Blood pressure drops when your heart pumps less forcefully, your blood vessels widen, or you don’t have enough fluid in your bloodstream. A reading below 90/60 mmHg is generally considered low. For some people that’s perfectly normal and causes no symptoms, but for others it triggers dizziness, fainting, or in severe cases, organ damage.
Understanding the specific cause matters because treatment depends entirely on what’s driving the drop. The triggers range from something as simple as standing up too fast to serious conditions like sepsis or heart failure.
Dehydration and Blood Volume Loss
Your blood is mostly water. When you lose fluid through sweating, vomiting, diarrhea, or simply not drinking enough, total blood volume shrinks. Less fluid in the bloodstream means less pressure pushing against artery walls, and blood pressure falls. As dehydration progresses, the body compensates by speeding up the heart rate and narrowing blood vessels, but if fluid loss continues, those mechanisms eventually fail and blood pressure drops further.
The type of fluid you lose also matters. When lost fluid contains a high concentration of sodium, water shifts out of the bloodstream and into surrounding tissues, making the drop in blood volume even worse than the amount of fluid lost would suggest. This is why severe diarrhea or heavy sweating can cause blood pressure to plummet quickly. Blood loss from injury, surgery, or internal bleeding works through the same basic mechanism: less volume in the system means lower pressure.
Medications That Lower Blood Pressure
Drugs are one of the most common causes of unexpected blood pressure drops, especially in older adults. The medication categories most frequently linked to low blood pressure include:
- Blood pressure medications (ACE inhibitors, calcium-channel blockers, beta-blockers) can overshoot their target, lowering pressure more than intended
- Diuretics reduce blood volume by increasing urine output
- Alpha-blockers prescribed for prostate enlargement relax blood vessel walls
- Antidepressants (particularly SSRIs) interfere with the nervous system signals that regulate blood vessel tone
The risk compounds when you take more than one of these at the same time. Research from UK primary care data found that older adults are frequently prescribed multiple drugs from this list simultaneously, dramatically increasing the chance of blood pressure drops. If you’ve recently started a new medication or changed your dose and feel lightheaded, the timing is probably not a coincidence.
Standing Up, Eating, and Other Positional Triggers
When you stand from a sitting or lying position, gravity pulls blood downward into your legs and abdomen. This temporarily reduces the amount of blood returning to your heart, which would cause a pressure drop in everyone if the body didn’t immediately correct it. Specialized pressure sensors near your heart and neck arteries detect the change within seconds and signal your brain to speed up your heart rate and tighten blood vessels. In most people, this correction happens so fast they never notice.
Orthostatic hypotension occurs when that correction system fails or responds too slowly. You feel dizzy or lightheaded within seconds of standing, and in some cases you faint. This is more common in older adults, people who are dehydrated, and anyone taking the medications listed above. Prolonged bed rest also weakens this reflex because the body loses its conditioning for handling gravity.
Eating can trigger a similar problem. After a meal, your body diverts extra blood to the digestive system. In some people, particularly older adults, blood pressure drops noticeably within one to two hours of eating. This is called postprandial hypotension, and it tends to be worse after large, carbohydrate-heavy meals.
Heart Problems
Your heart is the pump driving the entire system, so anything that weakens it can lower blood pressure. Heart failure means the muscle can’t pump with enough force to maintain adequate pressure. An abnormally slow heart rate (below about 60 beats per minute in some people) reduces the volume of blood pushed out per minute. Heart valve problems, where valves don’t open or close properly, reduce the efficiency of each pump cycle.
A heart attack can cause a sudden drop in blood pressure because damaged heart muscle loses its ability to contract. This is why a sudden, unexplained blood pressure drop accompanied by chest pain, shortness of breath, or cold sweats is treated as a medical emergency.
Hormonal and Endocrine Causes
Your adrenal glands, which sit on top of your kidneys, produce hormones that directly control blood pressure. Cortisol helps maintain pressure in blood vessels. Aldosterone keeps sodium and potassium balanced in the blood, which controls how much water your body retains and, by extension, your blood volume. When the adrenal glands don’t produce enough of these hormones, a condition called adrenal insufficiency (Addison’s disease), blood pressure can drop significantly.
In an adrenal crisis, cortisol levels plummet to dangerously low levels, causing life-threatening low blood pressure along with low blood sugar and dangerous shifts in sodium and potassium. Low blood sugar from any cause, including skipping meals or diabetes medication miscalculation, can also trigger a blood pressure drop because the body’s stress response becomes overwhelmed.
Thyroid disorders can play a role as well. An underactive thyroid slows the heart rate and weakens the force of heart contractions, both of which reduce blood pressure over time.
Severe Infections and Allergic Reactions
Sepsis and anaphylaxis cause some of the most dramatic and dangerous blood pressure drops. They work through a similar endpoint, but the triggers differ.
In sepsis, your immune system overreacts to an infection and floods the bloodstream with inflammatory signals. These signals force blood vessels to dilate widely and become leaky, allowing fluid to seep out of the bloodstream into surrounding tissues. At the same time, the heart muscle itself is directly weakened by these inflammatory compounds. The combination of widened, leaky vessels and a weakened heart causes blood pressure to collapse, sometimes within hours.
Anaphylaxis works faster. When someone with a severe allergy encounters their trigger (a bee sting, peanut, medication), immune cells release massive amounts of histamine almost instantly. Histamine forces blood vessels open and makes them permeable, causing a rapid plunge in blood pressure. This can happen within minutes of exposure, which is why people at risk carry injectable epinephrine to counteract the reaction.
Nervous System Disorders
Your autonomic nervous system, the part that runs on autopilot, constantly fine-tunes blood pressure by adjusting heart rate and blood vessel width. Diseases that damage this system remove the body’s ability to respond to changing conditions. Parkinson’s disease, diabetes-related nerve damage, and a group of conditions called autonomic neuropathies can all impair these reflexes. The result is blood pressure that swings unpredictably, often dropping when standing, exercising, or dealing with heat.
Some people experience a nervous system misfire called neurally mediated hypotension, where standing for long periods triggers a faulty signal that widens blood vessels and slows the heart at exactly the wrong moment. This is a common cause of fainting in otherwise healthy young adults.
Recognizing the Symptoms
Mild drops in blood pressure often cause no symptoms at all. When symptoms do appear, they typically include dizziness or lightheadedness, blurred vision, nausea, fatigue, and difficulty concentrating. These signal that your brain isn’t getting quite enough blood flow, and they usually resolve quickly once you sit or lie down.
More concerning signs include fainting, cold or clammy skin, rapid shallow breathing, and confusion. These suggest a more significant drop and point to possible shock, where organs aren’t receiving enough blood to function. A sudden, severe blood pressure drop with these symptoms, especially if paired with fever, chest pain, or signs of an allergic reaction, requires emergency care.