Low blood pressure, clinically defined as a reading below 90/60 mmHg, has a wide range of causes. Some are temporary and harmless, like standing up too quickly. Others signal something more serious, like a heart problem or severe infection. Understanding what’s behind a low reading helps you figure out whether it’s something your body handles on its own or something that needs attention.
Dehydration and Blood Volume Loss
Your blood pressure depends on two things: how much blood your heart pumps and how much resistance your arteries create against that flow. When you don’t have enough fluid in your system, the volume of blood circulating through your body drops, and your heart simply has less to pump. The result is lower pressure.
Dehydration is one of the most common and straightforward causes. Vomiting, diarrhea, excessive sweating, or just not drinking enough water can all reduce your blood volume enough to bring your pressure down. This is usually mild and resolves once you rehydrate. But in more extreme cases, like heavy bleeding from an injury, surgery, or internal bleeding, fluid loss becomes severe enough to cause a dangerous condition called hypovolemic shock. At that point, your heart can’t pump enough blood to keep your organs working.
Medications
A number of prescription drugs can lower blood pressure as either their intended effect or an unwanted side effect. The most obvious culprits are medications prescribed specifically for high blood pressure. These include diuretics (which reduce fluid volume), beta-blockers (which slow the heart rate), ACE inhibitors, calcium channel blockers, alpha-blockers, and vasodilators. All of these can cause dizziness, lightheadedness, or fainting, especially when you first start taking them or when a dose is adjusted.
But blood pressure medications aren’t the only ones to watch. Certain antidepressants, drugs for Parkinson’s disease, and erectile dysfunction medications can also lower blood pressure. If you’ve recently started or changed a medication and notice you’re feeling dizzy when you stand up, the medication is a likely explanation.
Positional Drops: Standing Up and Eating
When you stand up from sitting or lying down, gravity pulls blood into your legs and abdomen. Normally, specialized cells near your heart and neck arteries detect the temporary dip in pressure and send signals to your brain within seconds. Your heart speeds up, your blood vessels tighten, and pressure stabilizes. Orthostatic hypotension happens when this correction system doesn’t work properly. You feel dizzy, lightheaded, or even faint in the moments after standing.
This is more common in older adults, people who are dehydrated, and people taking blood pressure medications. It can also be caused by nervous system disorders like Parkinson’s disease, Lewy body dementia, and a condition called multiple system atrophy, all of which can disrupt the body’s automatic blood pressure regulation.
Some people also experience a blood pressure drop after eating, known as postprandial hypotension. Digestion requires extra blood flow to the stomach and intestines, and in some people, particularly older adults, the body doesn’t compensate well enough to keep pressure steady elsewhere.
Heart Problems
Since blood pressure depends on how effectively your heart pumps, any condition that weakens the heart or disrupts its rhythm can lower pressure. A very slow heart rate (bradycardia) means less blood is pushed out per minute. Heart valve problems can reduce the efficiency of each pump. Heart failure, where the heart muscle is too weak to maintain adequate output, is another cause. A heart attack can suddenly impair the heart’s ability to pump, leading to a sharp drop in pressure.
Heart-related causes of low blood pressure tend to come with other symptoms: chest pain, shortness of breath, fatigue, or swelling in the legs. These are not the kind of low blood pressure you shrug off.
Endocrine and Hormonal Causes
Several hormone-related conditions affect blood pressure. Addison’s disease, where the adrenal glands don’t produce enough cortisol and aldosterone, is a well-known cause. Aldosterone helps your body retain sodium and water, so when levels are low, blood volume drops and pressure falls. Thyroid disorders, both overactive and underactive, can also contribute. Low blood sugar (hypoglycemia) is another trigger, particularly in people with diabetes.
Pregnancy commonly causes lower blood pressure, especially during the first and second trimesters. The circulatory system expands rapidly to supply the developing baby, and blood pressure often dips before the body catches up with increased blood production. This is usually normal and resolves after delivery.
Severe Allergic Reactions and Infections
Two of the most dangerous causes of low blood pressure are anaphylaxis and sepsis. Both fall under a category called distributive shock, where blood vessels throughout the body suddenly relax and widen. This causes a dramatic drop in blood pressure because the same amount of blood is now filling a much larger space. The small blood vessels also become leaky, allowing fluid to seep out of the circulation and further reducing blood volume.
Anaphylaxis is a severe allergic reaction, often to foods like peanuts, insect stings, or certain medications. It can cause blood pressure to plummet within minutes. Sepsis happens when an infection triggers an overwhelming immune response that damages blood vessels throughout the body. Both are medical emergencies where the drop in pressure can prevent blood from reaching the brain, heart, and kidneys.
Nutritional Deficiencies
Not getting enough vitamin B12 or folate can contribute to low blood pressure, partly through their role in red blood cell production. When levels of these nutrients are too low, your body can’t make enough healthy red blood cells, leading to anemia. With fewer red blood cells carrying oxygen, the cardiovascular system is under strain, and blood pressure can fall.
Research published in the European Heart Journal found that people with vitamin B12 deficiency were more likely to experience fainting episodes. Those with the lowest B12 levels had significantly more lifetime fainting attacks compared to those with adequate levels. B12 and folate deficiencies may also affect blood pressure regulation through their influence on nervous system signaling. These deficiencies are especially common in older adults, vegetarians, and people with digestive conditions that impair nutrient absorption.
Nervous System Disorders
Your autonomic nervous system, the part that runs in the background controlling heart rate, digestion, and blood vessel tone, plays a central role in maintaining blood pressure. Conditions that damage this system can prevent your body from making the rapid adjustments needed to keep pressure stable. Parkinson’s disease, Lewy body dementia, pure autonomic failure, and amyloidosis all fall into this category.
People with these conditions often experience the most noticeable drops when changing position, but their blood pressure regulation can be unreliable throughout the day. The underlying issue isn’t the heart or blood volume, it’s the signaling system that coordinates the response. When those signals are disrupted, the heart doesn’t speed up when it should, and blood vessels don’t tighten when they need to.