Taking paracetamol or other painkillers during pregnancy is not responsible for increase in the risk of asthma in children, finds a study of nearly 500,000 women.

Although women taking paracetamol during pregnancy are more likely to have children with asthma, painkillers are not the cause of the risk. Rather it could be the result of chronic pain that the women suffer from, say researchers. "Our interpretation of this is that it's less likely that drugs are responsible for the asthma. It seems another factor that we have not measured is linked to use of these drugs and asthma risk," said lead author Seif Shaheen, Professor at the Queen Mary University of London.

"For example, women who are taking prescribed painkillers are likely to suffer from chronic pain," Shaheen said. The study, published in the European Respiratory Journal, found the risk of asthma in kids was similar when women had been prescribed opioids such, as codeine and tramadol or migraine medication.

The increase in risk for asthma at five years of age was 50 per cent for paracetamol, 42 per cent for codeine and 48 per cent for migraine medication. "Severe pain, and the stress that it causes, have profound effects on the body, including on levels of some hormones, and there is evidence for a link between high levels of mothers' stress in pregnancy and increased risk of asthma in the offspring," Shaheen said.

For the study, the team examined 492,999 mothers and their children. They looked at the data on prescriptions for different types of painkillers during pregnancy and compared it with the rate of asthma diagnosis in children. Researchers say results should give women reassurance to take painkillers during pregnancy when prescribed by a doctor, and that managing chronic pain during pregnancy is important.