Sinusitis, also known as a sinus infection or rhinosinusitis, is inflammation of the mucous membrane that lines the sinuses resulting in symptoms. Common symptoms include thick nasal mucus, a plugged nose, and facial pain. Other signs and symptoms may include fever, headaches, poor sense of smell, sore throat, and cough. The cough is often worse at night. Serious complications are rare. It is defined as acute sinusitis if it lasts less than 4 weeks, and as chronic sinusitis if it lasts for more than 12 weeks.Sinusitis can be caused by infection, allergies, air pollution, or structural problems in the nose. Most cases are caused by a viral infection. A bacterial infection may be present if symptoms last more than ten days or if a person worsens after starting to improve. Recurrent episodes are more likely in people with asthma, cystic fibrosis, and poor immune function. X-rays are not typically needed unless complications are suspected. In chronic cases confirmatory testing is recommended by either direct visualization or computed tomography.Some cases may be prevented by hand washing, avoiding smoking, and immunization. Pain killers such as naproxen, nasal steroids, and nasal irrigation may be used to help with symptoms. Recommended initial treatment for acute sinusitis is watchful waiting. If symptoms do not improve in 7–10 days or get worse, then an antibiotic may be used or changed. In those in whom antibiotics are used, either amoxicillin or amoxicillin/clavulanate is recommended first line. Surgery may occasionally be used in people with chronic disease.Sinusitis is a common condition. It affects between about 10% and 30% of people each year in the United States and Europe. Women are more often affected than men. Chronic sinusitis affects approximately 12.5% of people. Treatment of sinusitis in the United States results in more than US$11 billion in costs. The unnecessary and ineffective treatment of viral sinusitis with antibiotics is common.