If you are in a hurry
- Abnormal behavior can occur whether or not a medicine such as Tamiflu is taken, so take steps to prevent falls for at least 2 days from the start of the fever[1]
- If your child does not respond when called, has a seizure, or keeps behaving or speaking abnormally, call 119 (Japan’s emergency number) without hesitating[2]
- As a guide, start antiviral medicine within 48 hours of onset. See a doctor early and ask about it[1]
Hello. I am Shunichi Gamo, MD, PhD, a board-certified pulmonologist and the director of Sendai Airport North Clinic.
Your child has been diagnosed with influenza, and suddenly tries to run out of the room, or tries to open a window. For a parent, little is more frightening. The thought that comes to mind is
“Did giving Tamiflu make my child act strangely?”
In fact, official national documents give a clear answer to this question.
Let me give you the conclusion first. Abnormal behavior can occur with influenza itself, even when no medicine such as Tamiflu has been taken. The Ministry of Health, Labour and Welfare asks for measures to prevent falls for at least 2 days from the start of the fever, whether or not a medicine is taken and whichever medicine it is[1]. Rather than focusing only on worry about the medicine, making your home safe is the surest way to protect your child.
In this article, I explain, as a respiratory specialist, the thinking in the official national documents, what you can do at home, what research shows, the signs that suggest encephalopathy (serious brain dysfunction), and how to choose antiviral medicine and fever medicine.
First, a note about our fever clinic on Sundays and national holidays. PCR testing for influenza and COVID (a test that copies the virus’s genetic material in order to detect it) gives results the same day.
From October 4, 2026, we also run the fever clinic on Sundays and national holidays
10:00 to 15:00 (no lunch break). No appointment is needed; you check in on the day. We give you the PCR result the same day. On Sundays and holidays we see fever patients only, and we do not offer general outpatient care or IV drips. We are closed on Sunday, October 25, and from November we are generally open every Sunday and national holiday. Notice: fever clinic on Sundays and national holidays
Shunichi Gamo, MD, PhD
Director, Sendai International Airport North Clinic
Born and raised locally, he attended Yuriage Elementary School and Yuriage Junior High School in Natori City. After graduating from Tohoku University School of Medicine, he worked at Sendai Open Hospital and Kesennuma City Hospital, and then at Tohoku University Hospital. In April 2021, in the middle of the COVID-19 pandemic, he opened Sendai International Airport North Clinic. Holding to a fever clinic that never turns patients away, he cared for well over 20,000 patients with fever in the two years before COVID-19 was moved to Class 5 under Japan’s infectious disease law.
Society memberships and board certifications
Certified Member and Board Certified Fellow, Japanese Society of Internal Medicine
Board Certified Pulmonologist, Japanese Respiratory Society
Board Certified Allergist, Japanese Society of Allergology
Board Certified Specialist, Japanese Society of Anti-Aging Medicine
PhD in Medicine (Tohoku University)
In short: is abnormal behavior caused by Tamiflu? The national position and the 2018 revision
The Ministry of Health, Labour and Welfare says that a causal relationship (a cause-and-effect link) with the medicine is unclear, and explains that the same abnormal behavior also appears when people with influenza have not taken any medicine[1].
According to the Ministry’s Q&A, past surveys have reported the following two points[1].
- When people have influenza, the same abnormal behavior appears even if they have not taken an anti-influenza antiviral medicine
- Abnormal behavior appears regardless of which anti-influenza antiviral medicine was taken
For this reason, when someone has influenza, attention to abnormal behavior is considered necessary regardless of whether a medicine was taken or which one[1].
In 2018, the “avoid use in teenagers in principle” statement for Tamiflu was removed
In 2007, two cases in which junior high school students who had taken Tamiflu died after falling were widely reported[3]. As a result, a statement that use in patients in their teens should in principle be avoided was added to the “Warnings” section of the package insert (the medicine’s instruction sheet)[3].
