Phenylephrine is a medication primarily used as a decongestant, to dilate the pupil, to increase blood pressure, and to relieve hemorrhoids. While marketed as a decongestant, taken by mouth at recommended doses it is of unclear benefit for hay fever. It can be taken by mouth, given by injection into a vein or muscle, or applied to the skin.Common side effects when taken by mouth or injected include nausea, headache, and anxiety. Use on hemorrhoids is generally well tolerated. Severe side effects may include a slow heart rate, intestinal ischemia, chest pain, kidney failure, and tissue death at the site of injection. It is unclear if use during pregnancy or breastfeeding is safe. Phenylephrine is a selective α1-adrenergic receptor activator which results in the constriction of both arteries and veins.Phenylephrine was patented in 1927 and came into medical use in 1938. It is available as a generic medication. In the United Kingdom the injectable formulation costs the NHS £4 per vial. Unlike pseudoephedrine, abuse of phenylephrine is very uncommon.