Blood pressure is considered low when it drops below 90/60 mmHg. For many people, naturally low blood pressure causes no symptoms and is perfectly healthy. But when blood pressure drops suddenly or stays low enough to reduce blood flow to your organs, it can cause dizziness, fainting, and in severe cases, shock. The reasons range from everyday triggers like dehydration to underlying heart or hormonal conditions.
How Low Blood Pressure Works in Your Body
Your blood pressure depends on two things: how forcefully your heart pumps and how much resistance your blood vessels provide. Anything that reduces the volume of blood in your system, weakens your heart’s pumping ability, or causes your blood vessels to widen can lower your pressure. Your body has built-in correction mechanisms, including sensors in your arteries that detect pressure changes and signal your heart to beat faster or your vessels to tighten. When those mechanisms fail or get overwhelmed, blood pressure drops.
Dehydration and Low Blood Volume
One of the most common reasons for low blood pressure is simply not having enough fluid in your bloodstream. When you’re dehydrated from not drinking enough water, sweating heavily, vomiting, or diarrhea, the total volume of blood circulating through your body decreases. Less volume means less pressure pushing against your artery walls, and your heart has less blood to pump with each beat.
In its mildest form, this causes lightheadedness when you stand up too quickly. In severe cases, major fluid loss from bleeding, burns, or prolonged vomiting can lead to hypovolemic shock, where your heart simply can’t deliver enough blood to keep your organs functioning. This is a medical emergency.
Standing Up Too Fast
Orthostatic hypotension is the medical term for a blood pressure drop that happens when you go from sitting or lying down to standing. It’s diagnosed when your top number (systolic) falls by 20 mmHg or more, or your bottom number (diastolic) falls by 10 mmHg or more, within two to five minutes of standing.
When you stand, gravity pulls blood toward your legs. Normally, sensors in your blood vessels detect this shift and trigger your heart to beat faster and your vessels to tighten, keeping pressure stable. But this reflex can be sluggish or impaired in older adults, people who are dehydrated, and those taking certain medications. The result is a brief drop in blood flow to your brain, causing that familiar head rush or, in worse cases, a full faint.
Medications That Lower Blood Pressure
Dozens of commonly prescribed drugs can lower blood pressure as a side effect, sometimes intentionally and sometimes not. The most frequent culprits fall into a few categories:
- Blood pressure medications: Drugs prescribed specifically to treat high blood pressure, including diuretics (water pills), beta blockers, calcium channel blockers, and ACE inhibitors, can overshoot their target and bring pressure too low, especially when doses change or medications are combined.
- Diuretics: Both thiazide and loop diuretics reduce fluid volume by making you urinate more. This directly lowers the amount of blood circulating in your system, causing weakness and drops in pressure, particularly when standing.
- Antidepressants: Older tricyclic antidepressants commonly cause orthostatic hypotension. Some newer antidepressants that affect norepinephrine levels also lower blood pressure, though SSRIs do so only rarely.
- Prostate medications: Alpha blockers, often prescribed for enlarged prostate, are known for causing significant drops in blood pressure upon standing.
- Parkinson’s disease drugs: Dopamine-based medications used for Parkinson’s frequently cause orthostatic hypotension.
- Heart medications: Nitrates used for chest pain can cause sudden blood pressure drops and fainting.
If you’ve recently started a new medication and notice dizziness or lightheadedness, the timing is worth mentioning to your prescriber.
Heart Conditions
Your heart is the pump that generates blood pressure, so any condition that weakens it or disrupts its rhythm can lower pressure. Bradycardia, an abnormally slow heart rate, happens when the electrical signals that coordinate heartbeats slow down or get blocked. If your heart beats too slowly, it may not pump enough oxygen-rich blood to meet your body’s needs.
Heart valve problems can reduce the efficiency of each heartbeat. Heart failure, where the heart muscle is too weak to pump effectively, directly lowers the force driving blood through your arteries. Heart attacks can also cause sudden drops in blood pressure by damaging the muscle responsible for pumping.
