Inclusion Criteria:
* Age ≥18 years at time of signing ICF.
* At screening, meet all of the following:
1. Positive influenza antigen test or influenza virus nucleic acid test (other rapid molecular diagnostic methods are acceptable).
2. Axillary temperature ≥37.3°C at screening; if antipyretics have been taken, axillary temperature ≥37.3°C at least 4 hours after dosing;
3. At least one systemic influenza symptom of moderate or greater severity: muscle/joint aches, fatigue, headache, fever/chills;
4. At least one respiratory influenza symptom of moderate or greater severity: nasal congestion, sore throat, cough.
* Time from onset of first influenza symptom to randomization ≤48 hours.
* Female participants must meet:
1. Not of childbearing potential (i.e., hysterectomy, bilateral oophorectomy, documented ovarian failure, or postmenopausal defined as \>50 years and amenorrhea ≥12 months), or
2. Of childbearing potential, with negative pregnancy test at screening, and not pregnant, peripartum, or breastfeeding.
* Participants and their partners agree to complete abstinence or use effective contraception from screening until 1 month after study completion.
* Male participants agree not to donate sperm from screening until 1 month after study completion.
* Understand and voluntarily sign ICF, willing and able to comply with all study procedures, and able to complete the participant diary as required.
Exclusion Criteria:
* Influenza virus infection requiring hospitalization (any of the following):
1. Significant exacerbation of underlying diseases, e.g., COPD, diabetes, chronic heart failure, chronic renal failure, cirrhosis, etc.
2. Any of the following severe case criteria:
1. Respiratory rate ≥30/min;
2. Oxygen saturation ≤93% on room air at rest;
3. PaO2/FiO2 ≤300; for high-altitude areas (\>1000 m), correction: PaO2/FiO2 × \[760/atmospheric pressure (mmHg)\] (1 mmHg=0.133 kPa);
4. Progressive clinical deterioration with lung imaging showing \>50% lesion progression within 24-48 hours.
3. Any critical case criteria:
1. Respiratory failure requiring mechanical ventilation;
2. Shock;
3. Acute necrotizing encephalopathy;
4. Other organ failure requiring ICU monitoring.
* High-risk groups for severe/critical cases (any of the following):
1. Severe or poorly controlled underlying diseases, e.g., COPD, liver disease (ALT or AST ≥3×ULN, total bilirubin ≥1.5×ULN), chronic kidney disease (serum creatinine \>177 μmol/L or 2 mg/dL), severe hematological disorders, chronic congestive heart failure (NYHA class III-IV), neurological and neuromuscular diseases, metabolic diseases, etc.;
2. Clinically significant corrected QT interval abnormality on ECG (QTc \>450 ms for males or \>470 ms for females, QTcF by Fridericia formula);
3. Immunocompromised patients, e.g., malignancy, organ or bone marrow transplantation, HIV infection, or use of immunosuppressants within 3 months; Note: Participants with basal cell carcinoma, localized squamous cell carcinoma of skin, or cervical carcinoma in situ may be enrolled if curative treatment completed ≥12 months before ICF; other malignancies if curative treatment completed ≥5 years before ICF.
4. Concurrent conditions requiring aspirin or salicylate therapy;
5. Obesity (BMI \>30 kg/m²).
* Bronchitis, pneumonia, pleural effusion, or interstitial lung disease suspected by a clinician at screening, or confirmed by chest imaging (X-ray / CT) and judged by investigator at screening.
* Acute respiratory infection, otitis media, or sinusitis within 2 weeks before screening.
* Concurrent infection requiring systemic anti-infective therapy, or WBC \>10.0×10⁹/L at screening.
* Purulent sputum or suppurative tonsillitis.
* Difficulty swallowing medication or history of gastrointestinal diseases that significantly affect drug absorption (including but not limited to reflux esophagitis, chronic diarrhea, inflammatory bowel disease, intestinal tuberculosis, gastroinoma, short-bowel syndrome, post-gastrectomy, etc.).
* History of allergy to active ingredient or excipients of the study drug.
* Previous exposure to SYH2085.
* Body weight \<40 kg.
* Use of anti-influenza drugs within 7 days before screening (including but not limited to neuraminidase inhibitors, hemagglutinin inhibitors, M2 ion channel blockers, and CEN inhibitors, e.g., oseltamivir, zanamivir, peramivir, laninamivir, umifenovir, favipiravir, rimantadine, amantadine, arbidol, nitazoxanide, baloxavir marboxil, marboxil, etc.).
* Influenza vaccination within 6 months before screening.
* History of alcohol abuse within 3 months before screening (males \>14 units/week, females \>7 units/week; 1 unit = 10 g pure alcohol), or alcohol consumption within 48 hours before dosing, or history of drug abuse.
* Use of any prohibited medications within 2 weeks / 5 half-lives (whichever longer; excluding anti-influenza drugs) before screening and during the planned trial period.
* Participation in any clinical trial of investigational drug, biologic, or medical device within 30 days (or 5 half-lives, whichever longer) before screening.
* Positive SARS-CoV-2 antigen or nucleic acid test.
* Any other condition judged by the investigator as unsuitable for participation.
