Phenylephrine Other names: Neo-synephrine Phenylephrine hydrochloride

Phenylephrine is a sympathomimetic agent with mainly direct effects on adrenergic receptors. It has predominantly alpha adrenergic activity and is without significant stimulating effects on the central nervous system at usual doses.

The sympathomimetic effect of phenylephrine produces vasoconstriction which in turn relieves nasal congestion.

After injection it produces peripheral vasoconstriction and increased arterial pressure; it also causes reflex bradycardia.

Absorption

Phenylephrine is readily absorbed after oral administration but is subject to extensive presystemic metabolism, much of which occurs in the enterocytes. As a consequence, systemic bioavailability is only about 40%.

Following oral administration, peak plasma concentrations are achieved in 1-2 hours. The mean plasma half life is in the range 2-3 hours. Penetration into the brain appears to be minimal.

When injected subcutaneously or intramuscularly, phenylephrine takes 10 to 15 minutes to act. Subcutaneous and intramuscular injections are effective for up to about one and up to about two hours respectively. Intravenous injections are effective for up to about 20 minutes.

Biotransformation

Phenylephrine is metabolised in the liver by monoamine oxidase. Both phenylephrine and its metabolites are excreted in the urine.

The volume of distribution is between 200 and 500 litres, but there are no data on the extent of plasma protein binding.

Elimination

The metabolites, their route and rate of excretion have not been identified.

Phenylephrine has been used to induce cardiac myocyte hypertrophy in cultures of rat neonatal mycocytes at doses of 100 µM and 10 µM. To the best of our knowledge there have been no human studies associating therapeutic phenylephrine use with the development of cardiac myocyte hypertrophy.