When your body reacts to a stimulation of the vagus nerve, usually through vasodilation. In other words, when your vagus nerve spasms, your blood vessel walls relax sharply. This causes a sudden large drop in blood pressure, and if you are upright, can cause fainting as the brain is deprived of blood.

This most commonly occurs to people when they are having their veins pierced. While I don't know if it ever happens to IV drug users new or experienced, I can say with assurance that it can happen when having blood drawn, especially if they need to try multiple times. Not fun. Symptoms typically include blackouts, shortness of breath, cold sweat, dizziness, nausea.

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