Dose-Finding Study of Favipiravir in the Treatment of Uncomplicated Influenza

Inclusion Criteria: 1. Patients who test positive for influenza A or B by a commercially available Rapid Antigen Test (RAT) of the nasopharynx. A patient with a negative RAT result may still enroll if the sponsor and investigator agree that there is a known influenza outbreak circulating in the community. 2. Patients ≥ 20 but \< 65 years of age with a temperature (oral) of 100.4°F (38.0°C) or more; patients ≥ 65 years of age with a temperature (oral) of 100.0°F (37.8°C) or more at the first visit, or in the 6 hours prior if antipyretics were taken. 3. Patients with 2 or more of the following symptoms (moderate to severe in intensity) at the time of enrollment: * Cough * Sore throat * Headache * Nasal congestion * Feeling feverish * Body aches and pains * Fatigue (tiredness) Exclusion Criteria: 1. Patients who have had influenza symptoms or fever (i.e., one of the symptoms listed under the inclusion criteria) for 36 hours or more prior to study screening. 2. Patients who have underlying chronic respiratory disease (e.g., chronic obstructive pulmonary disease \[COPD\], chronic bronchitis, diffuse panbronchiolitis, bronchiectasis, pulmonary emphysema, pulmonary fibrosis, bronchial asthma, or old tuberculosis). 3. Patients who at the beginning of the study are suspected of having bacterial respiratory infection (i.e., expectoration of purulent or mucopurulent sputum and/or infiltrate in lung in a chest x-ray, etc.). 4. Patients who have serious chronic diseases. For example, patients with HIV, cancer (i.e., requiring chemotherapy within the preceding 6 months), cirrhosis (end-stage), and/or unstable renal, cardiac, pulmonary, neurologic, vascular, or endocrinologic disease states requiring medication dose adjustments within the last 30 days.