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Evidence linking a child’s exposure to paracetamol in utero and during early infancy to an increased risk of asthma is “often overstated”, a new review of 11 observational cohort studies concludes.

According to five studies, using paracetamol during the first trimester increased the risk of asthma when the child was five years of age or older by 39 per cent. However the risk varied markedly between studies. Only one study adjusted for maternal respiratory tract infections, another possible explanation for the link.

Two studies, which examined the link between paracetamol exposure during the second trimester and asthma, saw a non-significant increase in one investigation (6 per cent), while the risk more than doubled in the other.

Based on two studies, using paracetamol during the third trimester increased the risk by 17 per cent and, based on three studies considering the second and third trimesters together, by 49 per cent.

Three studies examined the link between paracetamol exposure during the entire pregnancy and asthma, but the results varied markedly (ORs of 1.65, 1.22 and 0.74). Based on three studies, the risk of childhood asthma increased by 15 per cent for each doubling of the number of days children were exposed to paracetamol during infancy. Adjusting for respiratory tract infections reduced the strength of the association (OR 1.06).

The reasons for the wide variations in the studies are unknown – although respiratory tract infections probably contribute. Future studies need to account for respiratory infections and assess the effects of paracetamol exposure on lung function.

In the meantime, the authors conclude that, “the evidence of an association between earlylife paracetamol [exposure] and asthma is often overstated and there is currently insufficient evidence to support changing guidelines”.