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Viral infections, such as a common cold
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Allergies
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Air pollution and cigarette smoke
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Dental infections
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Narrowed nasal passages from nasal polyps
When the sinuses become inflamed or infected, the mucus thickens and clogs the openings to one or more sinuses. Fluid builds up inside the sinuses causing increased pressure. Also bacteria can become trapped, multiply and infect the lining. This is sinusitis.
Pseudoephedrine (Sudafed) is available without prescription