What is a decongestant and how do they work?

Tracey Sullivan Pharmacy Features Writer

Feeling stuffy and congested can be one of the most uncomfortable symptoms of a cold or allergy. Enter decongestants, the over-the-counter medicines that many of us turn to for relief. But what exactly are decongestants, and how do they work? In this article, we’ll explore the science behind them, their benefits, potential side effects, and how to use them effectively to ensure you can breathe easy again. Whether you’re battling a seasonal allergy or a stubborn cold, understanding decongestants can help you make informed decisions about your health and wellness.


What is congestion and why does it happen?
The common viruses that make us unwell often enter our body via our nose. Once in there, the virus starts to multiply inside the linings of our nasal passages. Our sinuses (the spaces behind the nose, eyes and cheeks) become irritated and our immune system activates by making mucus to get rid of the virus and moisten the irritated tissues. The mucus filling up the sinus cavities causes pressure. Our immune system also increases blood flow to the area to send in cells to attack the virus. This causes inflammation and more pressure. So, congestion is actually a defence mechanism against respiratory viruses.


Which decongestant should I choose?
Decongestants are helpful medications used to treat the annoying symptoms of colds, flu, hayfever and other allergies, mild Covid infections, sinusitis and catarrh (mucus build-up in sinuses and phlegm in throat).

Available orally as tablets, and also as nasal drops and sprays, decongestants reduce the feeling of pressure and pain that comes with congestion.


Nasal drops and sprays:

  • contain oxymetazoline or xylometazoline
  • give short-term relief of nasal congestion within minutes for up to several hours
  • decrease swelling of nasal blood vessels, which helps open the airways
  • decrease mucus build up in the nose
  • decrease mucus dripping down the back of the throat (which can cause cough or sore throat)
  • are very useful short-term (no longer than three days)
  • used long-term can cause damage to the nasal lining resulting in “rebound congestion”, ultimately more congestion that you started with!


Oral decongestants:

  • decrease the swelling in the blood vessels of the nose, decrease mucus production and increase drainage in the sinus cavities. They also relax the smooth muscle lining the airways, making it easier to breathe
  • pseudoephedrine is effective orally, phenylephrine is not
  • oral pseudoephedrine provides relief within 15 to 30 minutes and lasts 3 to 8 hours
  • pseudoephedrine makes you feel good – almost like it has taken your cold away. This is because it acts as a central nervous system stimulant with a very similar chemical structure to amphetamine. It eliminates drowsiness and fatigue and improves concentration by increasing the release of brain neurotransmitters serotonin and dopamine. The pleasant feeling can give people a false sense of feeling well – something to be aware of, as it enables us to “soldier on” while sick, when really we should be resting and keeping our germs at home!


The return of pseudoephedrine
Pseudoephedrine is now back on pharmacy shelves to give New Zealanders wider access to effective cold and flu medications. It was removed because of its connection with the illegal manufacture of P (Meth), however it didn’t have the impact on reducing P consumption that was intended, and therefore has now been reintroduced. Pseudoephedrine is effective at reducing cold & flu symptoms because it works quickly – often starting to relieve symptoms in as little as 15 minutes, and is longer-acting than nasal decongestants. Unlike nasal decongestants, pseudoephedrine also has the ability to relax the smooth muscle lining of the airways, making it easier to breathe.


Side effects of oral decongestants:

  • increased blood pressure and blood sugar
  • aggravates glaucoma
  • increases seizure risk
  • affects heart conditions
  • fast/irregular/pounding heart
  • anxiety, confusion, restlessness
  • shaking
  • difficulty urinating


Not suitable for people with these conditions:

  • High blood pressure
  • Heart disease
  • Diabetes
  • Prostatic hyperplasia
  • Urinary obstruction
  • Kidney disease
  • Glaucoma
  • Thyroid disorder
  • Pregnancy, breastfeeding
  • Seizure disorders.


Pseudoephedrine has several medication interactions. People taking medication for high blood pressure or ADHD and people on some heart medications or anti-depressants will not be able to take an oral decongestant.

Consumer Infomation

Talk to your pharmacist if you are needing treatment for congestion. Not all pharmacies will choose to stock pseudoephedrine. Because it is classified as a pharmacist-only medicine, pharmacists are legally obliged to record the name and address of people they sell this medication to, and you may be asked to provide proof of ID at the time of sale. Pharmacists have the right to refuse a sale of any medicine that they think is not suitable for your symptoms or safe for your health.

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