Hormonal and Endocrine Causes
Your adrenal glands produce hormones that play a direct role in regulating blood pressure. In adrenal insufficiency (Addison’s disease), the adrenal glands don’t make enough cortisol or aldosterone. Aldosterone controls how much sodium and water your kidneys retain, so when levels are too low, your body loses fluid and blood volume drops. Cortisol deficiency compounds the problem by reducing your body’s ability to produce adrenaline and weakening the response of blood vessels to signals that would normally keep them constricted.
People with adrenal insufficiency tend to have noticeably lower systolic blood pressure compared to people with low blood sugar from other causes, roughly 120 versus 128 mmHg in one study of emergency department patients. They’re also more likely to experience blood pressure drops when standing.
Thyroid disorders and low blood sugar can also contribute. When blood sugar drops too low, it can trigger a cascade that affects circulation and pressure regulation.
Blood Pressure Drops After Eating
Postprandial hypotension is a blood pressure drop that occurs after meals, and it primarily affects older adults. Here’s why: digesting food requires a large blood supply to the intestines. In younger people, the body compensates automatically by speeding up the heart rate and tightening blood vessels elsewhere. In some older adults, these compensating mechanisms don’t work well enough. Blood flows to the gut as expected, but the heart doesn’t speed up sufficiently and blood vessels elsewhere don’t constrict enough, so pressure falls.
This tends to be worse after large, carbohydrate-heavy meals. Eating smaller, more frequent meals can reduce the effect.
Fainting From Emotional or Physical Triggers
If you’ve ever fainted at the sight of blood or while standing in a hot, crowded room, you’ve experienced neurally mediated hypotension. Up to 25% of healthy young adults have at least one episode in their lifetime, often triggered by fear, pain, anxiety, or disgust.
The underlying problem is a miscommunication between your heart and brain. Normally, pressure sensors in your arteries detect changes and relay information to your brainstem, which adjusts your heart rate and blood vessel tone to keep pressure steady. In susceptible people, emotional or physical triggers hijack this system. The brain paradoxically tells the heart to slow down and the blood vessels to relax at exactly the wrong moment, causing a rapid drop in pressure and reduced blood flow to the brain. The result is lightheadedness or a full faint.
Specific physical triggers can also set this off, including coughing, straining during a bowel movement, swallowing, or even urinating. These are sometimes called “situational” fainting episodes, and while they’re alarming, they’re usually not dangerous in otherwise healthy people.
Severe Causes: Shock and Sepsis
The most dangerous cause of low blood pressure is shock, where blood flow drops so severely that organs begin to fail. There are several types. Hypovolemic shock results from massive blood or fluid loss, whether from trauma, internal bleeding, severe burns, or extreme dehydration. Septic shock occurs when a bloodstream infection triggers a widespread inflammatory response that causes blood vessels to dilate dramatically, collapsing pressure. Anaphylactic shock is a severe allergic reaction, often to insect stings, foods, or medications, that causes rapid vessel dilation and airway swelling.
All forms of shock share the same core problem: not enough blood and oxygen reaching vital organs. Signs include cold or clammy skin, rapid breathing, confusion, and very low blood pressure. Shock is always a medical emergency requiring immediate treatment.
When Low Blood Pressure Is Normal
Not all low blood pressure is a problem. Some people, particularly those who are physically fit, naturally run blood pressures below 90/60 mmHg and feel perfectly fine. Athletes often develop lower resting blood pressures because their hearts pump more efficiently with each beat, requiring less force overall. Genetics also play a role; low blood pressure tends to run in families.
The distinction that matters is whether low blood pressure causes symptoms. A reading of 85/55 in someone who feels energetic and alert is not a medical concern. The same reading in someone who feels dizzy, fatigued, or unable to concentrate suggests that blood flow to the brain and other organs is inadequate and the cause needs to be identified.