I'm in the medical field. Just because they are physicians doesn't mean they are always right or know what they are doing. Just because you call DAN doesn't mean you are going to get proper advice, in spite of the constant refrain here to do so. I figure if the folks WERE going to call DAN or see a physician they would do so. So they are here taking their chances.
I STRONGLY believe EACH diver has the responsibility to determine the validity of claims and suggestion made on forums like this. It's YOUR life. You also may be affecting your buddies safety and other people on a dive. Don't take it lightly.
HOW DOES REBOUND WORK?
There are tiny muscles that surround the capillaries in the mucosal membrane in the nose. These are controlled by the autonomic system to moderate blood flow depending on the requirements for humidification of the air you breathe. There is a MASSIVE transfer of water out of circulation and heat into the breathed air. By the time air hits your carina (bifurcation/split of the trachea) it is 100% humidified and warmed to body temperature.
By tightening and relaxing the muscles this requirement is met depending on environmental conditions and airflow demands. When the muscles are relaxed (histamine release from allergies can do this, as well as viral induced inflammation) the mucosa swells up leading to congestion. Afrin tells the muscles to tighten up.....a LOT. Being smooth muscle they can do this, for hours, and the mucosa shrinks down correspondingly. When the medication wears off the muscle relaxes, and being tired from 12+ hours of hard contraction it may relax MORE than it was before > congestion returns, maybe worse.
Rebound. At first, not a big deal since the muscle is not over fatigued.
This annoys the person with the returned congestion so > more Afrin. Begin the cycle of tighten, relax, rebound, more Aftrin. And away we go.....
Repeat cycle, each time the muscle being more and more exhausted from the ordeal. At some point those little muscles simply cannot cope anymore, the Afrin no longer works, or works poorly for a short time. The nose may even rebel from the abuse and get worse, MUCH worse, a condition called
Rhinitis medicamentosa
The simple cure > quit using it and put up with the congestion until things normalize, unless you've pushed it too far and really screwed things up.
Yes, you can do that.
That link is well worth reading for Afrin (ab)users.
One big advantage of Sudafed is not having rebound. A big disadvantage it that it is a systemic (whole body) drug that has far more side effects.