Phenylephrine and pseudoephedrine are both sympathomimetic drugs which means that they imitate the action of neurotransmitter substances of the sympathetic nervous system like epinephrine, dopamine, catecholamines etc.

Difference in action

Both these drugs act on adrenergic receptor system. The adrenergic receptor system stimulates the sympathetic nervous system which is responsible for fight or flight responses in the body. Pseudoephedrine has direct action on adrenergic receptor system i.e. it has action on α and β2-adrenergic receptors which cause vasoconstriction i.e. constriction of blood vessels and relaxation of smooth muscles in the bronchial tubes of lungs respectively, whereas phenylephrine is a selective α1-adrenergic receptor agonist.

Use in medical conditions

Both these drugs are mainly used as decongestants i.e. to relieve nasal congestion, but their result varies due to difference in mode of action. As pseudoephedrine acts on α- receptors situated on the muscle lining the walls of blood vessels and cause vasoconstriction, due to which the blood vessels constrict. Therefore the quantity of fluid that leaves the blood vessels and enters the nose, throat and sinuses is reduced. As a result, inflammation of the nasal membranes and the mucus production is reduced, thereby relieving the nasal congestion. At the same time due to its action on β2-adrenergic receptors, the smooth muscles of bronchi relax leading to dilation of the the bronchial tubes; hence relieving both the congestion and difficulty in breathing.

Since phenylephrine acts only on α1-adrenergic receptors, it only helps in relieving the nasal congestion. Hence, we see that pseudoephedrine is a better choice as a decongestant, but as it belongs to the class of amphetamine drugs, it acts as a good stimulant too and promotes sleeplessness. Due to this stimulant action it causes side effects like insomnia, dizziness, nervousness, anxiety, palpitations etc. It is not given to patients suffering from diabetes mellitus, hypertension, coronary artery disease, panic attacks etc.

Pseudoephedrine is used in treating nasal, sinus and Eustachian tube congestion.

Phenylephrine is used in treatment of hemorrhoids because of its vasoconstricting properties, which causes narrowing of the blood vessels and thereby reduces the pain caused due to hemorrhoids. It is used in treatment of priapism i.e. painful erection of penis, which does not return to its flaccid state owing to dilation of blood vessels. When phenylephrine is injected, it reduces the blood flow and relieves priapism.

It is also used as a vasopressor i.e. an agent that is used to increase the reduced blood pressure due to its ability to cause constriction of blood vessels. At the same time, this causes increase in blood pressure, hence limiting the use in people suffering from high blood pressure. This drug is also used as an eye drop to dilate pupils i.e. as a mydriatic agent. It is generally used by ophthalmologists for visualization of the retina.

Summary

Despite pseudoephedrine being a more effective decongestant, it is being replaced by phenylephrine. This is because of it being used in illegal manufacturing of methamphetamine as it belongs to the amphetamine class of drugs. In low doses methamphetamine causes mood elevation, increases alertness, concentration and energy in fatigued people and can lead to addiction too. Often, pseudoephedrine is used for its stimulant properties by various athletes to enhance their performance. In order to curb this rampant usage of this drug, various laws have been passed in different countries for procuring this drug from the pharmacies.

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