Low blood pressure, defined as a reading below 90/60 mmHg, happens when your body can’t maintain enough force to push blood through your vessels efficiently. The causes range from something as simple as not drinking enough water to serious conditions like heart problems or severe infections. For many people, naturally low blood pressure is harmless. It only becomes a concern when it causes symptoms like dizziness, fainting, or fatigue, or when it signals an underlying problem.
How Your Body Controls Blood Pressure
Your blood pressure depends on a balancing act between three things: how much blood your heart pumps with each beat, how much total blood is circulating in your system, and how tightly your blood vessels squeeze. Your nervous system constantly adjusts all three in real time. When you stand up, for instance, your brain signals your blood vessels to tighten and your heart to beat a little faster so blood doesn’t pool in your legs.
The sympathetic nervous system (your “fight or flight” wiring) is the main driver here. It speeds up your heart rate and constricts blood vessels to raise pressure when needed. The opposing parasympathetic system slows things down. When either side of this system malfunctions, or when the raw materials it needs (like blood volume or healthy red blood cells) run short, your pressure drops.
Dehydration and Low Blood Volume
One of the most common and straightforward causes of low blood pressure is not having enough fluid in your bloodstream. Your cardiovascular system is essentially a closed loop of pipes, and blood volume is what fills them. When you’re dehydrated from sweating, vomiting, diarrhea, or simply not drinking enough, the volume inside those pipes drops. Less fluid means less pressure pushing against vessel walls.
This is also why significant blood loss from an injury or internal bleeding causes a sharp drop in pressure. Your heart can pump harder and faster to compensate for a while, but if the volume deficit is large enough, it can’t keep up.
Medications That Lower Blood Pressure
Prescription drugs are one of the most frequent culprits behind low blood pressure, especially in older adults. Several broad categories of medication can cause it:
- Diuretics (water pills): These reduce blood volume by making you urinate more. Both thiazide and loop diuretics can cause low pressure, weakness, and dizziness.
- Blood pressure medications: Beta blockers can slow the heart rate enough to lower pressure too much. Calcium channel blockers, ACE inhibitors, and related drugs all carry the same risk, particularly when doses are adjusted.
- Heart medications like nitroglycerin: Drugs used for chest pain work by relaxing blood vessels, which can cause sudden pressure drops.
- Antidepressants: Older tricyclic antidepressants and SNRIs commonly cause blood pressure to drop when standing. Even SSRIs do this occasionally.
- Parkinson’s disease medications: Dopamine-related drugs used for Parkinson’s frequently cause low blood pressure as a side effect.
- Drugs for psychosis or severe agitation: These block receptors in blood vessels that help maintain pressure.
If you’ve recently started a new medication or changed a dose and notice dizziness or lightheadedness, the drug is a likely explanation.
Heart Conditions
Your heart is the pump driving the whole system, so problems with how it beats or how its valves function can directly reduce blood pressure. Bradycardia, an unusually slow heart rate, is one example. Your heart has a natural pacemaker that generates electrical signals to trigger each beat. When those signals slow down or get blocked on their way through the heart’s chambers, the heart may not pump enough oxygen-rich blood to meet your body’s needs. Symptoms include dizziness, confusion, extreme fatigue during activity, and fainting.
Heart valve problems work differently but produce a similar result. If a valve doesn’t open fully or doesn’t close properly, the heart moves blood less efficiently with each beat, reducing the overall pressure in your arteries. Heart failure and heart attacks can also weaken the pump itself.
Hormonal and Endocrine Problems
Your adrenal glands, which sit on top of your kidneys, produce hormones that are essential for blood pressure regulation. Cortisol helps control blood pressure directly, and aldosterone manages 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 (sometimes called Addison’s disease), these hormone levels fall too low. Without enough cortisol, blood pressure can drop dangerously during physical stress like illness or injury, sometimes alongside low blood sugar and dangerously high potassium levels. Thyroid disorders can also affect blood pressure. An underactive thyroid slows the heart, while other hormonal shifts can reduce blood volume or relax blood vessels.
Nutritional Deficiencies and Anemia
Your body needs vitamin B12 and folate to produce healthy red blood cells. When you’re deficient in either nutrient, your body makes red blood cells that are too large and don’t carry oxygen efficiently. This type of anemia forces the heart to work harder to deliver oxygen, and in many cases, blood pressure falls. Iron deficiency anemia works through a similar pathway. The telltale signs overlap heavily with low blood pressure symptoms: tiredness, shortness of breath, and dizziness.
Orthostatic Hypotension: Pressure Drops When You Stand
If you feel dizzy or lightheaded specifically when you stand up from sitting or lying down, you likely have orthostatic hypotension. It’s diagnosed when your top blood pressure number drops by 20 mmHg or more, or the bottom number drops by 10 mmHg or more, within two to five minutes of standing.
When you go from horizontal to vertical, gravity pulls blood toward your legs. Normally your nervous system compensates almost instantly by tightening blood vessels and bumping up your heart rate. But if that reflex is sluggish (due to aging, medications, dehydration, or nerve damage from conditions like diabetes), your brain temporarily loses adequate blood flow. The result is that familiar head rush, or in worse cases, a full faint. Orthostatic hypotension becomes more common with age and is a significant risk factor for falls.
Postprandial Hypotension: Pressure Drops After Eating
After a meal, your body directs extra blood to your digestive tract to process food. Your heart rate normally rises to compensate, and blood vessels elsewhere in your body tighten to keep overall pressure stable. In some people, particularly older adults, this compensatory response doesn’t work well enough. The heart doesn’t speed up as much as it should, and blood vessels don’t constrict adequately, so systemic blood pressure falls. Large, carbohydrate-heavy meals tend to trigger this more than smaller ones.
Severe Drops: Shock and Emergencies
In emergency situations, blood pressure can plummet to life-threatening levels. During distributive shock, which includes septic shock from severe infection and anaphylaxis from allergic reactions, blood vessels throughout the body suddenly relax and become extremely dilated. This drastically reduces the pressure driving blood to vital organs like the brain, heart, and kidneys. On top of that, the smallest blood vessels (capillaries) often become leaky, allowing fluid to escape the circulation entirely, which compounds the volume loss.
Severe bleeding, major heart attacks, and extreme dehydration can all push blood pressure low enough to become a medical emergency. The common thread is that the body’s normal compensatory mechanisms get overwhelmed.
When Low Blood Pressure Is Normal
Not every low reading is a problem. If your blood pressure consistently runs at or just below 90/60 and you feel fine, it’s likely just your baseline. Athletes and people who exercise regularly often have lower resting blood pressure as a sign of cardiovascular efficiency. Low blood pressure without symptoms, or with only mild symptoms, rarely needs treatment. The reading matters most when it’s a change from your usual pattern or when it comes with dizziness, fainting, blurred vision, nausea, or unusual fatigue. Those symptoms mean your organs aren’t getting the blood flow they need, and that’s worth investigating.