Blood pressure is considered low when it drops below 90/60 mmHg. Many things can cause this, from something as simple as not drinking enough water to serious conditions like severe infection or blood loss. For some people, naturally low blood pressure is normal and causes no symptoms at all. It only becomes a concern when it drops enough to reduce blood flow to your brain and organs, causing dizziness, fainting, or fatigue.
Dehydration and Fluid Loss
Your blood pressure depends partly on having enough fluid circulating through your blood vessels. When you’re dehydrated, your total blood volume shrinks, and there’s less fluid pushing against artery walls. The result is lower pressure. This can happen from not drinking enough water, but it also happens during bouts of vomiting or diarrhea, heavy sweating, or excessive urination.
Mild dehydration might cause a slight dip you barely notice. But significant fluid loss can progress to a dangerous condition called hypovolemic shock, where your heart can no longer pump enough blood to your organs. This is one reason severe dehydration, whether from illness, heat exposure, or blood loss, requires urgent treatment.
Standing Up Too Quickly
When you go from lying down or sitting to standing, gravity pulls blood toward your legs. Your body has a built-in system to handle this: pressure sensors in your arteries (called baroreceptors) detect the change within a couple of heartbeats and signal your brain to tighten blood vessels and speed up your heart rate. This keeps blood flowing to your brain.
Sometimes this reflex doesn’t work fast enough or strongly enough. The result is orthostatic hypotension, a temporary drop in blood pressure that causes lightheadedness or even fainting when you stand. It’s more common in older adults, people who are dehydrated, and those taking certain medications. If it happens to you occasionally on a hot day, it’s probably nothing serious. If it happens regularly, it can signal an underlying problem with your nervous system or fluid balance.
Medications
Low blood pressure is a side effect of many common medications. Blood pressure drugs themselves can sometimes overshoot their goal, especially diuretics (water pills), which lower pressure by reducing fluid volume. Beta-blockers slow the heart rate, which can drop pressure too far in some people. Other culprits include medications for depression, erectile dysfunction drugs, and drugs used to treat Parkinson’s disease. Even over-the-counter pain relievers can contribute if combined with other medications.
If you’ve recently started or changed a medication and notice dizziness or lightheadedness, the timing is worth paying attention to. Dosage adjustments often solve the problem without needing to switch drugs entirely.
Heart Conditions
Your heart is the pump that generates blood pressure, so anything that weakens its pumping ability can lower your numbers. A very slow heart rate, called bradycardia, means fewer beats per minute pushing blood through your system. This can develop when the heart’s natural pacemaker malfunctions or when electrical signals have trouble traveling from the upper to lower chambers of the heart.
Heart valve problems, heart failure, cardiomyopathy (a weakened heart muscle), and even prior heart attacks can all reduce the heart’s output enough to cause chronically low blood pressure. These conditions don’t just cause low readings on a monitor. They reduce blood flow to your brain, kidneys, and other organs, which is why symptoms like fatigue, confusion, and shortness of breath often accompany them.
Blood Pressure Drops After Eating
Your digestive system needs a surge of blood flow after a meal. Normally, your body compensates by increasing your heart rate and tightening blood vessels elsewhere to keep overall pressure steady. In some people, this compensation falls short, and blood pressure drops noticeably within one to two hours of eating. This is called postprandial hypotension.
It’s surprisingly common in older adults. Research across multiple studies found that roughly 40% of people between ages 65 and 86 experience it. Large, carbohydrate-heavy meals tend to make it worse. Eating smaller, more frequent meals and staying hydrated can help blunt the drop.
Hormonal and Endocrine Problems
Several glands in your body produce hormones that directly influence blood pressure. Your adrenal glands, which sit on top of your kidneys, produce hormones that help regulate salt and fluid balance. When they don’t produce enough (a condition called adrenal insufficiency or Addison’s disease), your body can’t retain enough sodium and water, and blood pressure falls. The drop can be dramatic, particularly when standing.
Thyroid disorders can also play a role. An underactive thyroid slows down many body processes, including heart rate and the responsiveness of blood vessels, both of which contribute to lower blood pressure. In clinical cases, the combination of low thyroid function and adrenal problems has caused blood pressure to plummet from normal ranges while lying down to dangerously low levels upon standing.
Pregnancy
Blood pressure naturally drops during pregnancy, particularly in the first and second trimesters. Research published by the American Heart Association found that in healthy pregnant women, blood pressure steadily decreases until roughly the midpoint of pregnancy, then gradually rises back toward early-pregnancy levels by delivery. The second trimester shows the lowest readings.
This happens because pregnancy triggers the circulatory system to expand rapidly. Blood vessels relax and widen to accommodate increased blood flow to the uterus and placenta, and the overall volume of blood hasn’t yet caught up. For most women, this dip is mild and causes nothing more than occasional lightheadedness. Staying well hydrated and standing up slowly during this period helps.
Severe Infections and Allergic Reactions
Some causes of low blood pressure are medical emergencies. In sepsis, a life-threatening response to infection, the body releases chemicals that cause blood vessels throughout the body to relax and widen dramatically. Tiny blood vessels become leaky, allowing fluid to escape from the bloodstream into surrounding tissues. The combination of widened vessels and lost fluid volume causes blood pressure to plummet, starving organs of the blood they need.
A severe allergic reaction, called anaphylaxis, triggers a similar cascade. Exposure to an allergen like peanuts, bee stings, or certain medications can cause widespread blood vessel dilation and fluid leakage within minutes. Both septic shock and anaphylactic shock require immediate emergency treatment because organs begin to fail quickly without adequate blood flow.
Major blood loss works through a different mechanism but has the same result. Whether from trauma, surgery, or internal bleeding, losing a large volume of blood directly reduces the amount of fluid available to circulate, and pressure drops accordingly.
Nervous System Disorders
Your autonomic nervous system, the part that runs on autopilot, constantly adjusts your heart rate and blood vessel tone to keep pressure stable. Conditions that damage these nerves can impair this regulation. Diabetes is one of the most common causes of this type of nerve damage, but Parkinson’s disease, multiple system atrophy, and certain autoimmune conditions can also disrupt the signals that keep blood pressure where it needs to be.
People with autonomic nerve damage often experience dramatic swings in blood pressure throughout the day, with particularly sharp drops when standing or after meals. The body simply loses its ability to make the rapid adjustments that healthy circulation requires.
Nutritional Deficiencies
Your body needs adequate iron, vitamin B12, and folate to produce red blood cells. When these nutrients are lacking, you develop anemia, meaning your blood carries fewer oxygen-transporting cells. Severe anemia reduces the blood’s ability to deliver oxygen effectively, and the resulting strain on the cardiovascular system can lower blood pressure. Fatigue, pale skin, and shortness of breath typically appear alongside the pressure drop, since the underlying problem is insufficient oxygen reaching your tissues.