In anaphylactic shock, your blood vessels dilate to the point where blood can drain out of your vital organs, including the brain, if you sit or stand up. To prevent this from happening, it is important to lie down with your feet elevated if you are experiencing anaphylaxis.
After using your epinephrine auto-injector, it is important to stay lying down so that the blood (and the medication) can circulate through your body. A recent study found 10 fatalities after people had sat up or been propped upright after using epinephrine. It is also important to be seen by a doctor as soon as possible, in case of further reaction.
Have someone near you call 911 immediately if you are experiencing symptoms of anaphylaxis.
Simons, Keith J; Estelle R. Simons. Epinephrine and its Use in Anaphylaxis: Current Issues Curr Opin Allergy Clin Immunol. 2010;10(4):354-361
Pumphrey RSH. Fatal posture in anaphylactic shock. J Allergy Clin Immunol 2003; 112:451-452.