Fever in children: the temperature thresholds that matter and what to do

What is fever and at what temperature does it start?

Fever is not a disease in itself. It is a deliberate defence response your child's immune system launches to fight infection. When a new microbe enters the body, immune cells release chemical signals that raise core temperature intentionally, because most bacteria and viruses reproduce less efficiently in a warmer environment. A mild fever is therefore often a sign of an immune system doing its job, not evidence that something is going seriously wrong.

Normal temperature in a child ranges from 36.5 to 37.5 degrees Celsius measured at the armpit and from 36.6 to 38 degrees measured rectally. A multi-site diagnostic accuracy study published in Nursing open in 2021 confirmed that the exact threshold at which fever begins varies with age and measurement site, and that drawing firm lines too early leads either to over-treatment or to missing genuine illness.

The practical definition accepted by most paediatric guidelines is 38 degrees Celsius or above measured rectally, or 37.5 degrees or above measured at the armpit. But the number alone is never enough. A child with a temperature of 38.5 who is running around and demanding food is a very different situation from a child with a temperature of 38 who is motionless, pale and will not respond to their name.

When is fever dangerous in a young baby?

A baby under three months old is the most vulnerable situation you will encounter. At this age the immune system is still immature, and doctors cannot easily determine the source of fever from symptoms alone, because a very young baby can have a serious bacterial infection such as meningitis or sepsis and appear only mildly unwell in the first hours. The rule that most paediatricians apply is clear: any fever above 38 degrees Celsius in a baby younger than three months needs same-day medical assessment, not in the morning and not tomorrow.

Between three and six months, the situation is less immediately urgent but a first episode of fever should still be evaluated by a doctor. After six months, a generally healthy child who is alert, drinking fluids and has a known cause such as an upper respiratory infection can usually be monitored at home for 24 to 48 hours with careful observation. But fever that is not responding to medication, or that extends beyond three days without clear improvement, always needs a clinic visit.

Some features alongside the fever matter more than the number on the thermometer. A child with fever and a rash that spreads rapidly and does not disappear when you press a clear glass against it, or a child with fever and a stiff neck that will not bend forward, or a child who was crying and has gone suddenly and abnormally quiet, these are emergency presentations regardless of what the thermometer says.

How to give paracetamol and ibuprofen correctly

The single most common error parents make is giving a dose based on the child's age instead of their weight. The correct dose of paracetamol is 15 milligrams per kilogram of the child's actual weight, given every four to six hours when needed. Ibuprofen is 10 milligrams per kilogram, given every six to eight hours. Weigh your child and calculate from there, not from the age bracket printed on the box.

A 2020 systematic review in JAMA Network Open examined over 19 studies comparing paracetamol and ibuprofen in children under two years old and found both drugs similarly effective at reducing fever. Ibuprofen may keep temperature lower for slightly longer. A 2024 meta-analysis in Pediatrics found that alternating between the two drugs provides longer stretches of comfort but is not necessarily superior to one drug used consistently and correctly.

Ibuprofen should not be given to babies under three months, or to children who are significantly dehydrated, or to those with kidney disease or who are taking blood-thinning medicines. In these situations paracetamol is the only safe choice. Always check the concentration printed on the bottle, because infant drops are a different strength from children's liquid suspension and mixing them up is a common source of accidental overdose.

Never give aspirin to children with fever, under any circumstances. Aspirin used in children with viral infections such as influenza or chickenpox is associated with Reye syndrome, a rare but serious condition affecting the liver and brain. This rule has no exceptions.

Tepid sponging: what works and what harms

Tepid sponging means wiping your child's body with a sponge dampened with lukewarm water, roughly body-temperature water, and it is acceptable as a supplement to medication rather than a replacement for it. Gentle strokes over the torso, arms and legs are enough. You do not need to sponge the face or head.

Cold water is a common mistake that should be avoided entirely. Cold water causes peripheral blood vessels to constrict, which traps heat inside the body rather than releasing it. The skin feels cool while core temperature rises. Cold water also provokes shivering, which generates even more heat and exhausts the child. The result is the opposite of what you are trying to achieve.

Rubbing alcohol on the skin to reduce temperature is a practice that persists in some communities and it is genuinely dangerous. Alcohol is absorbed through the skin quickly in infants and young children and can cause real alcohol poisoning. Avoid it entirely regardless of how high the fever climbs. Excessive wrapping in blankets is another error: a feverish child needs light clothing and a ventilated room, not extra layers that prevent heat from escaping.

Febrile seizures: what they are and what to do

Febrile seizures are convulsions triggered by a rapid rise in temperature, occurring in children between six months and five years of age. They affect between two and five percent of children in this age group. The episode is terrifying to witness: the child goes rigid, then shakes rhythmically in the limbs, and may lose consciousness for seconds to a couple of minutes. They almost always stop on their own in under five minutes, after which the child is drowsy and limp.

