Blood pressure can drop for dozens of reasons, from something as simple as standing up too fast to something as serious as a severe infection. A reading below 90/60 mmHg is generally considered low. For some people that’s perfectly normal and causes no symptoms at all. For others, it triggers dizziness, fainting, or dangerously poor blood flow to vital organs.
Dehydration and Fluid Loss
The most common everyday cause of a blood pressure drop is not having enough fluid in your bloodstream. Your cardiovascular system depends on a certain volume of blood to maintain pressure in your arteries. When that volume falls, whether from not drinking enough water, sweating heavily, vomiting, diarrhea, or significant bleeding, pressure drops because there simply isn’t enough fluid to fill the system.
Mild dehydration might only cause a brief dizzy spell when you stand up. Severe fluid loss is a different story. When you lose a large amount of blood or fluid quickly, your heart can’t pump effectively, and the result is hypovolemic shock, a life-threatening emergency where organs begin shutting down from lack of blood flow. This can happen after a traumatic injury, surgery, or prolonged severe illness with heavy fluid losses.
Medications
Prescription drugs are one of the most frequent culprits behind unexpectedly low blood pressure, especially in older adults taking multiple medications. Several broad categories of drugs lower blood pressure either as their intended effect or as a side effect:
- Blood pressure medications. This includes ACE inhibitors, beta-blockers, calcium channel blockers, and angiotensin receptor blockers. These are designed to lower pressure, but the dose can sometimes overshoot, particularly if you become dehydrated or your kidney function changes.
- Diuretics (water pills). These reduce fluid volume by increasing urine output, which directly lowers the amount of blood circulating in your body.
- Antidepressants. Tricyclic antidepressants in particular can cause blood pressure to drop, especially when standing.
- Parkinson’s disease drugs. Dopamine-based medications used for Parkinson’s are well known for causing low blood pressure episodes.
- Opioid pain medications. These can dilate blood vessels and reduce blood pressure as a side effect.
- Erectile dysfunction drugs and nitrates. Both relax blood vessel walls, and combining them can cause a dangerous pressure drop.
If you’ve recently started a new medication or had a dosage change and notice dizziness or lightheadedness, the timing is worth mentioning to your prescriber.
Positional Changes (Orthostatic Hypotension)
You’ve probably felt this one: you stand up quickly from sitting or lying down and the room briefly swims. That’s orthostatic hypotension, a temporary drop in blood pressure caused by gravity pulling blood into your legs faster than your body can compensate. Normally, your nervous system detects the shift and tightens blood vessels in your lower body within a second or two. When that reflex is slow or impaired, pressure falls and your brain briefly doesn’t get enough blood.
This is especially common in older adults, people with diabetes (which can damage the nerves controlling blood vessel tone), and anyone who is dehydrated. Prolonged bed rest can also weaken the reflex, which is why hospitalized patients are often encouraged to sit up slowly before standing.
Blood Pressure Drops After Eating
Postprandial hypotension is a blood pressure drop that happens within 30 to 60 minutes of eating, though it can occur up to two hours after a meal. It’s defined as a drop of about 20 mmHg or more in the top (systolic) number. After you eat, your body diverts extra blood to your digestive tract. Normally, your heart rate increases slightly and blood vessels elsewhere tighten to keep overall pressure steady. When those compensating mechanisms don’t work well, pressure falls.
Symptoms include dizziness, lightheadedness, fainting, weakness, fatigue, nausea, and black spots in your vision. Some people also experience chest pain. This condition is most common in older adults and people with conditions that affect the autonomic nervous system, like Parkinson’s disease or diabetes. Eating smaller, more frequent meals and limiting high-carbohydrate foods can reduce the severity of these episodes.
Heart Problems
Your heart is the pump that maintains blood pressure, so any condition that weakens or disrupts it can cause pressure to fall. A very slow heart rate (bradycardia) means fewer pumps per minute, which reduces the total output of blood. Heart valve problems, particularly a leaky or narrowed valve, can prevent the heart from moving blood forward efficiently. Heart failure, where the heart muscle is too weak to pump with adequate force, directly lowers pressure. A heart attack can suddenly reduce the heart’s pumping ability, causing a rapid and dangerous pressure drop.
