The treatment for influenza (Type A and B) is primarily categorized into two classes of drugs: neuraminidase inhibitors (e.g., Tamiflu, Relenza) and endonuclease inhibitors (Xofluza). Based on the guidelines from the Japanese Association for Infectious Diseases and the Ministry of Health, Labour and Welfare, early administration within 48 hours of onset is recommended, and is expected to shorten the duration of symptoms by 1 to 2 days. For high-risk patients (elderly, those with underlying conditions, pregnant women), emphasis is also placed on the effect of preventing severe illness. The focus of this summary will be on Tamiflu (Oseltamivir), Xofluza (Baloxavir), and Nasal Sprays (Relenza).
Overview and Comparison of Efficacy and Side Effects
| Drug Name | Mechanism of Action | Dosage (Adult) | Efficacy | Main Side Effects | Important Notes |
| Tamiflu(Oseltamivir) | Neuraminidase Inhibitor (Inhibits viral release) | Oral, 75mg/dose, twice daily for 5 days | Shortens symptom duration by 1 day. High efficacy in preventing severe illness, low resistance rate (<1%). Effective against both A/B types. | Nausea/Vomiting (10−15%), Diarrhea, Headache. Reports of abnormal behavior (though mainly due to the fever of the flu itself). | Recommended during pregnancy/lactation. Short half-life, easier side effect management. Generics available, low cost. |
| Xofluza(Baloxavir) | Endonuclease Inhibitor (Inhibits viral replication) | Oral, single dose (40−80mg, based on weight/age) | Shortens symptom duration by 1−2 days. Rapid and sharp reduction in viral shedding (1/100 by the next day). Effective against both A/B types. | Diarrhea (5−10%), Nausea, Headache, Liver dysfunction (rare). Resistance rate 5−10%. | Not recommended during pregnancy/lactation due to insufficient data. Granules for infants available. Long half-life, potential for prolonged side effects. |
| Nasal Spray (Relenza)(Zanamivir, Inhalation) | Neuraminidase Inhibitor (Inhibits viral release) | Inhalation, 10mg/dose, twice daily for 5 days | Shortens symptom duration by 1 day. Has an effect on preventing severe illness. Effective against both A/B types. | Bronchospasm/Cough(in 2−5% of asthma patients), Nasopharyngeal discomfort. Fewer gastrointestinal symptoms. | Contraindicated in asthma/respiratory diseases. Approved for ages 5 and up. Nasal spray treatment (e.g., FluMist) is not approved in Japan (only as a vaccine). |
Overall Efficacy
Maximum efficacy is achieved when administered within 48 hours of onset. A 2024-2025 meta-analysis (15 trials, 6295 cases) showed no significant difference in symptom improvement rates between Tamiflu and Xofluza (approx. 70−80%). In high-risk groups, hospitalization risk is reduced by 15−20%.
Overall Side Effects
Frequency is 1−15%, with severe cases being rare (shock/anaphylaxis <0.1%). Abnormal behavior is reported with all medications, but the causal link is weak, and the main cause is high fever from influenza. The 2025 guidelines recommend monitoring after administration.
Key Points for Latest Treatment Selection (2025 Guidelines)
- First Choice: Tamiflu
- Priority for typical cases (outpatient, mild). Low resistance and stable efficacy in preventing severe illness. Suitable for pregnant women, infants, and the elderly. Easier side effect management, and also usable for prophylaxis (post-exposure). Confirmed effect in shortening hospitalization for severe cases based on 2025 evidence (Clin Infect Dis).
- Option: Xofluza
- Used when compliance is critical (single dose eliminates missed doses) or for severe outpatient cases. Convenient for children (5 years and older) and when considering weight. However, due to the higher resistance rate (Nature Commun 2024: 5-10%), it should be avoided in areas with high resistance prevalence or when multi-drug resistance is a concern. Not recommended for pregnant women.
- Option: Nasal Spray (Relenza)
- Used when oral medication is not feasible (e.g., frequent vomiting) or to avoid gastrointestinal side effects. For patients aged 5 and up without complicating asthma. Confirm the patient’s ability to use the inhalation device. Severe cases may require switching to intravenous treatment (Rapiacta).
Overall Treatment Selection Flow:
- Confirm onset within 48 hours → Administer immediately.
- Patient Background Assessment: High-risk (underlying conditions) → Tamiflu preferred. Children/Concerns about missed doses → Xofluza. No respiratory complications → Relenza.
- Additional Consideration: Region with resistance prevalence (2025 Japan: Type A H3N2 strain shows Xofluza resistance↑) → Tamiflu. Pregnant women → Tamiflu.
Caveats and Advice
- Co-administration/Contraindications: May be used with antipyretics (Acetaminophen). Avoid live vaccines (FluMist nasal type) for 2 weeks after use.
- 2025 Update: Enhanced resistance monitoring for Xofluza (Japanese Association for Infectious Diseases). Increased use of IV drug (Rapiacta) in severe cases.
- Do not self-medicate; consult a doctor. Seek early medical attention to prevent complications (e.g., pneumonia).
