Asthma and paracetamol: Could we really know what happens between them?

An association between paracetamol use or exposure in different times of life, including gestation and childhood, and asthma has been observed in recent years. Causality cannot be established from observational studies because of the arguable presence of many confounding factors and biases. Randomised trials are needed to disclose the nature of the association, but are difficult to carry out because of ethic, economic and logistical issues as large patient samples should be involved for a long time in such studies. Pragmatic trials may be the best option to shed some light on this issue. Questions regarding the problems and difficulties of conducting such trials and the way to overcome them are discussed.