After that, the results of about 10 years of research were reviewed, and on August 21, 2018, an instruction to revise the package insert was issued[3]. The statement about avoiding use in patients in their teens in principle was deleted from Tamiflu’s “Warnings” section[3]. The following two facts were confirmed at the national safety measures committee (a panel of experts)[3].
| Facts confirmed in the 2018 review | Details |
|---|---|
| Fact 1 | Abnormal behavior occurs when people have influenza, regardless of whether or which anti-influenza antiviral medicine was taken |
| Fact 2 | For Tamiflu and for other medicines alike, there is no clear difference in how often abnormal behavior occurs between teenagers and children under 10 |
It was also pointed out that if strong warnings were given only for Tamiflu, people might wrongly think other medicines are safer and take abnormal behavior less seriously[3]. At the same time, the “Important precautions” for all anti-influenza antiviral medicines were amended to state the risk of abnormal behavior and to call for measures to prevent accidents such as falls for at least 2 days from the start of the fever[3].
Dr. GamoEven while the debate over “the medicine or the illness” continues, if you make your home safe, you are prepared for either case.
Who is affected, and when does it tend to happen?
Severe abnormal behavior that could lead to an accident such as a fall is known to be reported more often in boys and young men of school age and older, up to age 19[1]. However, it also occurs in girls and women[4]. It is also said that it often occurs within 2 days of the start of the fever[1].
Examples of abnormal behavior given by the Ministry of Health, Labour and Welfare are as follows[1].
- Suddenly standing up and trying to leave the room
- Becoming agitated, opening a window, going out onto the balcony, and trying to jump
- Leaving the house and walking outside, not responding when spoken to
- Feeling that someone is attacking, and rushing outside
- Saying strange things and moving around the room in tears
What should we do at home? Prevent falls for at least 2 days
The Ministry of Health, Labour and Welfare asks parents and other caregivers to take measures to prevent accidents such as falls for at least 2 days from the start of the fever, whether or not a medicine is taken and whichever medicine it is[1]. Examples of specific measures are as follows[1].
- Make sure the front door and the windows of all rooms are locked (including using inside locks and auxiliary locks if you have them)
- Have your child sleep in a room that does not face the balcony
- Have your child sleep in a room with bars on the windows (if you have one)
- Have your child sleep on the ground floor if at all possible (if you live in a detached house)
The Ministry’s leaflet says to take these measures “including while the child is asleep” and also gives using a door chain as an example[4]. Please do not let your guard down while your child sleeps at night.
What does “do not leave your child alone” mean?
The Ministry’s Q&A contains the following sentence[1].
“When a child or minor has been diagnosed with influenza and, after treatment has started, is recovering at home, care is needed, such as not leaving the child or minor alone, for at least 2 days from the start of treatment.”
The same Q&A also uses the wording “for at least 2 days from the start of the fever”[1]. Because the visit often happens the day after the fever starts, it is safer to go by the longer period and keep your child within sight for 2 days from the start of treatment[1].
If you live in an apartment and cannot move to the ground floor, choose a room that does not face the balcony and make locking the windows your top priority.
What does research show about Tamiflu and neuropsychiatric symptoms?
The studies that can be found on PubMed (a search site for medical papers worldwide) differ in their subjects and methods, and the results are not uniform. Here I introduce only what is written in the papers’ abstracts.
| Study | Subjects and method | Result stated in the abstract |
|---|---|---|
| United States (2025)[5] | Medicaid (public health insurance) enrollees aged 5 to 17. An observational study (cohort study) comparing periods of influenza | The rate of serious neuropsychiatric events requiring hospitalization was lower during periods of oseltamivir treatment than during periods of untreated influenza (rate ratio 0.53, 95% confidence interval 0.33 to 0.88) |
| South Korea (2020)[6] | People aged 8 and older diagnosed with influenza. An observational study comparing neuropsychiatric events within 30 days of diagnosis | The adjusted hazard ratio was 0.98 (95% confidence interval 0.96 to 1.01), with no difference in overall risk. The authors concluded that “oseltamivir does not increase the risk of neuropsychiatric events” |
| Japan (2017)[7] | From a survey of about 10,000 influenza patients under 18, an analysis of the 28 who had severe abnormal behavior (self-controlled case series) | 24 of the 28 (86%) had taken oseltamivir. Events tended to occur around the time when blood levels peak after starting the medicine, but that period overlaps with the period of high fever, so a role of influenza itself could not be ruled out |
| South Korea (2019)[8] | 5,322 people who had a neuropsychiatric event and had been prescribed oseltamivir beforehand (case-crossover study) | The adjusted odds ratio for oseltamivir use in the 2 days before the event was 1.90 (95% confidence interval 1.29 to 2.81). The authors concluded that short-term use triggers neuropsychiatric events and that early observation may be necessary |
The rate ratio, hazard ratio, and odds ratio are all numbers that compare how likely an event is to occur. A value below 1 means it is less likely to occur, and a value above 1 means it is more likely.
