The most common side effects of Tamiflu (oseltamivir) are nausea and vomiting, which affect roughly 8 to 10% of people who take it. These stomach symptoms typically show up within the first two days of treatment and resolve on their own within a day or two. Beyond the digestive issues, there are less common but more serious reactions worth knowing about, particularly in children.
Nausea and Vomiting
Stomach problems are by far the most frequent complaint. In a large meta-analysis of randomized controlled trials published in The Lancet, about 9.9% of adults taking Tamiflu experienced nausea compared to 6.2% on placebo, and 8% had vomiting compared to 3.3% on placebo. That means Tamiflu roughly doubles the chance of vomiting. These numbers sound significant in percentage terms, but for most people the symptoms are mild and short-lived.
Taking Tamiflu with food can reduce the likelihood of stomach upset. If you’re giving it to a child who refuses the capsule, a pharmacist can compound it into a liquid suspension, which is also easier to pair with a meal or snack.
Headache, Fatigue, and Other Mild Effects
Some people report headaches, fatigue, and general body aches while on Tamiflu. These overlap heavily with flu symptoms themselves, which makes it hard to tell whether the drug or the illness is to blame. In clinical trials, headache rates were similar between people taking Tamiflu and those taking a placebo, suggesting the flu itself is the more likely culprit. Most of these side effects were reported just once per patient, appeared on the first or second day of treatment, and cleared up within 48 hours.
Neuropsychiatric Symptoms in Children
Reports of unusual behavior in children taking Tamiflu, including confusion, hallucinations, delirium, and self-harm, have drawn attention since the drug’s early years on the market. These reports were especially prominent in Japan, where Tamiflu was prescribed widely. The FDA reviewed these cases and noted that the events were “primarily related to unusual neurologic or psychiatric events such as delirium, hallucinations, confusion, abnormal behavior, convulsions, and encephalitis” in children 16 and younger.
However, a large study of nearly 700,000 children ages 5 to 17 enrolled in Tennessee Medicaid found that influenza itself, not Tamiflu, appears to be the bigger driver of these events. Children treated with Tamiflu actually had about a 50% lower rate of serious neuropsychiatric events compared to children with untreated flu. The most common events seen across the study period were mood disorders (36.3%) and suicidal or self-harm behaviors (34.2%), but these occurred more often in children who did not receive treatment.
The FDA has not established a direct causal link between Tamiflu and neuropsychiatric events. There is strong evidence that the flu itself can cause brain inflammation and behavioral changes in children, a phenomenon documented in the medical literature since the mid-1990s. Still, it’s reasonable to keep an eye on children and teenagers taking the drug, particularly during the first couple of days.
Allergic and Severe Skin Reactions
Serious allergic reactions to Tamiflu are rare but real. Post-marketing reports have included anaphylaxis (a severe, whole-body allergic reaction) and dangerous skin conditions like Stevens-Johnson syndrome and toxic epidermal necrolysis, where the skin blisters and peels. An FDA review identified 43 serious skin reaction cases and 75 hypersensitivity reactions, including anaphylactic shock, from voluntary post-marketing reports. Because these reports are voluntary, the true frequency is unknown, but these reactions are clearly uncommon given the millions of prescriptions filled each year.
When anaphylaxis does occur, it tends to happen fast. Most reported cases developed within 20 minutes to two hours after the first dose. Signs to watch for include swelling of the face, tongue, or throat, difficulty breathing, chest tightness, hives, and blistering or sores in the mouth. If any of these appear, the drug should be stopped immediately and emergency care sought.
Kidney Function and Drug Clearance
Tamiflu is processed and eliminated through the kidneys. If your kidneys aren’t working well, the drug can build up in your system and increase the chance of side effects. People with significantly reduced kidney function need a lower dose. Those on dialysis face an additional challenge because there is no established dosing regimen for them, so the prescribing decision is more complex and case-specific.
If you have known kidney problems, your prescriber will likely adjust the dose or frequency. This isn’t something you need to figure out yourself, but it’s worth mentioning any kidney history when Tamiflu is prescribed, especially in older adults where reduced kidney function is common but sometimes undiagnosed.
How Long Side Effects Last
For the vast majority of people, Tamiflu’s side effects are front-loaded. They appear on day one or two of the five-day treatment course and clear within a day or two, often while you’re still taking the medication. Stomach symptoms are the most predictable and the most manageable, especially when you take each dose with food. Serious reactions like skin blistering or anaphylaxis, while extremely rare, require immediate attention because they can escalate quickly. Once you finish the course and stop taking the drug, lingering side effects are uncommon.