Paracetamol is an effective treatment for high fever
Use of paracetamol in babies increases the risk of developing asthma five years later, a study of more than 200,000 children suggests.
Those given the painkiller for fever in the first year of life had a 46% increased risk of asthma by the age of six or seven, The Lancet reported.
Researchers do not know if the drug directly increases asthma risk or another underlying factor is to blame.
Experts said parents should still use the drug for high temperatures.
Increasing use of paracetamol in children has coincided with rising cases of asthma over the past 50 years, the researchers said.
The latest study, carried out in 31 countries, is the largest to date looking at paracetamol use and childhood asthma.
Parents of children aged six and seven were asked questionnaires about symptoms of asthma, eczema and related allergic conditions in addition to details on paracetamol use for fever in the child's first year of life and the past 12 months.
The results also showed that higher doses and more regular use of the drug are associated with a greater risk of developing asthma.
Analysis of current use in 103,000 children showed those who had used paracetamol more than once a month in the past year had a three-fold increased risk of asthma compared with those who had not taken the drug in the past 12 months.
Use of paracetamol was also associated with more severe asthma symptoms.
And risk of eczema and hayfever was also increased.
Cause and effect
One explanation for the findings is that paracetamol may cause changes in the body that leave a child more vulnerable to inflammation and allergies.
Another is that the use of paracetamol in children may be a marker for something else which is causing increased rates of asthma, such as lifestyle issues or the underlying infection causing the fever, experts said.
Study leader Professor Richard Beasley from the University of Auckland said: "We stress the findings do not constitute a reason to stop using paracetamol in childhood.
"However the findings do lend support to the current guidelines of the World Health Organization, which recommend that paracetamol should be reserved for children with a high fever (38.5C or above)."
Professor Jeffrey Aronson, president of the British Pharmacological Society, said the dose relationship with paracetamol and asthma suggested there was a real association between the two.
"This confirms previous findings and underlines the importance of a current recommendation that paracetamol should not be used regularly in young children and should be reserved for times when they have a fever and are in obvious discomfort or pain."
Leanne Male, Asthma UK's assistant director of research, said: "Despite a great deal of research being carried out, we still don't know how important different lifestyle and genetic factors are in affecting the development of asthma.
"If we can establish the mechanisms behind how paracetamol might affect it, this could go some way towards helping to prevent the condition in the first place.
"At this stage however, the use of paracetamol should not be a concern for parents or carers who are worried about the development of asthma in their children."