Фенилэфрин: Эффективность под вопросом и многообразие применений.

Decongestant medication

Phenylephrine is used as an alternative to pseudoephedrine as a decongestant, whose availability has been restricted in some countries due to a potential for use in the illicit synthesis of methamphetamine. Its efficacy as an oral decongestant has been questioned, with several independent studies finding that it provided no more relief to sinus congestion than a placebo. A 2007 meta analysis concluded that the evidence for its effectiveness is insufficient, though another meta analysis published shortly thereafter by researchers from GlaxoSmithKline found the standard 10 mg dose to be more effective than a placebo; however, the fact that GSK markets many products containing phenylephrine has raised some speculation regarding selective publishing and other controversial techniques. A 2007 study by Wyeth Consumer Healthcare notes that 7 studies available in 1976 support the efficacy of phenylephrine at a 10 mg dosage. The Food and Drug Administration withdrew the indication "for the temporary relief of nasal congestion associated with sinusitis" in 2007. Phenylephrine has been off patent for some time, and many generic brands are available. In September 2023, an independent advisory committee to the US Food and Drug Administration (FDA) unanimously agreed that there is insufficient evidence showing that "orally administered phenylephrine is effective as a nasal decongestant". The committee also unanimously believes that this does not need further study. The FDA responded to the committee, stating it would take its advice under advisement.

Hemorrhoids are caused by swollen veins in the rectal area. Phenylephrine can be used topically to prevent symptoms of hemorrhoids. Phenylephrine causes the constriction of vascular smooth muscle and is often used in the treatment of hemorrhoids to narrow the swollen veins and relieve the attendant pain. However, veins contain less vascular smooth muscle in their walls than arteries. Products for treatment may also include substances that will form a protective barrier over the inflamed area, resulting in less pain when feces are passed. Phenylephrine hydrochloride at 0.25% is used as a vasoconstrictor in suppository formulations for hemorrhoid treatment.

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 eye drops are applied to the eye after a topical anesthetic is applied.

Phenylephrine has been used as an intracameral injection into the anterior chamber of the eye to arrest intraocular bleeding occurring during cataract and glaucoma surgery.

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 spinal anesthesia, as well as the vasodilating effect of bacterial toxins and the inflammatory response in sepsis and systemic inflammatory response syndrome. The elimination half life of phenylephrine is about 2.5 to 3.0 hours. Because of its vasoconstrictive effect, phenylephrine 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 may cause side effects such as headache, reflex bradycardia, excitability, restlessness and cardiac arrhythmias. The low concentration eye drops do not cause blood pressure changes and the changes with the higher dose drops do not last long.

Prostatic hyperplasia can also be worsened by use, and chronic use can lead to rebound hyperemia. People with a history of anxiety or panic disorders, or on anticonvulsant medication for epilepsy 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. Phenylephrine is pregnancy category C. Due to the lack of studies done in animals and in humans, it is not known whether there is harm to the fetus. Phenylephrine should only be given to pregnant women who have a clear need.

The increase in blood pressure effect of phenylephrine may be increased by drugs such as monoamine oxidase inhibitors, tricyclic antidepressants, and hydrocortisone. Patients taking these medications may need a lower dose of phenylephrine to achieve a similar increase in blood pressure. Drugs that may decrease the effects of phenylephrine may include calcium channel blockers, ACE inhibitors and benzodiazepines. Patients taking these medications may need a higher dose of phenylephrine to achieve a comparable increase in blood pressure.

Phenylephrine is a sympathomimetic drug, which means that it mimics the actions of epinephrine (commonly known as adrenaline) or norepinephrine. Phenylephrine selectively binds to α1 adrenergic receptors which causes venous and arterial vasoconstriction. Whereas pseudoephedrine causes both vasoconstriction and increase of mucociliary clearance through its non specific adrenergic activity, phenylephrine's selective α1 adrenergic receptor agonism causes vasoconstriction alone, creating a difference in their methods of action.

Oral phenylephrine is extensively metabolized by monoamine oxidase, an enzyme that is present on the mitochondrial membrane of cells throughout the body. Compared to oral pseudoephedrine, phenylephrine has a reduced and variable bioavailability; only up to 38%.