As you can see, the conclusions differ between studies. For the self-controlled case series study, the authors themselves state that this method is not the best way to examine the relationship between a medicine and abnormal behavior[7].
There is also a study that used nationwide health insurance claims data in Japan. It reported that the risk of abnormal behavior does not appear to rise when acetaminophen is used alone or together with a neuraminidase inhibitor (an antiviral medicine such as Tamiflu)[9].
Dr. GamoPrecisely because study results are split, I want you to put home preparations such as locking the windows first, whether or not you use the medicine.
As I described above, the Ministry’s position is that care is needed regardless of whether a medicine is taken or which one[1]. If you are worried, please talk to the doctor who prescribed it.
How do I tell abnormal behavior from “influenza encephalopathy”? When to call 119
Influenza encephalopathy (a serious complication in which a viral infection severely damages the brain) also begins with impaired consciousness, seizures, and abnormal speech and behavior. Even doctors can find it hard to tell apart[2].
The treatment strategy from the research group states that abnormal speech and behavior are often seen in the early stage of influenza encephalopathy, and that it must be distinguished from conditions such as febrile delirium (incoherent talk caused by a high fever)[2]. Encephalopathy is said to start suddenly and progress quickly[10].
Examples of abnormal speech and behavior listed in the treatment strategy as warning signs of encephalopathy are as follows[2].
- Does not recognize their parents, or says there are people who are not there
- Cannot tell food from non-food, for example by biting their own hand
- Reports seeing things or hallucinating, such as cartoon characters or animals
- Says things that make no sense, or slurs their words
- Complains of fear, or looks frightened
- Suddenly gets angry, starts crying, or starts singing loudly
Call 119 right away
If even one of the following is present, call 119 without hesitating, even at night or on a holiday. The height of the fever does not matter.
- Your child does not respond when called, is limp, or is dazed (consciousness is abnormal)
- A seizure has occurred (for what to do in a seizure, please also see our article on febrile seizures)
- Abnormal speech or behavior does not settle and continues
- Your child tried to jump out of a window or off the balcony, fell, or hit their head hard
The treatment strategy sets the following as cases for referral to an advanced medical facility[2]. Abnormal speech or behavior that continues, either continuously or on and off, for roughly 1 hour or more. Consciousness that is clearly poor or getting worse. A combination of seizures and abnormal speech or behavior is also grounds for referral[2].
This is a guide for medical facilities deciding where to send a patient. It does not mean that you should wait an hour at home. If you feel it is not settling, call 119 right away.
If you are unsure, phone for advice and be seen
- The abnormal speech settles in a short time and your child answers normally when called: contact a medical facility and be seen the same day
- You want to know at night whether to be seen: phone Miyagi Prefecture’s children’s night-time advice line (#8000). Every day from 7:00 p.m. until 8:00 a.m. the next morning, nurses advise parents of children, generally under 15 years old[11]
- From a phone that cannot reach #8000 (a landline that is not a push-button line), call 022-212-9390[11]
- If any of the “119” signs above apply, call 119 without stopping to ask for advice
Even after things settle down, please do not relax too much. The treatment strategy states that in biphasic encephalopathy (a course in which the child gets better once and then gets worse again), seizures or impaired consciousness can appear 3 to 5 days later[2]. If neurological symptoms come back after you get home, or new symptoms appear, be sure to be seen again[2].
The number of encephalopathy cases each year varies with the size of the flu season, but is put at 100 to 300. The fatality rate is 7 to 8% and about 15% have lasting after-effects, which is improving but still not good enough[10]. That is exactly why I ask you to get in touch early whenever your child is “not like themselves.”
By when should antiviral medicine be started? Within 48 hours
As a guide, antiviral medicine should be started within 48 hours of onset[1]. Please see a doctor early and talk about whether to use it.
According to the Ministry’s Q&A, when it is used at the right time (within 48 hours of onset), the fever period is usually shortened by 1 to 2 days and the amount of virus shed from the nose and throat also falls[1]. It is also said to help prevent severe illness in people thought to be at high risk of it, such as children[1].
The doctor decides whether to use it, and when it is used, it is important to follow the method, dose, and duration (number of days to take it)[1]. For your child’s diagnosis and a discussion about treatment, please see a doctor early rather than putting it off because of worry about abnormal behavior.
