Low blood pressure, or hypotension, is generally defined as a reading below 90/60 mmHg. But the number alone doesn’t always tell the full story. A drop of just 20 mmHg from your normal baseline can be enough to make you feel dizzy or faint, even if your reading technically stays above that threshold. The causes range from everyday factors like dehydration to serious medical conditions that need prompt attention.
Dehydration and Low Blood Volume
Your blood pressure depends partly on how much fluid is circulating through your blood vessels. When you lose more fluid than you take in, whether from not drinking enough water, heavy sweating, vomiting, or diarrhea, your blood volume drops. With less fluid pushing against artery walls, your pressure falls. This is one of the most common and most fixable causes of low blood pressure, especially during hot weather or after intense exercise.
Blood loss works the same way. Significant bleeding from an injury, surgery, or internal source like a stomach ulcer rapidly reduces blood volume and can cause a dangerous pressure drop. Even heavy menstrual periods can contribute to lower blood pressure over time if the resulting blood loss isn’t offset by adequate hydration and nutrition.
Heart Problems
Your heart is the pump that generates blood pressure, so anything that weakens or disrupts its pumping ability can lower your readings. A very slow heart rate (bradycardia) means less blood is pushed out per minute. Heart valve problems can prevent efficient blood flow. Heart failure, where the heart muscle becomes too weak to pump adequately, directly reduces the force behind each beat. A heart attack can cause a sudden drop if it damages enough muscle tissue to impair output.
Medications
Several common types of medication lower blood pressure as either their intended effect or a side effect. Diuretics (water pills) reduce blood volume by increasing urine output. Blood pressure medications can sometimes work too well, pushing readings lower than intended. Antidepressants, drugs for Parkinson’s disease, and erectile dysfunction medications can all contribute. If you notice dizziness or lightheadedness after starting or adjusting a medication, the drug itself is a likely culprit.
Hormonal and Endocrine Causes
Several glands in your body produce hormones that help regulate blood pressure, and problems with those glands can cause it to drop. The adrenal glands, which sit on top of your kidneys, produce a hormone called aldosterone that balances sodium and potassium levels to keep blood pressure in a healthy range. In Addison’s disease, the adrenal glands don’t produce enough of these hormones, leading to low blood pressure, fatigue, and dizziness upon standing.
An underactive thyroid can also lower blood pressure, as can low blood sugar. Diabetes sometimes damages the nerves that help regulate blood pressure, creating drops that are especially noticeable when changing positions.
Drops When Standing Up
If your blood pressure falls primarily when you stand up from sitting or lying down, you’re experiencing orthostatic hypotension. When you stand, gravity pulls 500 to 1,000 mL of blood (roughly one to two pints) into your legs and abdomen. Normally, your body compensates within seconds by tightening blood vessels and slightly increasing heart rate. When that reflex is too slow or too weak, your brain briefly loses adequate blood flow, causing dizziness, blurred vision, or fainting.
This type of drop becomes more common with age, dehydration, prolonged bed rest, and certain medications. It’s also a hallmark of conditions that damage the autonomic nervous system, like Parkinson’s disease and some forms of diabetes.
Drops After Eating
Postprandial hypotension, a blood pressure drop after meals, primarily affects older adults. One study found that 24% of hospitalized elderly patients who experienced fainting or falls had this condition. Digestion naturally redirects blood flow toward the gut, and in most people the body compensates by raising heart rate and tightening blood vessels elsewhere. In postprandial hypotension, that compensation is inadequate, not because an unusual amount of blood pools in the digestive tract, but because the body’s corrective response simply isn’t strong enough.
Eating smaller, more frequent meals and limiting high-carbohydrate foods can reduce these episodes.
Pregnancy
Blood pressure commonly drops during the first and second trimesters of pregnancy. The circulatory system expands rapidly to supply the developing baby, and blood vessels dilate in response to hormonal changes. In a study tracking blood pressure patterns throughout pregnancy, about 80% of women had consistently low readings. Blood pressure typically dips lowest during the second trimester before gradually returning to pre-pregnancy levels in the third trimester. This drop is usually harmless, though it can cause dizziness, especially when standing quickly.
Nutritional Deficiencies
Your body needs vitamin B12 and folate to produce properly functioning red blood cells. Without enough of these nutrients, it produces red blood cells that are oversized and inefficient at carrying oxygen. The resulting anemia can cause dizziness, fatigue, and shortness of breath, and it contributes to lower blood pressure because the blood is less effective at delivering oxygen to tissues. This type of deficiency is more common in people with restricted diets, digestive conditions that impair nutrient absorption, and older adults.
Severe Infections and Allergic Reactions
Some of the most dangerous causes of low blood pressure involve sudden, body-wide emergencies. In septic shock, a severe infection triggers an overwhelming immune response. Bacterial toxins cause blood vessels to dilate dramatically, creating what’s essentially a mismatch: the same amount of blood now has to fill a much larger vascular space. The result is a sharp drop in pressure even though blood volume hasn’t changed. Excess nitric oxide production during sepsis further relaxes blood vessels and can damage the energy-producing structures inside cells.
Anaphylaxis, a severe allergic reaction to foods, insect stings, or medications, works through a similar mechanism. The immune system floods the body with chemicals that cause widespread vessel dilation and fluid leakage from blood vessels into surrounding tissues. Blood pressure can plummet within minutes. Both conditions are medical emergencies requiring immediate treatment.
When Low Blood Pressure Is Normal
Not everyone with readings below 90/60 has a problem. Some people, particularly young, fit adults, naturally run low and feel perfectly fine. Athletes often have lower resting blood pressure because their hearts pump more efficiently. In the absence of symptoms like dizziness, fainting, blurred vision, nausea, or unusual fatigue, low blood pressure on its own is generally not a concern and may even be protective against long-term cardiovascular damage.
The key distinction is whether your blood pressure is causing symptoms or dropping from its usual level. A reading of 85/55 in someone who always runs low is very different from that same reading in someone who normally sits at 120/80.