Low blood pressure, or hypotension, happens when blood moves through your arteries with less force than normal. A reading below 90/60 mmHg is generally considered low. For many people, naturally low blood pressure causes no problems at all. But when it drops suddenly or produces symptoms like dizziness, fainting, or blurred vision, something specific is usually driving it.
Dehydration and Blood Volume Loss
One of the most common and straightforward causes is simply not having enough fluid in your bloodstream. Your blood pressure depends partly on blood volume: less fluid means less pressure pushing against artery walls. Dehydration from illness, heat exposure, heavy exercise, or not drinking enough water is a frequent culprit.
Mild dehydration typically doesn’t change your blood pressure reading at rest. At moderate levels, your pressure may still look normal while sitting but drops when you stand up. Only with severe dehydration does blood pressure fall noticeably in all positions. Blood loss from an injury, surgery, or internal bleeding works the same way, reducing the volume of fluid your heart has to pump.
Medications That Lower Blood Pressure
Several common medications can push blood pressure lower than intended. The most obvious are blood pressure drugs themselves, including diuretics (water pills), alpha blockers, and beta blockers. If your dose is slightly too high or you become dehydrated while taking these, blood pressure can dip below a comfortable range.
Other medications that can cause low readings include:
- Parkinson’s disease medications
- Tricyclic antidepressants, an older class of depression medications
- Erectile dysfunction medications, particularly when combined with nitrate heart medications, which can cause a dangerous drop
If you suspect a medication is behind your symptoms, don’t stop taking it on your own. A dose adjustment or timing change often resolves the problem.
Heart Problems
Your heart is the pump behind blood pressure, so anything that weakens its pumping ability can lower your numbers. A very slow heart rate (bradycardia) means the heart moves less blood per minute. Heart valve problems can prevent blood from flowing efficiently. Heart failure, where the muscle can’t pump with enough force, also reduces the pressure in your arteries.
Heart-related low blood pressure tends to produce persistent symptoms rather than occasional dips. Fatigue, shortness of breath, and swelling in the legs often accompany it, which helps distinguish it from other causes.
Hormonal and Endocrine Disorders
Your adrenal glands sit on top of your kidneys and produce hormones that are directly involved in blood pressure regulation. Cortisol helps your body respond to stress and maintain blood pressure. Aldosterone controls the balance of sodium and potassium in your blood, which in turn controls your body’s salt and water balance to keep pressure stable.
In adrenal insufficiency (Addison’s disease), the adrenal glands don’t produce enough of these hormones. The result can be chronically low blood pressure that worsens during physical stress like illness or injury. During an adrenal crisis, the severe lack of cortisol can cause a life-threatening drop in blood pressure along with dangerously low blood sugar and sodium levels. Thyroid disorders and low blood sugar from other causes can also affect blood pressure, though less dramatically.
Nutritional Deficiencies and Anemia
Your body needs adequate red blood cells to carry oxygen and maintain healthy circulation. Deficiencies in vitamin B12 and folate (vitamin B9) can lead to anemia, a condition where you don’t have enough healthy red blood cells. This reduces the oxygen-carrying capacity of your blood and can contribute to low blood pressure.
B12 deficiency sometimes happens because of a condition called pernicious anemia, where the immune system attacks stomach cells needed to absorb the vitamin. Folate deficiency is more often tied to diet (it’s found mainly in dark green leafy vegetables and liver) or intestinal conditions like celiac disease that interfere with absorption. Heavy alcohol use and certain antiseizure medications can also deplete folate levels.
Pregnancy
Blood pressure naturally drops during pregnancy, and for most women this is completely normal. The circulatory system undergoes major changes starting surprisingly early. Blood vessels begin dilating as early as five weeks of gestation, well before the placenta is fully developed. Arterial pressure typically falls 5 to 10 mmHg below pre-pregnancy levels, reaching its lowest point during the second trimester. Most of the decrease actually happens between six and eight weeks. Pressure usually returns to pre-pregnancy levels by the time of delivery.
Drops After Standing or Eating
Two specific patterns of low blood pressure are tied to everyday activities. Orthostatic hypotension happens when your blood pressure drops after you stand up from sitting or lying down, causing a head rush or brief dizziness. Normally, your body compensates for gravity by tightening blood vessels and slightly increasing heart rate. When that reflex is sluggish, whether from aging, dehydration, medications, or nervous system problems, blood pools in your legs and pressure falls.
Postprandial hypotension is a drop that occurs after eating, most commonly in older adults. Digesting a meal requires extra blood flow to your gut. Your body is supposed to compensate by speeding up heart rate and tightening blood vessels elsewhere, but when those adjustments fall short, blood pressure drops. Large meals, hot weather, dehydration, and not eating enough salt all make it more likely. As arteries stiffen with age, the body has a harder time making these quick adjustments.
Severe and Life-Threatening Causes
Some causes of low blood pressure are medical emergencies. In septic shock, a severe bacterial infection triggers widespread inflammation that causes blood vessels to relax and become leaky. Blood pressure plummets because those flaccid vessels can’t maintain enough pressure to push blood to vital organs like the brain, heart, and kidneys.
Anaphylactic shock works similarly. A severe allergic reaction, to food, insect stings, or medications, causes rapid, extreme dilation of blood vessels along with swelling that can obstruct breathing. Both conditions involve what’s called distributive shock: blood is still in the body, but the vessels have opened so wide that pressure collapses. Without emergency treatment, organs begin to fail from lack of blood flow.
Severe bleeding, whether from trauma or internal sources like a ruptured blood vessel, causes a different type of crisis. Here the problem is straightforward volume loss. The heart simply doesn’t have enough blood to maintain pressure, and symptoms escalate from dizziness to confusion to loss of consciousness as the deficit grows.