Asthma rates in New Zealand are amongst the highest in the world. Previous research has suggested that paracetamol given in the first year of life may be responsible for up to 22% of later asthma. However it is unclear whether this finding is due to paracetamol itself or to the illness which lead to it being given – for example a chest infection. We will conduct a randomised trial in 3,922 infants to determine if paracetamol or ibuprofen use in the first year of life is associated with asthma at age 6 years. If paracetamol use in infancy increases the risk of later asthma we will have provided high-quality evidence for the first universal prevention strategy to reduce current high asthma rates. If paracetamol use in infancy does not increase later asthma we will have provided reassurance to parents and medical professionals that this most commonly used medicine is safe.