Redosing Tamiflu After Vomiting at 1 Hour
Do not redose oseltamivir if vomiting occurs 1 hour after administration, as significant absorption has already occurred. The active metabolite reaches detectable plasma concentrations within 30 minutes and approaches peak levels by 1 hour post-ingestion 1, 2.
Pharmacokinetic Rationale
- Oseltamivir is rapidly absorbed from the gastrointestinal tract, with the active metabolite (oseltamivir carboxylate) detectable in plasma within 30 minutes of oral administration 1.
- Peak plasma concentrations occur at 3-4 hours, but substantial absorption has already taken place by 1 hour post-dose 1, 2.
- The absolute bioavailability of the active metabolite is 80%, and the majority of this absorption occurs in the first hour after ingestion 1.
Clinical Decision Algorithm
If vomiting occurs:
- <30 minutes post-dose: Consider redosing, as minimal absorption has occurred 1.
- 30-60 minutes post-dose: Clinical judgment required; may consider redosing if vomiting was profuse or immediate, but generally not necessary as partial absorption has occurred 1.
- >60 minutes post-dose (as in this case): Do not redose; adequate absorption has occurred 1, 2.
Strategies to Prevent Vomiting
- Administer oseltamivir with food or a light snack, which significantly reduces gastrointestinal adverse effects including nausea and vomiting without affecting drug absorption 3, 4, 5.
- Nausea and vomiting are the most common adverse events, occurring in approximately 5-15% of patients, but are typically mild and transient 3, 5.
- Taking the medication with meals improves gastrointestinal tolerability and should be standard practice 3, 4.
Important Caveats
- If the patient continues to vomit repeatedly and cannot retain any oral medications, consider alternative routes of administration such as IV peramivir (600 mg single dose for adults, 12 mg/kg for children 2-12 years) 6.
- For critically ill patients with gastric stasis or bleeding, enteral absorption may be compromised, and parenteral alternatives should be considered 7.
- Treatment efficacy is time-dependent: initiation within 48 hours of symptom onset provides maximum benefit, so ensuring the patient receives adequate dosing is critical 7, 3.