The structure of phenylephrine as a line drawing superimposed over a ball model of the same molecule.

Achoo! Sniff! Cough! Yes, we are approaching the season of coughs, colds and other related illnesses! For most of us, this leads to several days of feeling pretty rubbish, moping about, and whinging! We’ve already seen similar symptoms for hay fever and how cetirizine can help them!

A man sneezes while outside representing hay fever and allergies to grass pollen - maybe he needs some cetirizine!
This fetching self-portrait of Phil first appeared in the August 2019 molecule of the month about Cetirizine.

It also leads us to try a range of remedies, medicines and treatments: hot tea with lemon and honey, sunlight, hot showers, and even wet towels.

However, many of us reach for the cough syrup, cold tablets or sachets of powders. A key ingredient in these is the molecule phenylephrine.

An Alternative Medicine

Phenylephrine has been available for medical use since the late since 1930s. Its development came about as an alternative to the very similar molecule pseudoephedrine, which authorities worried was being used to make recreational drugs.

Patients can receive phenylephrine by mouth, injection, and even through the skin. As we will see later, there are many other uses for this molecule than just a decongestant.


Given below is the structure of phenylephrine. It consists of a phenol group (a benzene ring with an -OH group attached) bonded to a chain of carbon and nitrogen atoms. Another -OH group also exists along this chain.

The structure of phenylephrine

At room temperature it is a solid, with an off-white colour, in addition, it is very slightly soluble in water!

Is It Actually Any Good?

Phenylephrine’s main selling point is as a decongestion. The idea is that the molecule helps to shrink swollen nasal mucous membranes, clearing the airwaves and letting you breathe better. However, studies have shown it to be no more effective than a placebo. On the other hand, the closely related pseudoephedrine is effective as a decongestion.

In fact, a recent study presented to the American drug administration (FDA) showed strong evidence that phenylephrine taken orally is ineffective as a decongestant.

Despite this, the molecule is still widely found in cough medication and sold by pharmacies! Part of the reason is that even though pseudoephedrine is more effective, there is a real fear of people making recreational drugs from it!

It’s Not All Bad

The most common use of phenylephrine may be as a decongestion, but it has many other, less controversial, uses.

As a vasopressor, phenylephrine constricts blood vessels, making them narrower. As a result, blood pressure increases, making the molecule useful for people suffering with low blood pressure following shock or anaesthetic.

A Pain In The Bum

Another use is to treat hemorrhoids, a condition where the veins in the rectum (your bum) become swollen! The exact way they do this is still unknown, it may help narrow the swollen veins, but this may not be the whole story!

One part of body phenylephrine can dilate (make wider) is the eyes. This is useful for eye examinations or before and after eye operations!

Undesired Effects

Like most drugs, phenylephrine is not without side-effects.

Given one of its uses is a vasopressor, it is unsurprising that one side effect of phenylephrine is high-blood pressure. As such, people who already have high-blood pressure should avoid using it.

Other side effects include headaches, restlessness and can worsen feelings of anxiety and panic for those who already suffer with them. There are also links to vomiting and stomach cramps.

Who Nose If It Works?

Whether for good or for bad, most molecules usually have a use and are good at it. Phenylephrine, however, is a strange one. Sold as a decongestant, it has become a standard molecule in many cold syrups and tablets, yet there are numerous studies that indicate it as useless.

So next time you’re suffering with a cold or a runny nose, maybe don’t just reach for the phenylephrine!


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