Very low blood pressure, generally defined as a reading below 90/60 mmHg, can result from dozens of different causes ranging from simple dehydration to serious heart conditions. For some people, naturally low blood pressure is normal and causes no symptoms at all. But when blood pressure drops suddenly or low enough to starve your organs of blood flow, the cause matters and needs to be identified.
Dehydration and Low Blood Volume
The most common and most fixable cause of very low blood pressure is not having enough fluid in your system. Fever, vomiting, severe diarrhea, heavy sweating during exercise or heat exposure, and simply not drinking enough water all reduce your blood volume. Less fluid in your blood vessels means less pressure pushing blood to your organs. This is why people who are dehydrated often feel dizzy when they stand up quickly.
Nutritional deficiencies can contribute in a related way. Vitamin B12 deficiency and anemia both reduce the number of red blood cells your body can produce, which effectively lowers your blood volume and can drop your pressure. This type of cause tends to develop gradually over weeks or months rather than hitting suddenly.
Heart Problems That Reduce Output
Your heart is the pump that generates blood pressure. When it can’t pump efficiently, pressure falls. Several cardiac conditions do this in different ways. An extremely slow heart rate (bradycardia) means fewer beats per minute pushing blood out. Heart valve problems allow blood to leak backward instead of moving forward. Heart failure means the muscle itself is too weak to generate adequate force. A heart attack damages part of the muscle and can cause a sudden, dangerous drop.
These heart-related causes tend to produce low blood pressure that’s persistent rather than situational. You might notice fatigue, shortness of breath, or swelling in your legs alongside the low readings.
Medications
Medications are one of the most frequently overlooked causes, especially in people over 65 who take multiple prescriptions. Several broad categories of drugs lower blood pressure either as their intended effect or as a side effect.
- Blood pressure and heart medications: Diuretics (both thiazide and loop types), beta blockers, calcium channel blockers, ACE inhibitors, and alpha blockers all lower blood pressure by design. When the dose is too high or multiple types are combined, they can push pressure too low.
- Antidepressants: Older tricyclic antidepressants and MAOIs are well known for causing drops in blood pressure, particularly when standing. Even newer classes like SNRIs commonly cause this through their effect on norepinephrine.
- Parkinson’s disease drugs: Dopamine agonists and MAO-B inhibitors both lower blood pressure as a side effect.
- Antipsychotic medications: All drugs in this class have some ability to relax blood vessels and cause pressure drops.
- Nitrate medications: Drugs like nitroglycerin, used for chest pain, can cause sudden and significant drops in blood pressure.
If you’ve recently started a new medication or had a dose change and notice dizziness or lightheadedness, the timing is worth flagging with your prescriber.
Hormonal and Endocrine Causes
Your endocrine system plays a direct role in regulating blood pressure. When it malfunctions, pressure can drop significantly.
Addison’s disease (adrenal insufficiency) is one of the clearest examples. Your adrenal glands produce a hormone called aldosterone that balances sodium and potassium levels to keep blood pressure stable. When the adrenals fail, aldosterone production drops, and blood pressure falls with it. In an adrenal crisis, blood pressure can plummet dangerously alongside low blood sugar and high potassium levels. Dizziness or fainting when standing is a hallmark symptom of Addison’s disease.
Thyroid conditions, particularly an underactive thyroid, and low blood sugar (hypoglycemia) can also trigger drops in blood pressure. Diabetes deserves special mention because over time it damages the nerves responsible for sending blood pressure control signals, making it harder for your body to adjust pressure when you change position.
Positional and Situational Drops
Some people have normal blood pressure most of the time but experience sharp drops in specific situations. Orthostatic hypotension, the most common type, happens when you stand up from sitting or lying down. Gravity pulls blood into your legs and abdomen, temporarily reducing the amount flowing back to your heart. Your body normally compensates within seconds by tightening blood vessels and slightly increasing heart rate. When that automatic correction fails, your blood pressure stays low and you feel dizzy or faint.
This positional drop becomes more common with age. Specialized pressure-sensing cells near the heart and neck arteries slow down as you get older, and an aging heart may not speed up fast enough to compensate. Prolonged bed rest from illness or injury weakens these reflexes too, which is why people often feel unsteady after being hospitalized.
Postprandial hypotension is a related pattern where blood pressure drops after eating, as blood is redirected to the digestive system. And neurally mediated hypotension occurs after standing for long periods, when miscommunication between the heart and brain causes a sudden pressure drop. This is the classic reason people faint during long ceremonies or while standing in line on a hot day.
Pregnancy
Low blood pressure during pregnancy is extremely common and usually harmless. It happens because the body’s blood vessels relax and widen dramatically starting as early as five weeks of gestation, well before the placenta is fully developed. The resistance in your blood vessels decreases by 35% to 40% compared to pre-pregnancy levels. Blood pressure typically hits its lowest point during the second trimester, dropping 5 to 10 mmHg below baseline, though most of that decrease actually happens between 6 and 8 weeks. Pressure generally returns to normal by the third trimester or after delivery.
Severe Allergic Reactions and Infections
Two emergency situations can cause blood pressure to crash rapidly. In anaphylaxis (a severe allergic reaction) and sepsis (the body’s overwhelming response to infection), blood vessels throughout the body suddenly dilate and become extremely relaxed. At the same time, the walls of tiny blood vessels called capillaries become leaky, allowing fluid to seep out of the bloodstream. The combination of flaccid vessels and fluid loss causes blood pressure to plummet, and not enough blood reaches vital organs. This type of collapse, called distributive shock, requires immediate emergency treatment.
Alcohol and Heat Exposure
Alcohol relaxes blood vessels and increases the risk of blood pressure drops, particularly when standing. The effect compounds if you’re also dehydrated from drinking. Heat exposure works through a similar two-pronged mechanism: hot environments cause your blood vessels to widen to release heat through your skin, while heavy sweating depletes your fluid volume. Together, these effects can produce significant drops in blood pressure, which is why fainting is more common on hot days.
How to Tell What’s Causing Yours
The pattern of your low blood pressure often points toward the cause. If it only happens when you stand up, the issue is likely orthostatic, driven by medications, dehydration, aging reflexes, or nerve damage. If your blood pressure is consistently low regardless of position, the cause is more likely cardiac, hormonal, or nutritional. If it dropped suddenly and severely, infection, allergic reaction, or acute blood loss are the urgent possibilities.
Symptoms to pay attention to include persistent dizziness, blurred vision, nausea, unusual fatigue, difficulty concentrating, and cold or clammy skin. Fainting is a clear sign your brain isn’t getting enough blood. If low blood pressure comes with confusion, rapid shallow breathing, or a weak pulse, those are signs of shock and a medical emergency.