What is the most common cause of syncope?

A trigger such as prolonged standing, heat, pain, fear, seeing blood, or emotional stress can cause the nervous system to widen blood vessels and slow the heartbeat. Blood pressure then falls temporarily, reducing blood flow to the brain. People may feel warm, nauseated, sweaty, light-headed, or notice narrowing vision before they lose consciousness.

Fainting can also occur with dehydration, low blood sugar, anaemia, medicines, or a heart-rhythm problem. Atrial fibrillation and other rhythm disturbances are among the conditions a clinician may consider, although the circumstances and examination guide the assessment.

If warning symptoms begin, lie down and raise your legs if possible. If lying down is unsafe, sit and lower your head between your knees. Loosen tight clothing, move away from heat, and do not stand quickly after the symptoms settle.

Someone who has fainted should be placed on their back with their breathing checked. Do not give food or drink until they are fully alert. Arrange medical review after an unexplained faint, a first episode, repeated episodes, or an injury, and record what happened immediately before and after it.

Call emergency services if the person does not wake promptly, is not breathing normally, has chest pain, severe breathlessness, a severe headache, new weakness, confusion, or a serious injury. Urgent assessment is also needed when fainting occurs during exercise, while lying down, or with a racing or irregular heartbeat.

Do not drive until a clinician has assessed an unexplained episode and advised you. A faint may be benign, but the surrounding details can point to a heart, neurological, metabolic, or bleeding-related problem.

Ask which features make reflex fainting more or less likely, whether you need an electrocardiogram or blood tests, and whether any medicine, fluid loss, or meal pattern could have contributed. Ask when you can safely drive, exercise, work at height, or be alone.