Heart-related causes of low blood pressure tend to come with other symptoms: chest pain or tightness, shortness of breath, swelling in the legs, or an irregular heartbeat. These causes generally develop gradually (with the exception of a heart attack), so a slow downward trend in your blood pressure readings over weeks or months, paired with new symptoms, is worth investigating.
Endocrine and Hormonal Conditions
Several glands in your body produce hormones that help regulate blood pressure. When they malfunction, pressure can drop. Addison’s disease, where the adrenal glands don’t produce enough cortisol and aldosterone, is a classic cause. Aldosterone’s job is to help your kidneys retain sodium and water, so when levels are low, you lose fluid volume and pressure falls. Thyroid disorders, both underactive and severely overactive thyroid, can also affect heart rate and blood vessel tone enough to lower pressure. Low blood sugar (hypoglycemia) triggers a similar effect, causing dizziness and weakness alongside the pressure drop.
Severe Infections and Allergic Reactions
Two of the most dangerous causes of sudden blood pressure drops are sepsis and anaphylaxis. Both can be fatal without rapid treatment.
Sepsis occurs when your body’s response to an infection spirals out of control, releasing chemicals into the bloodstream that trigger widespread inflammation. Blood vessels dilate dramatically, and fluid leaks out of the bloodstream into surrounding tissues. The result is a sharp fall in blood pressure. Severe cases progress to septic shock, where pressure drops to dangerous levels and multiple organs can fail. Warning signs include fever or abnormally low temperature, confusion, rapid breathing, and feeling much sicker than you’d expect from a routine infection.
Anaphylaxis is a severe allergic reaction to foods, insect stings, medications, or other triggers. It causes a massive release of chemicals that widen blood vessels and can make them leak fluid. Blood pressure can plummet within minutes. This is typically accompanied by hives or swelling, difficulty breathing, a rapid pulse, and nausea or vomiting.
Nutritional Deficiencies
Not getting enough of certain nutrients can gradually lower your blood pressure by reducing your body’s ability to produce healthy red blood cells. When you’re deficient in vitamin B12 or folate, your body can’t make enough red blood cells (a condition called anemia), which means less oxygen-carrying capacity in your blood. Your heart tries to compensate by pumping faster, but overall pressure in the vessels can still fall, especially during physical activity or position changes.
Iron deficiency works through the same mechanism. Without adequate iron, red blood cells are smaller and carry less oxygen, forcing the cardiovascular system to work harder with less to show for it. These deficiencies tend to develop slowly, so the blood pressure drop is gradual and often accompanied by fatigue, pale skin, and shortness of breath with exertion rather than dramatic fainting episodes.
Pregnancy
Blood pressure commonly drops during the first 24 weeks of pregnancy. The circulatory system expands rapidly to supply the growing placenta, and blood vessels relax under the influence of pregnancy hormones. This means the same volume of blood is filling a larger network of vessels, which lowers pressure. Most pregnant people notice this as occasional dizziness or lightheadedness, particularly when standing quickly or after hot showers. Blood pressure typically returns to pre-pregnancy levels in the third trimester.
Prolonged Bed Rest and Nervous System Disorders
Spending days or weeks in bed, whether recovering from surgery, illness, or injury, deconditions the reflexes that keep blood pressure stable when you’re upright. The longer you’re horizontal, the more your body “forgets” how to compensate for gravity when you finally stand. This is one reason physical therapy starts early after major surgery or hospitalization.
Neurological conditions that damage the autonomic nervous system cause a more permanent version of this problem. Parkinson’s disease, multiple system atrophy, and certain types of nerve damage (particularly from long-standing diabetes) can all impair the body’s ability to regulate blood pressure in real time. People with these conditions may experience frequent drops when standing, after meals, or during warm weather when blood vessels naturally dilate.