The Chemistry of Decongestants

It’s that time of year where a fair few of us (in the Northern Hemisphere, at least) are probably suffering from a cold of some description. The common symptom, that of a blocked nose, is probably one of the most irritating, but thankfully, nasal decongestants exist to provide relief. But how do they work?

Generally, decongestants exert their effects by constricting blood vessels. They act on alpha-adrenergic receptors in the nose and sinuses in order to cause this vasoconstriction, which subsequently helps to reduce inflammation and the formation of mucus in the nose. A number of different drugs can be used to induce these effects, but some of the most commonly used are shown in the graphic. Durations are variable, and can be affected by the precise formulation of the decongestant used.

One of the most effective decongestants is pseudoephedrine. This compound actually occurs naturally in the Ephedra species of plants, but commercially it is produced using a combination of yeast, dextrose and benzaldehyde. As well as acting on alpha-adrenergic receptors, pseudoephedrine acts on receptors which relax smooth muscle in the airways, further aiding the easing of congestion.

Pseudoephedrine can be obtained without a prescription, but in many countries the amount of medication containing it that can be purchased at one time is limited. This is due to the fact that it is a possible chemical precursor to methamphetamine, and can be used to synthesise the potent drug. As a result of this, a number of pharmaceutical companies have reformulated their decongestants to use other alternative drugs as the active ingredient.

One such alternative is phenylephrine. Whilst phenylephrine, like the other decongestants, acts as a vasoconstrictor, there has been dispute over its effectiveness; several studies have suggested it may be no more effective in easing nasal congestion than a placebo. However, another review suggested that collected studies indicated its effectiveness at the doses used in decongestant medications, so the verdict is still inconclusive.

Other alternatives are xylometazoline and oxymetazoline, both of which are supported as effective by research; their duration as decongestants is also roughly comparable to that of pseudoephedrine. Xylometazoline is on the World Health Organisation’s list of essential medicines, whilst oxymetazoline can also be used in the treatment of nosebleeds and eye redness.

Whilst nasal decongestants may seem like a no-brainer when it comes to clearing a blocked nose, they can only be used for a limited amount of time. For the most part, use of nasal sprays for more than a few days can induce rhinitis medicamentosa, more commonly referred to as rebound congestion. The exact manner in which this occurs is largely unknown, but the consequence is that, if these nasal sprays are overused, nasal congestion can worsen when use is stopped. This can lead to a vicious circle, where more of the decongestant is taken to alleviate the congestion, and the rebound congestion continues.

It’s not all bad news though – oral tablets seem to avoid, or at least mitigate, the effects of rebound congestion. So, if you’re currently suffering from a cold and looking for relief, the trick to avoiding continued nasal congestion may well be oral formulations, rather than nasal sprays. That said, anything’s better than nothing, and you can still use nasal sprays for a few days without your congestion worsening.



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References & Further Reading