Simple febrile seizures, defined as those lasting under five minutes, occurring only once during a single fever illness, in a child with no underlying neurological history, very rarely cause lasting harm. A 2021 Cochrane review concluded that preventive drug treatment is not recommended routinely for febrile seizures because the benefit is limited and the seizures themselves are generally benign. A 2019 review in American Family Physician found that the risk of developing epilepsy after a simple febrile seizure is not meaningfully higher than in the general population.

What to do when a seizure happens: stay calm, because your clarity of action depends on it. Turn the child onto their side to keep the airway clear. Do not put anything in their mouth. Do not hold the limbs down forcibly. Look at the clock and note the time. If the seizure does not stop within five minutes, or if it repeats twice in the same day, call for emergency help or go to an emergency department. If it stops on its own before five minutes, take the child to a doctor on the same day to assess the cause of the fever.

One important distinction to understand: the convulsion itself is not what makes febrile seizures serious in uncomplicated cases. What matters is identifying and treating the underlying infection causing the fever. A child who has had a febrile seizure and has been assessed by a doctor and sent home is not at high risk of permanent harm from the seizure. The fever is the issue to address.

Common fever myths that cause harm

The bundling myth: many grandparents advise covering a feverish child in heavy blankets to sweat out the illness. This does the opposite of what is intended. Extra layers prevent the body's normal heat-dissipation from the skin surface and raise core temperature further. A feverish child needs light clothing and good air circulation, not insulation.

The rapid cooling myth: there is a fine line between reducing fever and stressing the body by cooling it too fast. The goal is gradual, comfortable reduction and child comfort, not reaching a particular number on the thermometer as quickly as possible. Cold baths and ice packs to the body cause peripheral vasoconstriction and shivering that can worsen the situation.

The high number means serious illness myth: not always. A child with a temperature of 40 from a straightforward streptococcal throat infection who is still drinking and recognises their parents is in a very different situation from a child with a temperature of 38 from early meningitis who is drowsy and will not meet your eyes. Your child's behaviour and responsiveness tell you more than the thermometer does.

The stop feeding myth: it is true that fever reduces appetite and forcing food on a sick child is unhelpful. But refusing fluids for hours is a serious problem. Prioritise drinking over eating consistently. Warm broth is useful because it replaces both salt and fluids. Breast milk remains the best fluid for a feverish infant.

When to go to emergency versus when to see a doctor tomorrow

There is an important practical distinction between what needs the emergency department tonight and what can wait for a morning clinic appointment. Emergency now means: any fever in a baby under three months, a rash that is dark purple or dark red and does not fade when you press a clear glass firmly against it, a neck that is too stiff to bend the chin towards the chest, difficulty breathing or an unusual change in breathing rate, or a bluish tinge around the lips.

Also emergency: a seizure lasting more than five minutes, a child who is confused and does not recognise you within twenty minutes of a seizure stopping, or a child who has been unable to keep any fluid down for more than eight hours and is showing signs of dehydration such as a dry mouth, no tears when crying, sunken eyes, or no urine in the last six to eight hours.

See a doctor on the same day but not as an emergency: fever above 39 degrees in a baby between three and six months who is otherwise alert, or fever above 40 degrees in any child even if they appear reasonably well, or fever that has continued for more than 48 hours without clear improvement despite treatment at the correct dose.

Fluids and feeding during a fever

Dehydration is the most common and most preventable complication of fever in children. Fever raises the breathing rate and increases sweating, both of which lose fluid faster than normal. A sick child often refuses food, which is acceptable, but refusing to drink for extended periods needs active attention.

Offer fluids in small, frequent amounts rather than large volumes at once. For breastfed babies, continue feeding more frequently than usual. For older children, water, diluted fruit juice and oral rehydration solution are all suitable. Warm broth is useful because it replaces both salt and fluid together. Do not force food but do insist on drinking.

Signs that dehydration needs attention in a feverish child: a dry mouth and sticky tongue, crying with no tears, no urination in more than six hours, eyes that appear sunken and deeper set than usual, and skin that stays tented briefly when pinched rather than snapping back immediately. These signs in a feverish child need a same-day doctor visit.

Sources

• Tan E, Braithwaite I, McKinlay CJD et al: Comparison of Acetaminophen (Paracetamol) With Ibuprofen for Treatment of Fever or Pain in Children Younger Than 2 Years: A Systematic Review and Meta-analysis, JAMA network open, 2020
• Section on Clinical Pharmacology and Therapeutics, Committee on Drugs, Sullivan JE et al: Fever and antipyretic use in children, Pediatrics, 2011
• De la Cruz-Mena JE, Veroniki AA, Acosta-Reyes J et al: Short-term Dual Therapy or Mono Therapy With Acetaminophen and Ibuprofen for Fever: A Network Meta-Analysis, Pediatrics, 2024
• Smith DK, Sadler KP, Benedum M et al: Febrile Seizures: Risks, Evaluation, and Prognosis, American family physician, 2019
• Kasbekar R, Naz A, Marcos L et al: Threshold for defining fever varies with age, especially in children: A multi-site diagnostic accuracy study, Nursing open, 2021
• Offringa M, Newton R, Nevitt SJ et al: Prophylactic drug management for febrile seizures in children, The Cochrane database of systematic reviews, 2021