| Systematic (IUPAC) name | |
|---|---|
| (R)-3-[-1-hydroxy-2-(methylamino)ethyl]phenol | |
| Clinical data | |
| AHFS/Drugs.com | monograph |
| MedlinePlus | a606008 |
| Pregnancy cat. | B2(AU) C(US) |
| Legal status | GSL (UK) OTC (US) |
| Routes | Oral, intranasal, ophthalmic, intravenous, intramuscular |
| Pharmacokinetic data | |
| Bioavailability | 38% through GI tract |
| Protein binding | 95% |
| Metabolism | Hepatic (monoamine oxidase) |
| Half-life | 2.1 to 3.4 hours |
| Identifiers | |
| CAS number | 59-42-7 61-76-7 (hydrochloride) |
| ATC code | C01CA06 R01AA04, R01AB01, R01BA03, S01FB01, S01GA05 |
| PubChem | CID 6041 |
| IUPHAR ligand | 485 |
| DrugBank | APRD00365 |
| ChemSpider | 5818 |
| UNII | 1WS297W6MV |
| KEGG | D08365 |
| ChEBI | CHEBI:8093 |
| ChEMBL | CHEMBL1215 |
| Chemical data | |
| Formula | C9H13NO2 |
| Mol. mass | 167.205 g/mol |
| SMILES | eMolecules & PubChem |
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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 has recently been marketed as a substitute for pseudoephedrine (e.g., Sudafed (Original Formulation)), but there are claims that oral phenylephrine may be no more effective as a decongestant than a placebo[1] (see questions about effectiveness below).
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Phenylephrine is used as a decongestant sold as an oral medicine, as a nasal spray, or as eye drops. Phenylephrine is now the most common over-the-counter (OTC) decongestant in the United States; oxymetazoline is a more common nasal spray. Pseudoephedrine was historically more common, although its notoriety as a methamphetamine precursor has led some governments to restrict its sale.
Oral phenylephrine is extensively metabolised by monoamine oxidase,[2] an enzyme that is present in the gastrointestinal tract and in the liver. Therefore, compared to IV pseudoephedrine, it has a reduced and variable bioavailability; only up to 38 percent.[3][4] Because phenylephrine is a direct selective α-adrenergic receptor agonist, it does not cause the release of endogenous noradrenaline, as pseudoephedrine does. Therefore, phenylephrine is less likely to cause side effects such as central nervous system stimulation, insomnia, anxiety, irritability, and restlessness. Phenylephrine's effectiveness as a decongestant stems from its vasoconstriction of nasal blood vessels, thereby decreasing blood flow to the sinusoidal vessels, leading to decreased mucosal edema.
Some popular cold remedies containing phenylephrine include:
Australia
Austria
Brazil
China
Shenzhen Oriental Pharmaceutical co.,Ltd.
Canada
India
Iran
Israel
Italy
Mexico
New Zealand
Philippines
South Africa
South Korea
Spain
United Kingdom
United States
As a nasal spray, phenylephrine is available in 1 percent and 0.5 percent concentrations. It causes some rebound congestion effects, similar to oxymetazoline, although possibly to a lesser degree.
Phenylephrine is used as an eye drop to dilate the pupil in order to facilitate visualization of the retina. It is often used in combination with tropicamide. Narrow-angle glaucoma is a contraindication to phenylephrine use.
Phenylephrine is sometimes used as a vasopressor to increase the blood pressure in unstable patients with hypotension. Such use is more common in anesthesia or critical-care practices; phenylephrine is especially useful in counteracting the hypotensive effect of epidural and subarachnoid anesthetics. It also has the advantage of not being inotropic or chronotropic, and 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 hours.
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.
The primary side effect of phenylephrine is hypertension. Patients with hypertension are typically advised to avoid products containing phenylephrine. Prostatic hyperplasia can also be symptomatically worsened by use, and chronic use can lead to rebound hyperemia.[9] Patients with a history of epilepsy and on anticonvulsant medication should not take this substance. The drug interaction might produce seizures. Some patients have been shown to have an upset stomach, severe abdominal cramping, and vomiting issues connected to taking this drug.
Extended use may cause rhinitis medicamentosa, a condition of rebound nasal congestion.
Pseudoephedrine and phenylephrine are both used as decongestants; and, until recently, pseudoephedrine was much more commonly available in the United States. This has changed because provisions of the Combat Methamphetamine Epidemic Act of 2005 placed restrictions on the sale of pseudoephedrine products in order to prevent the clandestine manufacture of methamphetamine. Since 2004, phenylephrine has been increasingly marketed as a substitute for pseudoephedrine; some manufacturers have changed the active ingredients of products to avoid the restrictions on sales.[10] Phenylephrine has been off patent for some time, and there are many generic brands available.
Pharmacists Leslie Hendeles and Randy Hatton of the University of Florida suggested in 2006 that oral phenylephrine is ineffective as a decongestant at the 10-mg dose used, arguing that the studies used for the regulatory approval of the drug in the United States in 1976 were inadequate to prove effectiveness at the 10-mg dose and safety at higher doses.[11] Other pharmacists have expressed concerns over phenylephrine's effectiveness as a nasal decongestant,[1] and other clinicians have indicated concern for regulatory actions that reduced the availability of pseudoephedrine.[12][13] A subsequent meta-analysis by the same researchers concluded that there is insufficient evidence for its effectiveness,[14] though another meta-analysis published shortly thereafter by researchers from GlaxoSmithKline found the standard 10 mg dose to be significantly more effective than a placebo.[15] Additionally, two studies published in 2009 examined the effects of phenylephrine on symptoms of allergic rhinitis by exposing sufferers to pollen in a controlled, indoor environment. Neither study was able to distinguish between the effects of phenylephrine or a placebo.[16][17] Pseudoephedrine[16] and loratadine-montelukast therapy[17] were found to be significantly more effective than both phenylephrine and placebo.
The Food and Drug Administration has stood by its 1976 approval of phenylephrine for nasal congestion as the debate continues.[10]
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