Roxithromycin is an antibiotic, used commonly in Europe and Japan. It is not used in the United States, so most physicians there never heard of it, which leads them to uninformed responses.
Although it is an antibiotic, Roxithromycin has a mildly suppressive effect in some components of the immune system. This is why it can be helpful in asthma. The following is a small part of the bibliography, you can find more articles on pubmed.gov.
Shoji T, Yoshida S, Sakamoto H, Hasegawa H, Nakagawa H, Amayasu H., Clin Exp Allergy. 1999 Jul;29(7):950-6, Anti-inflammatory effect of roxithromycin in patients with aspirin-intolerant asthma.
Shimizu T, Kato M, Mochizuki H, Tokuyama K, Morikawa A, Kuroume T., Chest. 1994 Aug;106(2):458-61., Roxithromycin reduces the degree of bronchial hyperresponsiveness in children with asthma.
There is no such thing as an "antibiotic for asthma".
Asthma is a chronic disease defined by constriction of the air passageways and production of thick mucus as part of an immune response to the presence of an antigen, or "non-self" substance. This often results in "air hunger", difficulty breathing, and insufficient oxygen being transferred to the red blood cells in the lung air/blood interface within the alveoli..
Antibiotics are used to treat bacterial infections (which may, on ocasion precipitate asthmatic attacks). Different anti-biotics are used for different kinds of bacteria.
If you are prescribed an antibiotic, don't hesitate to ask your physician for the reason. When filling a prescription in the United States always ask the pharmacist for the "paclage insert" which has full information. Do not allow you to give you a "condensed version" issued by the drug store.
Reading these inserts will give you all of the information you need about the drug.