Most symptoms of a fever in young children can be managed at home
Ibuprofen is better at alleviating childhood fever than paracetamol and should be the drug of first choice, say UK researchers.
The Bristol-based trial involving 156 children aged between six months and six years showed ibuprofen reduced temperature faster than paracetamol.
The British Medical Journal work also says alternating the two drugs could help, which some GPs already recommend.
But experts advised against this, in line with official guidance.
The concern is the relative ease with which children could receive an overdose.
Fever is very common in young children, affecting seven in every 10 preschool children each year.
It can be miserable for the child and cause anxiety for parents. Most fevers will settle by themselves but a few are caused by serious infections such as pneumonia.
Guidelines published last year by the National Institute for Health and Clinical Excellence (NICE) say either ibuprofen or paracetamol can be used for children unwell or distressed with fever.
But they say that, due to the lack of evidence, the two drugs should not be given together or alternated.
The researchers from the University of Bristol and the University of the West of England, recruited children who had a temperature between 37.8 and 41 degrees centigrade, due to an illness that could be managed at home.
Children were randomised to receive either paracetamol plus ibuprofen, just paracetamol, or just ibuprofen.
The medicines were given over a 48-hour period, with the group of children on both paracetamol and ibuprofen receiving them as separate doses.
This group received one dose of paracetamol every four to six hours (maximum of four doses in 24 hours) and then one dose of ibuprofen every six to eight hours (maximum of three doses in 24 hours).
A normal temperature is between 36-36.8C (96.8-98.24F)
In children, any temperature of 38C (100.4F) or above is considered high and is called a fever
To find out if your child has a fever, place a thermometer under your child's armpit or use a special ear thermometer
The children's condition was followed up at 24 hours, 48 hours and at day five.
The researchers found that in the first four hours children given both medicines spent 55 minutes less time with fever compared to those given paracetamol alone.
But giving two medicines was not markedly better than just giving ibuprofen.
However, over a 24 hour period, children given both medicines experienced 4.4 hours less time with fever than those given just paracetamol, and 2.5 hours less time with fever than those just given ibuprofen.
Dr Alastair Hay, consultant senior lecturer in primary health care at the University of Bristol, who led the study, said: "Doctors, nurses, pharmacists and parents wanting to use medicines to treat young, unwell children with fever should be advised to use ibuprofen first.
"If more sustained symptom control over a 24-hour period is wanted, giving both medicines alternately is better than giving one on its own.
"However, parents should keep a careful record of when doses are given to avoid accidentally giving too much."
He said he thought it would be appropriate for NICE to review its guidance in light of the new study, saying the current guidance was too cautious.
In an accompanying editorial in the BMJ, Dr Anthony Harnden from the University of Oxford, warned of the relative ease with which children could receive an overdose.
He said that a "more complicated alternating regimen of paracetamol and ibuprofen may be less safe than using either drug alone".
A spokeswoman for NICE said the 2007 guidance recommended that more research should be conducted on the effectiveness and safety of alternating doses of paracetamol and ibuprofen in reducing fever in children who remain febrile after the first fever-reducing medicine.
She said: "Any newly published research will need to be thoroughly assessed by independent experts as part of the process of updating clinical guidelines.
"This is essential to ensure that any new evidence is of the highest standards before any potential updates can be made to existing guidance."
Professor Steve Fields, chairman of the Royal College of General Practitioners, advised parents and carers of children with fever to follow the NICE guidance.
"We believe parents should keep it simple. We do not see at this moment any need to change the advice.
"However, this paper does demonstrate that using ibuprofen initially is more effective at reducing temperature and may demonstrate that using both ibuprofen and paracetamol together could have a positive effect."