Have you seen any other symptoms such as swelling around an eye or the face, nasal drainage, or wheezing or rattles? If all you have seen is sneezing, you could be dealing with a virus, infectious bronchitis, and although antibiotics may prevent secondary bacterial infections, they won't cure...
Tylan 50 injectible is a really good antibiotic, and some give it orally, although it is best given by injection into the breast 1/4 ml daily for 5 days for a silkie. Here is a medicine chart with dosage of oxytetracycline and a list of respiratory diseases and symptoms...