I believe they’re basically steroids for the glands in your nose, forcing the nasal walls to contract shortly after contact.
As to why it’s bad to use them for longer than 3 days, your nose will develop rebound congestion, which is worse than the congestion that was treated initially and very uncomfortable.
These sprays contain a chemical similar to adrenaline. So similar that your body can’t tell the difference, and so it responds the same way. And the way that your body response to adrenaline is, in a nutshell, to prepare it for action. This means sending blood to your muscles, and away from places like your digestive tract or, indeed, the tissue inside your nose. This is convenient because congestion (true congestion, not just a snotty nose that you can blow to get relief) is caused by the tissue inside your nose being inflamed and swelling up. And the way that works is that the blood vessels inside your nose open up wide to let as much blood into the tissue as possible. Take the blood away, and the swelling goes down.
This is also why doing any kind of exercise will usually provide relief from nasal congestion as well. You’re putting your entire body into action mode, and so blood gets diverted away from your nasal passages. Nasal sprays work more locally. Instead of affecting your entire body, they specifically trick your nose into thinking the body is in action mode.
The reason they work quickly is because the blood vessels respond very quickly to (fake) adrenaline. It doesn’t take long for them to constrict and the swelling to subside. (This might seem magical to you if you thought that you were congested because your nose was somehow plugged up with snot or some other substance, which couldn’t just disappear into thin air that fast.)
Nasal decongestants work by shrinking blood vessels, so the tissues immediately shrink. But if you use them for too long, your body will get used to them and need more and more to get the same effect… the tissues get really swollen and inflamed, and you might need strong steroids to be able to counteract it. So follow doctor’s orders and only use for 3 days at a time!
Nasal steroid sprays like Flonase, on the other hand, work best when used for long periods of time. So start using Flonase a month before allergy season and use it daily for best results.
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