Paracetamol use in pregnancy and infancy linked to child asthma

"Babies given paracetamol are nearly a third more likely to develop asthma," the Mail Online reports. 

The study the news is based on also found a link between maternal use of the painkiller in pregnancy, and childhood asthma.

Pregnant women are advised not to take medicines if possible, but paracetamol is recommended as the best option if painkillers are needed to reduce a fever, because there is little evidence it can cause harm to the baby. Paracetamol is also recommended if painkilling or temperature reduction medicines are needed for babies.

However, recent research has found a potential link between paracetamol and asthma. This study set out to investigate the link further.

The researchers found paracetamol was linked to childhood asthma, both in cases when it had been taken by the pregnant woman and by the young baby (less than six months old). The study estimated that infant exposure to paracetamol increased the asthma risk by 29% and exposure in pregnancy led to a 13% increase; though this estimate was borderline significant.

They also found that the reason for taking the medication did not affect the chances of asthma. This suggests the increased chance of asthma may be due to paracetamol, not to the illness it is used to treat.

The results of this study will probably need to be replicated in larger populations before official advice on paracetamol use in pregnancy and infancy are changed. 

Where did the story come from?

The study was carried out by researchers from the Norwegian Institute of Public Health, the University of Oslo and the University of Bristol and was funded by the National Institutes of Health, the Norwegian Research Council and the Norwegian Extra Foundation for Health and Rehabilitation. 

The study was published in the peer-reviewed International Journal of Epidemiology, on an open-access basis, meaning it is free to read online.

Most of the UK media reported the study's findings accurately, but as is so often the case, the headline writers overstated the findings. A link between paracetamol use and asthma has only been suggested, not proven.

What kind of research was this?

This is a cohort study, which is a good type of study for investigating links between factors – in this case, between taking paracetamol in pregnancy or infancy, and children developing asthma. However, cohort studies cannot prove that one thing causes another. They can only show there is a link, and investigate factors that may or may not have affected the results.

What did the research involve?

Researchers took information about 114,761 children born in Norway between 1999 and 2008, and analysed it to look for links between paracetamol use and childhood asthma at the ages of three and seven. 

They adjusted their figures to take account of the illnesses that paracetamol had been used to treat, and other factors that might have affected the results. They then calculated the relative risk (RR) of the child having developed asthma.

The researchers included data on:

• 53,169 children who had information on asthma at age three
• 25,394 at age seven
• 45,607 who had records of whether or not they had been given asthma medications by age seven

Women in the study were asked about their own paracetamol use, and the reasons for it, at 18 and 30 weeks of pregnancy. When the child was six months old, the women were asked whether they had given paracetamol to the child, and if so what for.

The researchers checked whether the type of illness people took paracetamol for – pain, high temperature or chest infection/flu – had an effect on the child's chances of getting asthma. They also adjusted figures to take account of the mother's age, whether she had asthma, whether she smoked during pregnancy, antibiotic use, weight, education level and number of children.

They also looked for any effect from the father having taken paracetamol, or the mother having taken paracetamol when not pregnant. This is to see whether something else, such as the parents' attitudes to health and medicines, might affect the results.

What were the basic results?

About 28% of the children were born to mothers who had used paracetamol during pregnancy only, and 15% had taken paracetamol in the first six months of life only. A further 19% had been exposed to paracetamol both in the womb and in the first six months of life. A total of 5.7% of children had asthma at three years of age.

The researchers found "modest associations" between the use of paracetamol and childhood asthma, both for use during pregnancy and use by the child during the first six months of life.

Women who used paracetamol during pregnancy but did not give it to their infant were 13% more likely to have a child with asthma by age three (relative risk (RR) 1.13, 95% confidence interval (CI) 1.02 to 1.25).

A child was 29% more likely to have asthma at age three if the child had been given paracetamol before six months of age but was not exposed to it during the pregnancy (RR 1.29, 95% CI 1.16 to 1.45). While they were 27% more likely to have asthma with exposure during pregnancy and the first six months of life (RR 1.29, 95% CI 1.14 to 1.41). Results were similar for asthma at age seven.

The researchers said their results did not change when they adjusted them to take account of the reason for the medication. They found no link between the father's use of paracetamol or mother's use outside of pregnancy, and asthma in the child.

How did the researchers interpret the results?

The researchers say their study "is by far the largest study to provide evidence that prenatal and infant paracetamol exposures have independent positive associations with asthma development" – in other words, that the study found paracetamol was linked to a higher chance of getting asthma. 

They go on to say their findings "suggest that the associations cannot be fully explained" by other factors, such as the illness that the mother or baby was taking paracetamol for.

"Paracetamol is the most commonly used analgesic among pregnant women and infants, and uncovering potential adverse events is of public health importance," they say.

Conclusion

This study tells us more about a potential link between paracetamol and childhood asthma. Other studies have suggested that taking paracetamol in pregnancy, or giving it to babies, might increase the risk of the child getting asthma, but this study gives more detail.

One strong possibility before this study was published was that the problem was not paracetamol, but the reason for taking it – for example, that the baby's asthma was linked to the mother or baby having a chest infection, not to the medicine they took to relieve it. However, this study tests that possibility and finds that it cannot completely explain the link to paracetamol.

There are some weaknesses in the study. It relies on the mother's report of paracetamol use and whether their child has asthma, which might not be reliable. It is possible that women who decided to take paracetamol while pregnant might have been more ill than women who had that illness but didn't take medication, which might have affected the results. Another major limitation is that the study did not determine the amount or frequency that paracetamol was taken either by the mother or the infant, so we do not know how this might affect the risk.

However, it is a large study and the researchers carried out checks to make their findings as reliable as possible. While a cohort study can never confirm that one thing causes another, it does seem likely that there is a link between asthma and paracetamol.

It's important to remember that the relative increase in risk of asthma, especially for women taking paracetamol during pregnancy, is quite low. The researchers say that advice on paracetamol use for pregnant women and babies does not need to change as a result of their study.

Women in the UK are advised that paracetamol is the preferred choice to treat mild or moderate pain, or high temperature. They are advised to take paracetamol at the lowest effective dose, for the shortest possible time.

Babies can be given paracetamol to treat high temperature or pain if they are over two months' old. Check the packet or information leaflet for information about the correct dose.

Ibuprofen can be given for pain and fever in children of three months and over who weigh more than 5kg (11lbs) and, again, check the recommended dosage.