It is often forgotten that the nasal passages are an important part of the upper airway. Blockage of the nasal passages by a deviated septum, swelling, or excess mucus can lead to mouth breathing. Opening the mouth at night causes the muscles of the throat to relax and also allows the tongue to fall more easily into the back of the throat. This can be the critical factor in causing OSA in some patients who would only snore if they could breath through their noses at night. In patients who have blockage due to swelling or increased mucous, there are many effective nasal sprays that will alleviate the problem.
Antihistamines can also be effective, but use with certain antibiotics can cause fatal arrhythmias.
Again, consult with a physician and check with the pharmacist.