Low blood pressure, or hypotension, has a wide range of causes, from everyday triggers like dehydration and standing up too quickly to serious conditions like heart disease and severe infections. Blood pressure below 90/60 mm Hg is generally considered low. For some people, that’s perfectly normal and causes no symptoms. For others, it signals an underlying problem that needs attention.
Dehydration and Fluid Loss
One of the most common causes of low blood pressure is simply not having enough fluid in your body. Blood pressure depends on the volume of blood circulating through your vessels. When that volume drops, so does the pressure. Even mild dehydration from not drinking enough water, sweating heavily, or having a stomach bug can cause weakness, dizziness, and fatigue from a temporary blood pressure drop.
More serious fluid loss pushes things further. When total blood volume drops by about 10%, the body compensates by tightening blood vessels and speeding up the heart rate to keep pressure stable. But once blood volume falls by 20% to 25%, those backup systems get overwhelmed, and blood pressure drops noticeably. This can happen with significant bleeding, severe vomiting or diarrhea, or burns covering a large area of the body. At that point, the concern shifts from low blood pressure itself to whether enough blood is reaching vital organs.
Medications That Lower Blood Pressure
A surprisingly long list of medications can cause low blood pressure, and not just the ones designed to do so. Blood pressure drugs like diuretics (water pills), beta-blockers, and calcium channel blockers are obvious culprits, but medications prescribed for entirely different conditions can have the same effect.
- Antidepressants: Older tricyclic antidepressants are well known for causing blood pressure drops, particularly when standing. Newer antidepressants like SSRIs and SNRIs can also contribute, though less commonly.
- Antipsychotic medications can lower blood pressure by blocking signals that keep blood vessels constricted.
- Parkinson’s disease drugs containing levodopa activate receptors that widen blood vessels.
- Opioid pain medications reduce the nervous system’s ability to maintain vascular tone, and long-term use can trigger chemical release that further widens blood vessels.
- Sedatives like benzodiazepines may cause a bigger blood pressure drop when standing, partly by relaxing muscles and reducing nervous system activity.
- Nitrate medications used for chest pain relax blood vessels, especially veins, reducing the amount of blood returning to the heart.
If you’re taking any of these and experiencing dizziness, lightheadedness, or fainting, the medication is a likely contributor. The effect is often most noticeable when you stand up after sitting or lying down.
Heart Conditions
Your heart is the pump that drives blood pressure, so any condition that weakens its output can lower pressure. Heart failure reduces the heart’s ability to push enough blood with each beat. Heart valve problems can limit flow. A very slow heart rate, called bradycardia, means fewer pumping cycles per minute and less blood moving through the system. A heart attack can damage the muscle enough to impair pumping immediately.
These causes tend to produce persistently low blood pressure rather than the occasional dip you’d get from dehydration or standing too fast. They also usually come with other symptoms: shortness of breath, fatigue, swelling in the legs, or chest discomfort.
Drops When Standing Up
When you go from sitting or lying down to standing, gravity pulls blood into your legs and abdomen. Normally, pressure sensors near your heart and neck arteries detect the brief dip in blood pressure and signal your brain within seconds. Your heart rate increases, your blood vessels tighten, and pressure stabilizes. You don’t notice any of it.
Orthostatic hypotension happens when that correction fails or lags. You feel lightheaded, dizzy, or like the room is going dark. It’s especially common after prolonged bed rest, after sitting for a long time, in hot weather, and in older adults whose reflexes slow down. Dehydration, certain medications, and nervous system disorders all make it worse. For most people, it’s occasional and harmless. When it’s frequent or causes falls, it points to something that needs investigation.
Blood Pressure Drops After Eating
Postprandial hypotension is a blood pressure drop that occurs within one to two hours after a meal. Digesting food requires increased blood flow to the stomach and intestines, and in some people, the body doesn’t adequately compensate by tightening blood vessels elsewhere or speeding up the heart. This is most common in older adults and can cause dizziness or faintness after eating, particularly after large meals or meals high in carbohydrates.
Hormonal and Endocrine Problems
Several hormone-related conditions can lower blood pressure. Addison’s disease, where the adrenal glands don’t produce enough of certain hormones, is a classic cause. These hormones help regulate blood vessel tone and fluid balance, so when levels drop, blood pressure follows. Low blood sugar (hypoglycemia) can also trigger a drop in blood pressure, as can diabetes, which over time can damage the nerves that help regulate blood pressure responses.
Thyroid disorders, particularly an underactive thyroid, can contribute as well. The hormonal shifts are often gradual, so the blood pressure changes may develop slowly alongside other symptoms like fatigue, weight changes, or muscle weakness.
Pregnancy
Blood pressure typically falls during early pregnancy and stays lower throughout much of it. Toward the end of the first trimester, blood vessels widen significantly to accommodate the growing blood supply needed for the placenta and baby. This dilation outpaces the increase in blood volume, causing blood pressure to drop by 5 to 10 mm Hg on average. It generally stays at this lower level until the third trimester, when it gradually rises back toward pre-pregnancy numbers. Some lightheadedness is normal, especially when standing quickly.
Nutritional Deficiencies
Deficiencies in vitamin B12 and folate can cause megaloblastic anemia, a condition where the body produces fewer red blood cells than normal, and the ones it does make are abnormally large and less effective at carrying oxygen. With fewer functioning red blood cells, the blood’s capacity to maintain adequate pressure is reduced.
Vitamin B12 deficiency specifically can damage the nerves responsible for blood pressure regulation. Research dating back to the 1960s documented postural hypotension caused by B12 deficiency. When patients with this type of anemia stand up, their sympathetic nervous system releases less of the chemical signal that constricts blood vessels, so pressure drops. In documented cases, treating the deficiency with B12 and folate supplements led to a steady rise in blood pressure, confirming the deficiency as the direct cause.
Severe Infections and Allergic Reactions
Some of the most dangerous causes of low blood pressure involve the body’s own immune responses spiraling out of control.
In sepsis, a severe infection triggers a widespread inflammatory response. The immune system floods the bloodstream with signaling molecules that cause blood vessels throughout the body to relax and widen dramatically. At the same time, tiny blood vessels become leaky, allowing fluid to seep out of the bloodstream and into surrounding tissues. The combination of widened vessels and fluid loss causes blood pressure to plummet. The inflammatory chemicals can even weaken the heart muscle itself, compounding the problem.
Anaphylaxis, a severe allergic reaction to things like foods, insect stings, or medications, works through a similar mechanism but happens even faster. When someone with a sensitized immune system encounters the trigger again, specialized cells release large amounts of histamine throughout the body. Histamine causes rapid, widespread blood vessel dilation and fluid leakage from capillaries. Blood pressure can drop within minutes, which is why anaphylaxis is treated as a medical emergency.
When Low Blood Pressure Causes Symptoms
Low blood pressure only matters when it affects how well blood reaches your brain and organs. Some people walk around with a systolic pressure in the 80s and feel perfectly fine. The numbers alone don’t determine whether there’s a problem.
The symptoms that signal your blood pressure is too low for your body include dizziness or lightheadedness, blurred vision, nausea, fatigue, and difficulty concentrating. Fainting is the most dramatic sign, and it happens when blood pressure drops low enough that the brain temporarily doesn’t get adequate blood flow. If you’re experiencing these symptoms regularly, identifying the underlying cause from the list above is the key step, because treating the cause is almost always more effective than trying to raise blood pressure on its own.