What is the first‑line antiviral medication for an otherwise healthy adult presenting with influenza symptoms within 48 hours of onset?

First-Line Antiviral for Influenza in Healthy Adults

Oseltamivir (Tamiflu) 75 mg orally twice daily for 5 days is the first-line antiviral medication for otherwise healthy adults presenting with influenza symptoms within 48 hours of onset. 1, 2, 3

Optimal Timing and Initiation

• Treatment should be initiated as soon as possible within 48 hours of symptom onset for maximum benefit, with earlier initiation (within 12-24 hours) associated with significantly faster symptom resolution 1, 4
• Starting oseltamivir within the first 12 hours after fever onset reduces total illness duration by 74.6 hours (3.1 days) compared to intervention at 48 hours 4
• Do not wait for laboratory confirmation before initiating treatment—clinical diagnosis based on fever, cough, and systemic symptoms during influenza season is sufficient to start empiric therapy 1, 5

Expected Clinical Benefits

• Reduces illness duration by approximately 1-1.5 days (24-36 hours) when started within 48 hours 1, 6, 5
• Decreases symptom severity by 30-38% compared to placebo 1, 6
• Reduces risk of secondary complications requiring antibiotics by 35% 1
• Shortens time to return to normal activities and baseline health status 6, 7

Standard Dosing

• Adults and adolescents ≥13 years: 75 mg orally twice daily for 5 days 1, 2, 3
• The medication may be taken with or without food, though taking it with food enhances tolerability and reduces nausea 3, 6
• Complete the full 5-day course even if symptoms improve earlier—do not shorten treatment duration 2

Renal Dose Adjustment

• For creatinine clearance <30 mL/min: reduce dose to 75 mg once daily 1
• For creatinine clearance 10-30 mL/min: 30 mg once daily or 75 mg every other day 1

Common Adverse Effects

• Nausea and vomiting are the most common side effects, occurring in approximately 3.7-5.8% of patients (vs 9% with placebo) 1, 6
• Gastrointestinal symptoms are typically mild, transient, and occur primarily with the first dose 6, 4
• Taking oseltamivir with food significantly reduces nausea and vomiting 3, 6, 4

Alternative Agent

• Zanamivir (Relenza) 10 mg (2 inhalations) twice daily for 5 days is an alternative for patients who cannot tolerate oseltamivir or when oseltamivir resistance is suspected 2, 6
• Zanamivir carries a risk of bronchospasm and should be avoided in patients with underlying respiratory disease 8

Critical Pitfalls to Avoid

• Do not delay treatment while awaiting laboratory confirmation—rapid antigen tests have poor sensitivity, and negative results should not exclude treatment during influenza season 1, 5
• Do not withhold treatment in otherwise healthy adults presenting within 48 hours based on mild symptoms—earlier intervention provides progressively greater benefit 4, 7
• Do not discontinue treatment before completing the full 5-day course, even if symptoms resolve, as this may lead to incomplete viral clearance 2
• Do not reflexively extend treatment beyond 5 days in uncomplicated cases—there is no evidence supporting benefit in otherwise healthy patients 2

When Treatment Beyond 48 Hours Is NOT Indicated

• In previously healthy outpatients who are not deteriorating and present >48 hours after symptom onset, antiviral therapy should NOT be started—supportive care alone is appropriate 1
• Otherwise healthy adults (no chronic medical conditions, not pregnant, age 18-64 years) do not meet high-risk criteria that would justify late antiviral use 1