them carry this risk by design, including diuretics (water pills), alpha blockers, and beta blockers. The drop is usually most noticeable when a dose is too high or when you first start a new prescription.
- Parkinson’s disease medications, particularly those containing levodopa or similar compounds.
- Tricyclic antidepressants, an older class of depression medication.
- Erectile dysfunction medications, especially when combined with nitrate heart medications. This combination can cause a dangerous and sudden drop in pressure.
If you notice dizziness or lightheadedness after starting a new medication, that’s worth bringing up at your next appointment rather than stopping the drug on your own.
Heart and Endocrine Conditions
Several chronic conditions make low blood pressure an ongoing issue. Heart problems that reduce pumping strength, such as heart failure, heart valve disorders, or an unusually slow heart rate, directly lower cardiac output. When the heart can’t push enough blood forward, pressure in the arteries falls.
Endocrine disorders play a role too. Addison’s disease, where the adrenal glands don’t produce enough hormones, is a well-known cause. Thyroid disorders and low blood sugar can also contribute. These conditions affect the hormonal signals that help regulate blood vessel tone and fluid balance.
Drops After Standing or Eating
Orthostatic hypotension is the medical term for blood pressure that drops when you stand up. Gravity pulls blood into your legs and abdomen as you rise, temporarily reducing the amount flowing back to your heart. Normally, your body compensates within a second or two by tightening blood vessels and speeding up your heart rate. When that reflex is sluggish, blood pressure stays low for several seconds or longer, causing lightheadedness or even fainting. This is more common in older adults, in people taking blood pressure medications, and in people with nervous system conditions like Parkinson’s disease.
Postprandial hypotension is a similar drop that happens after eating, particularly large meals. Digestion redirects blood flow to the stomach and intestines, and in some people, the body doesn’t compensate quickly enough. This is also more common in older adults.
Pregnancy
Blood pressure naturally falls during the first half of pregnancy. The circulatory system expands rapidly to supply the placenta, and blood vessels relax in response to hormonal changes. This typically causes a noticeable dip in the first 20 weeks, with blood pressure gradually returning to pre-pregnancy levels by the third trimester. Some lightheadedness during early pregnancy is normal, though a significant or sudden drop is worth mentioning to your provider.
Severe and Life-Threatening Causes
In emergency situations, dangerously low blood pressure can result from shock. Septic shock occurs when a severe infection triggers widespread inflammation. The immune system releases signaling molecules that cause blood vessels throughout the body to dilate, collapsing blood pressure. Even with aggressive treatment, septic shock is fatal in up to 60% of cases. Early signs include rapid heartbeat, fast breathing, confusion, and falling blood pressure.
Anaphylaxis, a severe allergic reaction, causes a similar sudden drop through rapid vessel dilation and swelling. Hypovolemic shock happens when massive blood loss or fluid loss leaves too little volume in the circulatory system to maintain pressure. All three are medical emergencies.
How Low Blood Pressure Is Diagnosed
A standard blood pressure cuff identifies low readings, but finding the cause often requires more investigation. For people who faint or get dizzy when standing, a tilt table test is one of the most informative tools. You lie flat on a padded table with a blood pressure cuff and heart monitors attached. After resting for at least 10 minutes, the table tilts you upright to about a 70-degree angle within 10 seconds, simulating what happens when you stand. You stay upright for up to 45 minutes while monitors track your response.
A positive result means your blood pressure dropped significantly, your heart rate spiked abnormally, or you developed symptoms like dizziness or fainting. This test helps distinguish between orthostatic hypotension, vasovagal syncope (fainting from a nervous system overreaction), and postural tachycardia syndrome, where the heart races excessively on standing. Blood tests for anemia, thyroid function, and adrenal hormones can identify nutritional or endocrine causes.
What You Can Do About It
If your low blood pressure comes with symptoms, a few practical adjustments often help. Drinking more water increases blood volume. Adding a bit more salt to your diet (if your doctor agrees) helps your body retain fluid. Standing up slowly, especially in the morning, gives your body time to adjust. Compression stockings reduce blood pooling in the legs. Eating smaller, more frequent meals can prevent postprandial drops.
For people whose low blood pressure stems from a medication, adjusting the dose or timing often resolves the issue. When an underlying condition like anemia or an endocrine disorder is responsible, treating that condition typically brings blood pressure back up. Chronically low blood pressure that doesn’t cause symptoms generally doesn’t need treatment at all.