Low blood pressure has a wide range of causes, from everyday triggers like dehydration and standing up too quickly to underlying medical conditions affecting the heart, hormones, or nervous system. For most people, blood pressure below 90/60 mmHg is considered low. Some people naturally run on the lower end without any symptoms at all, but when blood pressure drops enough to reduce blood flow to the brain and organs, it becomes a problem worth investigating.
Dehydration and Blood Volume
Your blood pressure depends partly on how much fluid is circulating through your vessels. When you lose more fluid than you take in, blood volume drops and pressure follows. This can happen from not drinking enough water, sweating heavily, vomiting, diarrhea, or excessive urination. Even mild dehydration can produce noticeable lightheadedness, especially when you stand up. Severe dehydration from illness or heat exposure can cause a dangerous drop.
Blood loss works the same way. Whether from an injury, surgery, or internal bleeding (like a stomach ulcer), losing a significant amount of blood reduces the volume your heart has to pump, and blood pressure falls rapidly.
Heart Conditions
Your heart is the pump behind your blood pressure, so anything that weakens or disrupts it can bring the numbers down. Heart failure means the heart can’t pump forcefully enough to maintain adequate pressure. Heart valve disease allows blood to leak backward or restricts flow, reducing the output with each beat. A very slow heart rate, called bradycardia, means fewer pumps per minute, which can lower the total pressure in your arteries. A heart attack can also trigger a sudden drop by damaging the muscle that drives circulation.
Medications That Lower Blood Pressure
Medications are one of the most common causes of low blood pressure, especially in older adults. Some drugs are designed to lower blood pressure and can overshoot. Others lower it as a side effect. The main categories include:
- Diuretics (water pills): Both thiazide and loop diuretics reduce fluid volume and can cause low pressure along with weakness from potassium loss.
- Beta blockers: These slow the heart rate and can cause low blood pressure, particularly when standing.
- Alpha blockers: Often prescribed for prostate problems or high blood pressure, these commonly cause severe drops in pressure when standing.
- Calcium channel blockers: Used for blood pressure and heart conditions, these relax blood vessels and can lower pressure too much.
- Nitrates: Used for chest pain, these are a common cause of sudden blood pressure drops.
- Certain antidepressants: Older tricyclic antidepressants and some newer ones like venlafaxine and duloxetine commonly cause blood pressure to fall when you stand up.
- Antipsychotic medications: These have blood vessel-relaxing effects that frequently cause positional drops in pressure.
- Parkinson’s disease drugs: Dopamine-related medications used for Parkinson’s often lower blood pressure as a side effect.
If you take any of these and feel dizzy when standing, the medication is a likely contributor. The issue is often worse when multiple blood-pressure-lowering drugs are combined.
Orthostatic Hypotension: The Drop When You Stand
When you stand up, gravity pulls 500 to 800 mL of blood (roughly a pint and a half) into the veins below your heart. Normally, sensors called baroreceptors detect this shift within seconds and signal your nervous system to tighten blood vessels and speed up your heart rate, keeping blood flowing to your brain. When that reflex doesn’t fire properly, blood pressure drops and you feel dizzy or lightheaded.
This is called orthostatic hypotension, and it can happen for several reasons. Dehydration, medications, prolonged bed rest, and aging all weaken the reflex. Some people also have veins in their legs that pool blood more easily, making the gravitational shift worse. Conditions that damage the nerves controlling blood vessels, like diabetes or Parkinson’s disease, make this type of low blood pressure chronic and harder to manage.
Neurally Mediated Hypotension
Some people experience sudden blood pressure drops triggered by a miscommunication in the nervous system. Standing for long periods, emotional stress, or pain can activate areas in the brain that mistakenly slow the heart and widen blood vessels at the same time. The result is a rapid drop in blood pressure that can cause fainting.
This is the mechanism behind common fainting episodes, sometimes called vasovagal syncope. It can affect otherwise healthy people, particularly younger adults who faint in response to triggers like seeing blood, standing in hot environments, or experiencing sudden fear. In susceptible people, the nervous system essentially overreacts to what it perceives as a problem, and the “fix” it applies (slowing the heart, relaxing vessels) is the opposite of what’s needed. Interestingly, this reflex has even been observed in heart transplant patients, suggesting it doesn’t depend entirely on the nerve connections between the brain and heart.
Hormonal and Endocrine Causes
Several glands in your body help regulate blood pressure through hormones, and when they malfunction, pressure can fall. Addison’s disease is one of the clearest examples. The adrenal glands sit on top of your kidneys and produce cortisol (a stress and metabolism hormone) and aldosterone (a hormone that controls sodium and potassium balance to keep blood pressure stable). In Addison’s disease, the adrenal glands produce too little of both. Without enough aldosterone, the body can’t retain sodium properly, fluid balance shifts, and blood pressure drops. People with Addison’s disease often notice dizziness or faintness when standing.
An underactive thyroid can also contribute to low blood pressure by slowing the heart rate and reducing the force of each heartbeat. Low blood sugar, which can occur in people with diabetes or those who go long periods without eating, temporarily reduces blood pressure as well.
Blood Pressure Drops After Eating
After a meal, your body diverts extra blood to the digestive system. Normally, your heart rate increases and blood vessels elsewhere tighten to compensate. But when that compensation falls short, blood pressure drops. This is called postprandial hypotension, and it is surprisingly common in older adults. A review of multiple studies found that about 40% of people between ages 65 and 86 experience it.
Large meals and meals high in carbohydrates tend to cause the biggest drops. Symptoms usually appear within 30 to 60 minutes of eating and include lightheadedness, fatigue, and sometimes fainting. Eating smaller, more frequent meals and limiting simple carbohydrates can help reduce the effect.
Pregnancy
Blood pressure commonly drops during the first and second trimesters of pregnancy. This happens because blood vessels expand to accommodate increased blood flow to the growing baby, and hormonal shifts affect the entire circulatory system. The heart works harder during this time, but the expansion of vessels outpaces the increase in blood volume, so pressure falls.
This is generally considered a normal part of pregnancy rather than a medical problem. Blood pressure typically starts returning to pre-pregnancy levels during the third trimester and fully normalizes after delivery.
Severe Infections and Allergic Reactions
Two of the most dangerous causes of low blood pressure are sepsis and anaphylaxis. In sepsis, a severe infection triggers widespread inflammation throughout the body. Blood vessels dilate dramatically, fluid leaks out of the smallest vessels into surrounding tissues, and the normal distribution of blood flow breaks down. The result is a life-threatening drop in blood pressure called septic shock.
Anaphylaxis, a severe allergic reaction to foods, insect stings, or medications, causes a similar collapse. The immune system releases chemicals that cause blood vessels to widen suddenly and massively, and blood pressure can plummet within minutes. Both conditions require emergency treatment.
Nutritional Deficiencies
Not getting enough vitamin B12 or folate can lead to a type of anemia where the body doesn’t produce enough red blood cells. Fewer red blood cells means less oxygen-carrying capacity in the blood, and the body may not be able to maintain normal blood pressure. Iron deficiency anemia works similarly. These deficiencies develop slowly, so the blood pressure drop is usually gradual, with fatigue and weakness appearing alongside the lightheadedness.