Phenylephrine is a selective α1-adrenergic receptor agonist used primarily as a decongestant, as an agent to dilate the pupil, and to increase blood pressure. Phenylephrine is marketed as a substitute for the decongestant pseudoephedrine, though clinical studies differ regarding its effectiveness in this role.
Phenylephrine is used as a decongestant sold as an oral medicine, as a nasal spray, or as eye drops. It is now the most common over-the-counter decongestant in the United States; oxymetazoline is a more common nasal spray.
Oral phenylephrine is extensively metabolised by monoamine oxidase, an enzyme that is present in the intestinal wall and in the liver. Compared to intravenous pseudoephedrine, it has a reduced and variable bioavailability; only up to 38%. Because phenylephrine is a selective α-adrenergic receptor agonist it does not cause the release of endogenous noradrenaline. Nor does it increase the rate (chronotropy) and strength (inotropy) of heart contractions, as pseudoephedrine does. Phenylephrine may cause side effects such as headache, reflex bradycardia, excitability, restlessness and cardiac arrhythmias.
Phenylephrine is used as a replacement for pseudoephedrine in decongestant medicines due to pseudoephedrine’s use in the illicit manufacture of methamphetamine. Its efficacy as an oral decongestant has been questioned, with multiple studies not being able to come to an agreement. Whereas pseudoephedrine causes both vasoconstriction and increase of mucociliary clearance through its nonspecific adrenergic activity, phenylephrine’s selective α-adrenergic agonism causes vasoconstriction alone, creating a difference in their methods of action.
As a nasal spray, phenylephrine is available in 1% and 0.5% concentrations. It may cause rebound congestion, similar to oxymetazoline.
This medication is used to temporarily relieve swelling, burning, pain, and itching caused by hemorrhoids. It works by temporarily narrowing the blood vessels in the area. This effect decreases swelling and discomfort. Some products may also contain substances (e.g., cocoa butter, hard fat, mineral oil, shark liver oil) that form a protective barrier to prevent too much irritating contact with stool.
Phenylephrine is used as an eye drop to dilate the pupil to facilitate visualization of the retina. It is often used in combination with tropicamide as a synergist when tropicamide alone is not sufficient. Narrow-angle glaucoma is a contraindication to phenylephrine use. As a mydriatic, it is available in 2.5% and 10% minims.
Phenylephrine is commonly used as a vasopressor to increase the blood pressure in unstable patients with hypotension, especially resulting from septic shock. Such use is common in anesthesia or critical-care practices; it is especially useful in counteracting the hypotensive effect of epidural and subarachnoid anesthetics, as well as the vasodilating effect of bacterial toxins and the inflammatory response in sepsis and systemic inflammatory response syndrome. It has the advantage of not being inotropic or chronotropic, so it strictly elevates the blood pressure without increasing the heart rate or contractility (reflex bradycardia may result from the blood pressure increase, however). This is especially useful if the heart is already tachycardic and/or has a cardiomyopathy. The elimination half life of phenylephrine is about 2.5 to 3.0 hours.
Because of its vasoconstrictive effect, phenylephrine (Neo-Synephrine) can cause severe necrosis if it infiltrates the surrounding tissues. Because of this, it should be given through a central line if at all possible. Damage may be prevented or mitigated by infiltrating the tissue with the alpha blocker phentolamine by subcutaneous injection.
Phenylephrine hydrochloride at 0.25% is used as a vasoconstrictor in some suppository formulations.
Phenylephrine is used by urologists to abort priapism. It is diluted significantly and injected directly into the corpora cavernosa. The mechanism of action is to cause constriction of the blood vessels entering into the penis, thus breaking the pathophysiologic cycle that continues the priapism.