Whether or not antiviral medicine is used, keep up the fall prevention from the previous section for at least 2 days from the start of the fever[1].
For details, see Antiviral medicine: what to tell the doctor when you have a fever from influenza.
Under the Enforcement Regulations of the School Health and Safety Act, the exclusion period for influenza is “until 5 days have passed since onset and 2 days have passed since the fever went away (3 days for preschool children)”[1]. For details, see When can my child go back to school or nursery?
Which fever medicine should I use when the fever is hard to bear? Calonal and others
Acetaminophen (such as Calonal) can bring the fever down. Follow the instructions of your doctor or pharmacist for the amount and timing.
The Japan Pediatric Society considers acetaminophen preferable if a fever medicine is used for the fever of influenza[12]. The explanation from the Japan Institute for Health Security (JIHS) also says that the fever medicine used in the treatment of influenza encephalopathy is acetaminophen[10].
| Type of fever medicine | What the official sources say |
|---|---|
| Acetaminophen (such as Calonal) | The society says, “If a medicine is used for the fever of influenza, we consider acetaminophen preferable”[12] |
| Diclofenac, mefenamic acid, aspirin | The JIHS explanation contraindicates their use in treating influenza encephalopathy[10]. The society says the use of nonsteroidal anti-inflammatory drugs in influenza treatment should be approached with caution[12] |
| Other nonsteroidal anti-inflammatory drugs (such as ibuprofen) | The society says that the number of cases investigated is small and the link is not clear, so further investigation is needed[12] |
This society’s position was compiled in 2000[12]. For that reason, please do not use over-the-counter medicines, leftover painkillers at home, or adult medicines on your own judgment; check with a doctor or pharmacist before using them.
Dr. GamoIf the fever is high and your child looks miserable, please see a doctor early. We will help you, including with how to choose a fever medicine.
Frequently asked questions
- After I gave Tamiflu, my child started saying strange things. Should I stop the medicine?
-
First, make sure your child is safe and keep your child away from windows and the balcony. If your child does not respond when called, has a seizure, or the abnormal speech and behavior do not settle, call 119. Do not decide on your own whether to continue the medicine; contact the doctor who prescribed it and ask.
- If my child does not take the medicine, will abnormal behavior not happen?
-
No. The Ministry of Health, Labour and Welfare explains that when people have influenza, the same abnormal behavior appears even if they have not taken any medicine[1]. Whether or not a medicine is taken, take measures to prevent falls for at least 2 days from the start of the fever[1].
- Can Tamiflu also be used for teenagers?
-
With the 2018 revision of the package insert, the statement about avoiding use in teenage patients in principle was deleted from Tamiflu’s “Warnings” section[3]. The doctor decides whether to use it based on your child’s condition.
- Are we safe once the 2 days have passed?
-
Severe abnormal behavior is said to often occur within 2 days of the start of the fever[1], but that does not mean it cannot occur after 2 days. If your child seems not right, respond in the same way. In encephalopathy, symptoms can also appear 3 to 5 days later[2].
- The fever is high. Can I give the ibuprofen painkiller I have at home?
-
Do not use it on your own judgment; check with a doctor or pharmacist. The society says the use of nonsteroidal anti-inflammatory drugs in influenza treatment should be approached with caution[12]. Acetaminophen (such as Calonal) is what is used to bring down the fever.
- After influenza, when can my child go back to school or nursery?
-
Under the Enforcement Regulations of the School Health and Safety Act, the exclusion lasts until 5 days have passed since onset and 2 days have passed since the fever went away (3 days for preschool children)[1]. For details, please see this article.
Summary: prevent falls for 2 days from the start of the fever, whether or not a medicine is taken
- Abnormal behavior can occur whether or not a medicine such as Tamiflu is taken, so take steps to prevent falls for at least 2 days from the start of the fever[1]
- If your child does not respond when called, has a seizure, or keeps behaving or speaking abnormally, call 119 without hesitating[2]
- As a guide, start antiviral medicine within 48 hours of onset. See a doctor early and ask about it[1]
- Because study results are split, talk to your regular doctor about the medicine, and keep up the home preparations whether or not the medicine is used
- When the fever is hard to bear, use acetaminophen (such as Calonal) as your doctor or pharmacist directs
If you feel your child is “not like themselves,” trust that instinct. Even when you are unsure, please do not hesitate to talk to us. Leave the medical care to us.
