Decongestants are a class of medications used primarily to alleviate nasal congestion, a common symptom resulting from allergies, colds, sinusitis, and other upper respiratory tract infections. These drugs work by activating α-adrenergic receptors, constricting small blood vessels in the nasal membranes. This action results in the opening of clogged nasal passages, thereby facilitating sinus drainage and relieving congestion.

Most decongestants are readily available over-the-counter in various forms: nasal sprays, nasal drops, and tablets. These can be administered topically directly into the nasal passages or taken orally, depending on the specific medication and the severity of the symptoms.

Decongestants are commonly referred to as sympathomimetics because they are α-adrenergic agonists that mimic the effects of sympathetic nervous system stimulation. They can be classified into two groups based on their chemical composition: sympathomimetic amines (β-phenylethylamine derivatives) and sympathomimetic imidazolines (imidazoline derivatives). Sympathomimetic amines, such as epinephrine, phenylephrine, and pseudoephedrine, exert their effects on multiple receptors, but mainly acting on α₁-adrenoreceptors. On the other hand, sympathomimetic imidazolines, such as oxymetazoline, naphazoline, tetrahydrozoline, and xylometazoline, exert their effects on multiple receptors, but mainly, acting on α₂-adrenoreceptors.

While topical decongestants can provide rapid, short-term relief from nasal congestion, their usage should be limited to no more than three consecutive days. This is to avoid a phenomenon known as rebound nasal congestion, where the nasal congestion returns or even worsens after discontinuing the medication. Topical decongestants may occasionally cause side effects such as nasal burning, stinging, and dryness, although these are typically mild and temporary.

In contrast, oral decongestants can provide longer-lasting relief from nasal congestion. However, because they are systemically absorbed, oral decongestants may result in systemic side effects. These need to be discussed with patients, especially those with cardiovascular disease and uncontrolled hypertension, as these agents can cause cardiovascular stimulation. Common side effects include nervousness, tachycardia, nausea, and insomnia.