How do decongestants work? Decongestants like Sudafed or Afrin are essentially chemical versions of epinephrine or adrenaline. And what they do is they make the blood vessels in your nose go from being wide to being small. What that means is there's less blood flow to the structures in your nose. And it shrinks them.
So if there's any swelling in your nose from allergies or sinus infections, you're going to breathe better for the short term. But you can't use these molecules for the long term because long term, they can have bad side effects. Ultimately, it's better to try to fix the root cause of your nasal swelling, like fixing your sinusitis or fixing your allergies.