Blood pressure drops when your heart pumps less forcefully, your blood vessels relax too widely, or you don’t have enough fluid in your circulation. Sometimes it’s harmless, like standing up too fast after sitting for a while. Other times it signals something that needs attention, from dehydration to a heart condition. The causes range from everyday and temporary to serious and ongoing.
Dehydration and Blood Loss
Your blood is mostly water, so anything that shrinks your fluid volume directly reduces the pressure inside your vessels. Dehydration from illness, intense exercise, heat exposure, or simply not drinking enough is one of the most common reasons blood pressure dips. Vomiting, diarrhea, and heavy sweating can all pull enough fluid out of circulation to make you lightheaded.
Blood loss works the same way but more urgently. Whether from an injury, surgery, or internal bleeding (like a stomach ulcer), losing blood means less volume for your heart to pump. In its most severe form, called hypovolemic shock, the fluid loss is so significant that the heart can no longer deliver blood to organs effectively. A telling early sign is dizziness that appears when you shift from sitting to standing, because your depleted circulation can’t adjust to the change in posture.
Standing Up Too Quickly
When you stand, gravity pulls blood toward your legs. Normally, pressure sensors in your carotid arteries and aortic arch detect this shift instantly. They send signals through cranial nerves to your brain stem, which responds by tightening blood vessels and slightly increasing your heart rate. The whole correction happens in seconds, and you never notice it.
When that reflex doesn’t work properly, blood pools in your legs and your blood pressure falls. This is called orthostatic hypotension, and it’s especially common in older adults whose reflexes have slowed, people who are dehydrated, and those on blood pressure medications. The result is a wave of dizziness, blurred vision, or even fainting within a few seconds of standing.
Eating a Large Meal
Your digestive system needs a surge of blood flow after you eat, so your heart rate normally increases and blood vessels elsewhere in your body tighten to compensate. When that compensation falls short, blood pressure drops after meals. This is called postprandial hypotension, and roughly 40% of adults between ages 65 and 86 experience it. Risk factors include diabetes, heart failure, Parkinson’s disease, and high blood pressure itself, which may seem counterintuitive but reflects the same underlying problem with blood vessel regulation.
Medications
Several categories of medication lower blood pressure as either their intended effect or a side effect:
- Blood pressure drugs: Diuretics (water pills), beta blockers, and alpha blockers all reduce pressure through different mechanisms. If the dose is too high or you become dehydrated, they can overshoot.
- Parkinson’s medications: Drugs containing levodopa commonly cause blood pressure to drop, particularly when standing.
- Certain antidepressants: Older tricyclic antidepressants are known to lower blood pressure as a side effect.
- Erectile dysfunction drugs: These relax blood vessels by design. Combining them with heart medications like nitroglycerin can cause a dangerous pressure drop.
If you’ve recently started or changed a medication and notice dizziness or lightheadedness, the timing is worth mentioning to whoever prescribed it. Dose adjustments often solve the problem.
Heart Problems
Your heart is the pump behind your blood pressure, so anything that weakens it reduces the force driving blood through your body. A very slow heart rate, called bradycardia, means fewer beats per minute to push blood forward. It can result from aging-related damage to heart tissue, prior heart attacks, inflammation of the heart muscle, or congenital heart defects. When the heart rate drops low enough, it can’t deliver sufficient oxygen-rich blood to the brain and other organs, causing dizziness, fatigue, weakness, and shortness of breath.
Heart valve problems work differently but produce a similar result. If a valve doesn’t open fully or doesn’t close properly, less blood moves forward with each beat. Heart failure, where the heart muscle has weakened overall, also reduces the volume of blood pumped per contraction. All of these conditions lower what’s called cardiac output, and blood pressure falls as a consequence.
Nervous System Misfires
Sometimes the nervous system sends the wrong signal at the wrong time. In vasovagal syncope, the nerve that regulates heart rate and blood vessel tone overreacts to a trigger. Your heart rate slows, blood vessels in your legs widen, blood pools downward, and pressure plummets. The brain loses blood flow, and you faint.
Common triggers include standing for long periods, heat exposure, seeing blood, having blood drawn, fear of injury, and straining on the toilet. Most people recover quickly once they’re lying down, since gravity redistributes blood back toward the brain. Vasovagal episodes are usually not dangerous on their own, but the fall that comes with fainting can be.
Hormonal and Adrenal Causes
Your adrenal glands, which sit on top of your kidneys, produce hormones that play a direct role in blood pressure regulation. Cortisol helps control blood pressure. Aldosterone maintains the balance of sodium and potassium in your blood, which in turn controls how much water your body retains and how stable your pressure stays.
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 or illness. In severe cases, cortisol levels drop so low that blood pressure becomes life-threateningly low, blood sugar falls, sodium drops, and potassium climbs to dangerous levels. Thyroid disorders can also affect blood pressure, since thyroid hormones influence heart rate and blood vessel tone.
Pregnancy
Blood pressure typically falls during pregnancy, especially in the first and second trimesters. Your blood vessels expand to accommodate the increased blood volume needed to supply your growing baby, and hormonal shifts relax vessel walls further. Your heart works harder to keep up, but the expanded capacity of your circulatory system often outpaces the heart’s output temporarily. For most pregnant people, this is a normal and expected change. Pressure usually returns to pre-pregnancy levels in the third trimester or after delivery.
Severe Allergic Reactions and Infections
Some of the most dramatic blood pressure drops happen during anaphylaxis or severe infections. In both situations, blood vessels throughout the body dilate massively, becoming what’s described as flaccid or overly relaxed. The pressure driving blood to your organs collapses. On top of that, small blood vessels often become leaky, allowing fluid to escape from the circulation into surrounding tissues, which further reduces blood volume.
In sepsis, a widespread infection triggers this response. In anaphylaxis, a severe allergic reaction does the same. Both are medical emergencies. The combination of extreme vessel dilation and fluid leakage can prevent the heart, brain, and kidneys from receiving enough blood to function, even when the heart itself is pumping normally.
Nutritional Deficiencies
Not getting enough vitamin B12 or folate can lead to anemia, a condition where your body produces red blood cells that are too large and don’t carry oxygen efficiently. When your tissues aren’t getting adequate oxygen, your cardiovascular system is under strain. B12 deficiency can come from diet (it’s found mainly in animal products), from conditions like pernicious anemia where the immune system attacks cells needed to absorb B12, or from digestive disorders that interfere with nutrient absorption. Folate deficiency typically results from a diet low in leafy greens, beans, and fortified grains. Correcting the deficiency with dietary changes or supplements usually resolves the blood pressure issue over time.