That’s all.
Disclaimer
This article is intended to provide general medical information and does not replace an examination, diagnosis, or treatment by a doctor. The content is based on medical knowledge at the time of publication, but the right course of action differs for each person. If you are worried about symptoms, please consult a medical facility.
References
- Ministry of Health, Labour and Welfare. Q&A on the Comprehensive Measures against Acute Respiratory Infections (ARI) for Winter 2026 (Version of September 30, 2026). Ministry of Health, Labour and Welfare website (in Japanese). Accessed October 2026. https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/kenkou/kekkaku-kansenshou/infulenza/QA2026.html
- Japan Agency for Medical Research and Development research grant (Research Project for Promoting Development of Innovative Drugs for Emerging and Re-emerging Infectious Diseases). Treatment Strategy for Influenza Encephalopathy (February 2018). Japanese Society of Child Neurology website (in Japanese). Accessed October 2026. https://www.childneuro.jp/uploads/files/about/influenzaencephalopathy2018.pdf
- Ministry of Health, Labour and Welfare. Safety Measures for Anti-Influenza Antiviral Medicines (Pharmaceuticals and Medical Devices Safety Information No. 358). Pharmaceuticals and Medical Devices Agency website (in Japanese). Accessed October 2026. https://www.pmda.go.jp/files/000226773.pdf
- Ministry of Health, Labour and Welfare. For Influenza Patients, Their Families, and Those Around Them (Alert Leaflet). Ministry of Health, Labour and Welfare website (in Japanese). Accessed October 2026. https://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou01/dl/pamphlet181207_01.pdf
- Antoon JW, Williams DJ, Bruce J, Sekmen M, Zhu Y, Grijalva CG. Influenza with and without oseltamivir treatment and neuropsychiatric events among children and adolescents. JAMA Neurol. 2025 Oct;82(10):1013-21. doi:10.1001/jamaneurol.2025.1995 PMID: 40758339
- Huh K, Kang M, Shin DH, Hong J, Jung J. Oseltamivir and the risk of neuropsychiatric events: a national, population-based study. Clin Infect Dis. 2020 Dec;71(9):e409-14. doi:10.1093/cid/ciaa055 PMID: 31996920
- Fukushima W, Ozasa K, Okumura A, Mori M, Hosoya M, Nakano T, et al. Oseltamivir use and severe abnormal behavior in Japanese children and adolescents with influenza: is a self-controlled case series study applicable? Vaccine. 2017 Aug;35(36):4817-24. doi:10.1016/j.vaccine.2017.07.007 PMID: 28818475
- Kang HR, Lee EK, Kim WJ, Shin JY. Risk of neuropsychiatric adverse events associated with the use of oseltamivir: a nationwide population-based case-crossover study. J Antimicrob Chemother. 2019 Feb;74(2):453-61. doi:10.1093/jac/dky445 PMID: 30418537
- Sugawara T, Ohkusa Y, Taniguchi K, Miyazaki C, Momoi M, Okabe N. Association of severe abnormal behavior and acetaminophen with/without neuraminidase inhibitors. J Infect Chemother. 2019 Jun;25(6):423-6. doi:10.1016/j.jiac.2019.01.008 PMID: 30744989
- Japan Institute for Health Security. New Treatments for Influenza Encephalopathy (IASR Vol. 40 p101-103: June 2019 issue). Infectious Diseases Information site (in Japanese). Accessed October 2026. https://id-info.jihs.go.jp/niid/ja/typhi-m/iasr-reference/8947-472r06.html
- Miyagi Prefecture. Miyagi Prefecture Children’s Night-time Advice Line Program (#8000). Miyagi Prefecture website (in Japanese). Accessed October 2026. https://www.pref.miyagi.jp/soshiki/iryou/shouni04.html
- Japan Pediatric Society. On the Effect of Fever Medicines in Influenza Encephalitis and Encephalopathy (Notice from the Board of Directors to members, November 2000). Japan Pediatric Society website (in Japanese). Accessed October 2026. https://www.jpeds.or.jp/modules/activity/index.php?content_id=201
▶ Overview of this topic: Fever clinic on Sundays and national holidays (from October 4, 2026, 10:00 to 15:00)
More articles to read when you have a fever
For our Sunday and holiday fever clinic, see Notice: fever clinic on Sundays and national holidays.
