It's been a life saver for my Mac. From the moment I rescued him, he would make those respiratory noises and we were at loss with what more to do. Then a vet suggested Booster - and it disappeared immediately, never to come back. I love it!
It's helped with minor respiratory irritations with me as well but it does not replace antibiotics. If your rat is sick, you should not delay getting to the vet and starting antibiotics if they are needed but you can give it regularly (or with antibiotics) to boost the immune system.