Common side effects
| Side effect | How often (treatment trials) |
|---|---|
| Nausea, adults | About 10%[1] |
| Vomiting, adults | About 8%[1] |
| Headache, adults | About 2%[1] |
| Vomiting, children | 16%, versus 8% on placebo[1] |
Taking each dose with food may make it easier to tolerate.[1] CDC also lists nausea and vomiting as the most common side effects.[4]
Rare but serious
- Allergic and skin reactions. Anaphylaxis and serious skin reactions, including Stevens-Johnson syndrome, have been reported. Stop the medicine and get care right away for swelling of the face or throat, trouble breathing, or a blistering or peeling rash.[1]
- Confusion or unusual behavior. See below.
The behavior warning, in context
The FDA label says there have been reports after approval of delirium and abnormal behavior leading to injury, in some cases fatal, in people with flu who were taking Tamiflu. It also says "the contribution of TAMIFLU to these events has not been established," and that flu itself "can be associated with a variety of neurologic and behavioral symptoms," including hallucinations, delirium, and abnormal behavior.[1] Most reports came from children in Japan.[3]
A large 2025 study helps separate the two. Vanderbilt researchers followed 692,295 children and teens aged 5 to 17 in Tennessee Medicaid over several flu seasons and counted serious neurologic and psychiatric events that led to a hospital stay.[2]
- Compared with flu that was not treated, these events were about half as common while flu was being treated with oseltamivir (rate ratio 0.53).[2]
- The lower rate was driven mostly by fewer neurologic events rather than psychiatric ones.[2]
- The authors concluded the findings support oseltamivir for preventing these flu-related complications.[2]
This was an observational study, so it cannot prove cause and effect, and the label warning still stands. The practical advice is the same either way: watch anyone with flu, especially a child or teen, for sudden confusion or unusual behavior, and get care right away if it happens.[1]