Rats don't get colds the way we do, but can get serious respiratory infections very quickly. If your rat is sneezing very often, showing red porphyrin stains at the eyes and nose, sounding congested, losing weight, having difficulty breathing or labored breathing, see a vet. If you need a suggestion see the following lists that have been recommended by pet rat owners from all over.
<http://www.rmca.org/Resources/vets.htm>
<http://www.ratfanclub.org/vetref.html>

Here are some articles on respiratory disease and the RMCA Drug Chart (over the counter cold medications are not the best way to treat respiratory disease).
<http://www.rmca.org/Articles/dchart.htm>
<http://www.rmca.org/Articles/myco.htm>
<http://www.rmca.org/Articles/respir.htm>
<https://web.archive.org/web/20081009123140/http://members.aol.com/MMCart3475/Myco.html>