Orthostatic hypotension, also known as postural hypotension, is a medical condition wherein a person's blood pressure drops when they are standing up (orthostasis) or sitting down. Primary orthostatic hypotension is also often referred to as neurogenic orthostatic hypotension. The drop in blood pressure may be sudden (vasovagal orthostatic hypotension), within 3 minutes (classic orthostatic hypotension) or gradual (delayed orthostatic hypotension). It is defined as a fall in systolic blood pressure of at least 20 mmHg or diastolic blood pressure of at least 10 mmHg after 3 minutes of standing. It occurs predominantly by delayed (or absent) constriction of the lower body blood vessels, which is normally required to maintain adequate blood pressure when changing the position to standing. As a result, blood pools in the blood vessels of the legs for a longer period, and less is returned to the heart, thereby leading to a reduced cardiac output and inadequate blood flow to the brain.

Very mild occasional orthostatic hypotension is common and can occur briefly in anyone, although it is prevalent in particular among the elderly and those with known low blood pressure. Severe drops in blood pressure can lead to fainting, with a possibility of injury. Moderate drops in blood pressure can cause confusion/inattention, delirium, and episodes of ataxia. Chronic orthostatic hypotension is associated with cerebral hypoperfusion that may accelerate the pathophysiology of dementia. Whether it is a causative factor in dementia is unclear.

The numerous possible causes for orthostatic hypotension include certain medications (e.g. alpha blockers), autonomic neuropathy, decreased blood volume, multiple system atrophy, and age-related blood-vessel stiffness.

Apart from addressing the underlying cause, orthostatic hypotension may be treated with a recommendation to increase salt and water intake (to increase the blood volume), wearing compression stockings, and sometimes medication (fludrocortisone, midodrine, or others). Salt loading (dramatic increases in salt intake) must be supervised by a doctor, as this can cause severe neurological problems if done too aggressively.

To maintain sufficient blood pressure, the body has several compensatory mechanisms. Baroreceptors, a kind of mechanoreceptors, play a crucial role in conveying data about blood pressure in the autonomic nervous system. The data is conveyed to regulate the peripheral resistance and heart output, keeping blood pressure within an established normal limit. There are two kinds of baroreceptors: high-pressure arterial baroreceptors and low-pressure volume receptors, both activated by the stretching of vessel walls. Arterial baroreceptors are situated in the carotid sinuses and the aortic arch, while the low-pressure volume receptors, known as cardiopulmonary receptors, are in the atria, ventricles, and pulmonary vasculature. Arterial baroreceptors detect changes in blood pressure and transmit this information to the brainstem, the nucleus of the solitary tract, which modulates the activity of the autonomic nervous system (ANS). This results in decreased blood pressure, which leads to an increase in heart rate. What's more, the venoarteriolar axon reflex, which results in the constriction of arterial flow to muscles, skin, and adipose tissue also helps stabilize blood pressure.

Orthostatic hypotension is characterized by symptoms that occur after standing (from lying or sitting), particularly when done rapidly. Many report lightheadedness (a feeling that one might be about to faint), sometimes severe, or even actual fainting with associated fall risk. With chronic orthostatic hypotension, the condition and its effects may worsen even as fainting and many other symptoms become less frequent. Generalized weakness or tiredness may also occur. Some also report difficulty concentrating, blurred vision, tremulousness, vertigo, anxiety, palpitations (awareness of the heartbeat), unsteadiness, feeling sweaty or clammy, and sometimes nausea. A person may look pale. Some people may experience severe orthostatic hypotension with the only symptoms being confusion or extreme fatigue. Chronic severe orthostatic hypotension may present as fluctuating cognition/delirium. ^{[citation needed]} Women who are pregnant are also susceptible to orthostatic hypotension.

The disorder may be associated with Addison's disease, atherosclerosis (build-up of fatty deposits in the arteries), diabetes, pheochromocytoma, porphyria, long COVID, and certain neurological disorders, including autoimmune autonomic ganglionopathy, multiple system atrophy, and other forms of dysautonomia. It is also associated with Ehlers–Danlos syndrome and anorexia nervosa. It is also present in many patients with Parkinson's disease or Lewy body dementias resulting from sympathetic denervation of the heart or as a side effect of dopaminomimetic therapy. This rarely leads to fainting unless the person has developed true autonomic failure or has an unrelated heart problem.^{[citation needed]}

Another disease, dopamine beta hydroxylase deficiency, also thought to be underdiagnosed, causes loss of sympathetic noradrenergic function and is characterized by low or extremely low levels of norepinephrine, but an excess of dopamine.