 <unk> drugs mimic or enhance at least some of the effects of norepinephrine released by the sympathetic nervous system ; <unk> drugs , in contrast , block at least some of the effects . Both of these are large groups with diverse uses , depending on exactly which effects are enhanced or blocked . Norepinephrine itself is classified as a sympathomimetic drug : its effects when given by intravenous injection of increasing heart rate and force and constricting blood vessels make it very useful for treating medical emergencies that involve critically low blood pressure . 
