Low blood pressure, or hypotension, generally means a reading below 90/60 mmHg. It can result from something as routine as dehydration or standing up too fast, or it can signal an underlying heart condition, hormone disorder, or medication side effect. For many people, naturally low blood pressure causes no symptoms at all. It only becomes a problem when it drops enough to cause dizziness, fainting, or fatigue, which points to an underlying cause worth identifying.
Heart and Circulation Problems
Your heart is the pump that maintains blood pressure, so anything that weakens it or disrupts its rhythm can cause pressure to fall. Heart failure, heart valve disease, and a very slow heart rate (bradycardia) all reduce the volume of blood your heart pushes out with each beat. A heart attack can also cause a sudden drop in blood pressure because damaged heart muscle simply can’t pump as effectively.
These causes tend to produce persistent low blood pressure rather than brief episodes. You might notice fatigue, shortness of breath, or swelling in your legs alongside the low readings. If low blood pressure appears alongside these symptoms, it’s typically the heart issue driving it, not the other way around.
Medications That Lower Blood Pressure
Medications are one of the most common and most overlooked causes of low blood pressure. Drugs designed to treat high blood pressure, including diuretics (water pills) and beta-blockers, can sometimes push pressure lower than intended, especially when doses change or when you’re dehydrated. Certain antidepressants and medications for Parkinson’s disease also carry hypotension as a side effect.
The pattern is often predictable: symptoms worsen shortly after taking a dose, or they appear after a dose increase. If you notice dizziness that lines up with your medication schedule, that timing is an important clue. Alcohol and some erectile dysfunction drugs can compound the effect by relaxing blood vessels further.
Hormone and Endocrine Disorders
Your endocrine system helps regulate blood pressure through hormones that control fluid balance, blood vessel tone, and heart rate. When certain glands underperform, blood pressure can drop. Addison’s disease, where the adrenal glands don’t produce enough cortisol and aldosterone, is a classic example. Low thyroid function and low blood sugar can also contribute.
Pregnancy causes a well-documented dip in blood pressure too. During the second trimester, progesterone relaxes blood vessel walls, and the body’s blood volume expands faster than the heart compensates. Blood pressure typically reaches its lowest point around 23 to 24 weeks of pregnancy before gradually climbing back toward pre-pregnancy levels in the third trimester. This is considered normal, though it can cause lightheadedness, especially when standing quickly.
Dehydration and Blood Volume Loss
Blood pressure depends on having enough fluid in your blood vessels. When you lose fluid through sweating, vomiting, diarrhea, or simply not drinking enough water, blood volume drops and pressure follows. Even mild dehydration can cause noticeable symptoms in people who already run on the lower end of normal.
Severe blood loss from an injury, surgery, or internal bleeding causes a more dramatic and dangerous form of low blood pressure. The body tries to compensate by speeding up the heart and constricting blood vessels in the extremities (which is why people in shock often look pale and feel cold), but if volume loss continues, blood pressure falls to dangerous levels.
Nutritional Deficiencies
A shortage of vitamin B-12 or folate can lead to a type of anemia where the body produces red blood cells that are too large and don’t carry oxygen efficiently. With fewer functional red blood cells in circulation, blood pressure can gradually decline. This type of low blood pressure tends to develop slowly, so the symptoms, including fatigue and weakness, often creep in rather than hitting suddenly. Iron deficiency anemia works through a similar mechanism.
Standing Up Too Fast (Orthostatic Hypotension)
Orthostatic hypotension is the lightheadedness or near-blackout feeling you get when you stand up quickly from sitting or lying down. It’s diagnosed when the top number of your blood pressure drops by 20 mmHg or more, or the bottom number drops by 10 mmHg or more, within two to five minutes of standing.
Normally, your body compensates for the gravity shift by tightening blood vessels and slightly increasing heart rate. When that reflex is sluggish, blood pools in your legs and pressure in your upper body falls. Dehydration, prolonged bed rest, certain medications, diabetes, and nervous system conditions like Parkinson’s disease all make orthostatic hypotension more likely. It becomes more common with age because the reflexes that stabilize blood pressure slow down.
Low Blood Pressure After Eating
Digestion requires a large surge of blood to the intestines. In most people, the body compensates by speeding up the heart and constricting blood vessels elsewhere. In some older adults, those compensating mechanisms don’t keep pace. The result is postprandial hypotension, a drop in blood pressure within one to two hours of eating.
This affects up to one-third of older adults but is virtually unheard of in younger people. It’s especially common in people with high blood pressure, diabetes, Parkinson’s disease, or other conditions that impair the autonomic nervous system. Eating smaller, more frequent meals and limiting high-carbohydrate foods can reduce the post-meal drop.
Nervous System Misfires
Sometimes the brain and heart miscommunicate in a way that causes blood pressure to plummet. Neurally mediated hypotension happens when emotional triggers like pain, fear, the sight of blood, or anxiety activate a reflex that simultaneously slows the heart and dilates blood vessels. The result is a sudden drop in blood pressure, often ending in fainting. Up to 25% of healthy young adults experience at least one episode like this.
Situational triggers can also set it off: coughing forcefully, straining during a bowel movement, swallowing something large, or even standing in one place for a long time. The underlying problem is a reflex arc where sensors in the heart and blood vessels send the wrong signal to the brain, which responds by telling the heart to slow down precisely when it should be speeding up.
Severe Infections and Allergic Reactions
Some causes of low blood pressure are medical emergencies. Sepsis, a life-threatening response to infection, causes blood vessels throughout the body to dilate and leak fluid, which can send blood pressure plummeting. Anaphylaxis, a severe allergic reaction to foods, insect stings, or medications, triggers a chemical flood from the immune system that causes blood vessels to widen dramatically and airways to narrow. Both conditions cause blood pressure to drop within minutes and require immediate treatment.
The distinguishing feature of these causes is speed. Unlike the gradual pressure drop from anemia or dehydration, sepsis and anaphylaxis produce sudden, severe symptoms: confusion, rapid breathing, hives or swelling, and a sense that something is seriously wrong. These are not conditions where a wait-and-see approach is appropriate.