Hypotension, or low blood pressure, is a reading below 90/60 mmHg. It can result from something as routine as standing up too fast or as serious as a life-threatening infection. Understanding the cause matters because treatment depends entirely on what’s driving the drop.
Heart Conditions That Lower Blood Pressure
Your heart is the pump that generates blood pressure, so any condition that weakens its pumping ability can cause a sustained drop. Heart failure, heart valve disease, and a slow heart rate (bradycardia) all reduce the volume of blood pushed out with each beat. A heart attack can do this suddenly by damaging a section of heart muscle, leaving it unable to contract effectively.
These cardiac 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, depending on the underlying problem.
Dehydration and Blood Volume Loss
Blood pressure depends on having enough fluid in your blood vessels. When you lose volume through dehydration, heavy bleeding, severe burns, or prolonged vomiting and diarrhea, there simply isn’t enough blood to maintain normal pressure. Even mild dehydration from not drinking enough water on a hot day can cause lightheadedness when you stand.
Nutrient deficiencies can also quietly reduce blood volume over time. When your body lacks vitamin B-12, folate, or iron, it can’t produce enough red blood cells. The resulting anemia means less oxygen-carrying capacity in thinner blood, which contributes to lower pressure readings and symptoms like fatigue and dizziness.
Medications That Drop Blood Pressure
Several common medication classes can cause low blood pressure, sometimes intentionally and sometimes as a side effect. The most obvious culprits are blood pressure drugs themselves. Beta-blockers slow the heart rate and reduce the force of each heartbeat. Alpha-blockers relax blood vessel walls. Both can overshoot their target, especially when a dose is first increased or when combined with other medications.
Diuretics, often called “water pills,” lower blood pressure by pulling fluid out of the body. If they work too aggressively, they can cause dehydration and electrolyte imbalances that lead to dizziness, weakness, and drops in pressure when standing.
Less expected are antidepressants, which can cause low blood pressure and orthostatic hypotension (a sudden drop when you go from sitting to standing) as side effects. If you’ve recently started a new medication and notice dizziness or faintness, the timing is worth mentioning to your prescriber.
Hormonal and Endocrine Causes
Your endocrine system regulates blood pressure through hormones, and when certain glands underperform, pressure can fall. Addison’s disease is one of the clearest examples. The adrenal glands stop producing enough cortisol and, critically, enough aldosterone. Aldosterone is the hormone that balances sodium and potassium to keep blood pressure in a healthy range. Without it, the body loses sodium, fluid volume drops, and blood pressure falls. People with Addison’s disease often notice dizziness or faintness when standing.
An underactive thyroid can also contribute to low blood pressure by slowing the heart rate and reducing the force of heart contractions. Low blood sugar, whether from diabetes management or other causes, triggers a similar drop because the body diverts resources to keep essential organs fueled.
Nervous System Triggers
Vasovagal syncope is the most common form of neurally triggered low blood pressure, and it’s the reason some people faint at the sight of blood or while standing in a hot, crowded room. What happens is counterintuitive: the nervous system initially overreacts with a surge of activity, then abruptly reverses course. The heart slows dramatically, blood vessels widen, and blood pools in the legs instead of reaching the brain.
Several factors make this more likely to happen: fatigue, dehydration, emotional stress, pain, and prolonged standing. The trigger is essentially a misfiring reflex. Stretch receptors in the heart wall become overactivated, sending signals that slam the brakes on heart rate and blood vessel tone. The result is a rapid drop in pressure and, often, a faint.
Postprandial Hypotension
Some people experience a noticeable blood pressure drop after eating. Normally, your heart rate picks up after a meal to send extra blood to the digestive system, while blood vessels elsewhere tighten to compensate. In postprandial hypotension, that compensation fails. The heart doesn’t speed up enough, the vessels don’t tighten sufficiently, and pressure drops by roughly 20 mmHg or more within one to two hours of a meal.
This is far more common in older adults. As arteries stiffen with age, they lose the flexibility to widen and narrow quickly in response to shifting demands. The result is dizziness, lightheadedness, or even fainting after eating, particularly after large, carbohydrate-heavy meals.
Pregnancy
Blood pressure naturally falls during pregnancy, particularly in the first and second trimesters. The drop starts as early as five to six weeks of gestation, when blood vessels begin to relax and widen. Peripheral vascular resistance decreases substantially, reaching its lowest point around the middle of the second trimester. Diastolic pressure (the bottom number) typically drops 5 to 10 mmHg below pre-pregnancy levels.
At the same time, the body is expanding its blood supply dramatically. Total blood volume increases by roughly 45%, and cardiac output rises by up to 45% by 24 weeks. But plasma volume grows faster than red blood cell production, creating a “physiological anemia” from dilution. This combination of wider blood vessels and diluted blood explains why many pregnant people feel dizzy or faint, especially when standing quickly. Blood pressure usually climbs back toward pre-pregnancy levels during the third trimester.
Severe and Emergency Causes
Three forms of shock can cause dangerous, rapid drops in blood pressure that require immediate treatment.
Septic shock happens when the body’s response to a severe infection spirals out of control. Inflammatory signals flood the bloodstream, causing blood vessels to dilate widely and leak fluid. The body also ramps up production of nitric oxide, a powerful chemical that relaxes blood vessel walls even further. The combined effect is a catastrophic drop in pressure that starves organs of blood flow.
Anaphylactic shock is a severe allergic reaction. Immune cells release a massive wave of histamine, which causes blood vessels to expand and become leaky. Pressure drops within minutes, and the airways can swell shut simultaneously. Common triggers include foods, insect stings, and certain medications.
Hypovolemic shock occurs when the body loses so much blood or fluid that the heart can’t fill adequately between beats. Major trauma, internal bleeding, or severe dehydration from illness can all trigger it. All three types of shock share a core feature: there isn’t enough effective blood volume circulating to maintain pressure, and without intervention, organ damage follows quickly.