Adverse Effects of Oseltamivir

Common Adverse Effects

The Centers for Disease Control and Prevention recommends that gastrointestinal symptoms, particularly nausea (10%) and vomiting (8-9%), are typically mild, transient, and can be reduced by taking the medication with food 1, 2, 3
Nausea without vomiting occurs in approximately 10% of patients (vs. 6% with placebo) 1, 4
Vomiting occurs in approximately 9% of adults (vs. 3% with placebo) and 14% of children (vs. 8.5% with placebo) 1, 3
Diarrhea is reported in less than 5% of patients 1
Taking oseltamivir with food may reduce the severity of nausea and vomiting 1, 3
Dizziness is reported in less than 5% of patients 1

Transient neuropsychiatric events, including self-injury, delirium, abnormal behavior, hallucinations, agitation, anxiety, altered level of consciousness, confusion, nightmares, and delusions, have been reported during post-marketing surveillance 1, 3
The majority of these reports were among adolescents and adults in Japan 1, 3
The FDA advises close monitoring for abnormal behavior in patients receiving oseltamivir 1, 3
Allergic reactions, including oropharyngeal or facial edema, have been reported 1

Vomiting is the most common adverse effect in children (14% vs. 8.5% with placebo) 1, 3
Oseltamivir is classified as "Pregnancy Category C," indicating no clinical studies have assessed its safety in pregnant women, and should be used during pregnancy only if potential benefit justifies potential risk to the embryo or fetus 1, 4
No adverse effects have been reported among women who received oseltamivir during pregnancy or among infants born to such women 1, 4

Limited clinical data are available regarding drug interactions with oseltamivir, but there is potential for interaction with other drugs excreted by the anionic pathway via glomerular filtration and tubular secretion 1, 4
Coadministration with probenecid results in approximately 50% reduced clearance of oseltamivir carboxylate and approximately twofold increase in plasma levels 1, 4
No impairment of immunologic response to trivalent inactivated influenza vaccine has been demonstrated 3

The Centers for Disease Control and Prevention recommends that pregnant women with suspected or confirmed influenza should receive oseltamivir treatment immediately, without waiting for diagnostic test results, as they are at higher risk for influenza complications including maternal death, stillbirths, preterm delivery, and low birth weight 5
The American College of Obstetricians and Gynecologists implies that treatment should begin as early as possible after symptom onset, and the same dosing as nonpregnant patients should be used: 75 mg twice daily for treatment, 75 mg once daily for prophylaxis 5
Acetaminophen should be used for fever management, as fever itself can cause adverse fetal outcomes, according to the Centers for Disease Control and Prevention 5

The Advisory Committee on Immunization Practices stated that oseltamivir "should be used during pregnancy only if the potential benefit justifies the potential risk to the embryo or fetus" based on lack of clinical trial data, but this cautious language predated the extensive post-marketing surveillance data and the 2009 H1N1 pandemic experience, which demonstrated both the severe risks of influenza in pregnancy and the safety profile of oseltamivir 5
The Centers for Disease Control and Prevention explicitly states that "pregnancy should not be considered a contraindication to oseltamivir use" 5
One retrospective cohort study found no association between oseltamivir use during pregnancy and preterm birth, premature rupture of membranes, malformations, or abnormal fetal weight 5

In adults treated with oseltamivir, nausea occurs in about 10 % and vomiting in 8–9 % of patients; in children, vomiting occurs in 14–16 % of cases, reflecting a higher pediatric susceptibility【6】【7】【8】【9】.
Only roughly 1 % of patients discontinue oseltamivir because of gastrointestinal adverse effects, indicating that most tolerate the drug despite these symptoms【7】.

Administering oseltamivir with a meal or substantial snack markedly reduces the severity of both nausea and vomiting, providing a simple non‑pharmacologic mitigation strategy【6】【7】【8】【9】.
When ondansetron is given prophylactically together with food‑taken oseltamivir, the combined approach offers dual protection against emesis, further lowering the risk of treatment‑related vomiting【6】.

Recommended prophylactic ondansetron dose: 8 mg orally for adults (or 4 mg for children aged 4–11 years; weight‑based dosing for younger children)【10】.
After the initial ondansetron dose, continue 8 mg every 8 hours for the first 24–48 hours of oseltamivir therapy to maintain steady anti‑emetic coverage during the period of highest emetic risk【10】.

In interleukin‑2 (IL‑2) therapy protocols, scheduled ondansetron 8 mg IV every 8 hours is administered 30 minutes before each IL‑2 dose, demonstrating the benefit of pre‑emptive anti‑emetic timing【10】.
Prophylactic ondansetron use is more effective than waiting for vomiting to occur, supporting a preventive rather than rescue approach【10】.
Although ondansetron can increase diarrhea in gastroenteritis studies, this does not contraindicate its use with oseltamivir, which itself causes minimal diarrhea (<5 %)【6】【8】.
If vomiting persists despite prophylactic ondansetron, alternative anti‑emetics such as prochlorperazine 10 mg every 6 hours or lorazepam 0.5 mg every 6 hours may be considered【10】.

Administer oseltamivir with a substantial meal to maximize tolerability in the absence of ondansetron【6】【7】【8】【9】.
Approximately 99 % of patients tolerate oseltamivir without requiring any anti‑emetic therapy, underscoring that anti‑emetic prophylaxis is needed only for a minority of cases【7】.