Re-analysis of safety data supporting doxylamine use for nausea and vomiting of pregnancy

Antihistamines are commonly used to treat nausea and vomiting of pregnancy (NVP). We re-analyzed the 24 primary studies cited in a 1997 meta-analysis that concluded antihistamine use for NVP was safe as they had been studied in more than 200,000 participating women and the pooled odds ratio for congenital malformations was 0.76 (95% confidence interval [CI]: 0.60-0.94). Our analysis of this meta-analysis showed that 139,414 women were included in 22 original studies involving antihistamines, 129,108 of which were in studies involving doxylamine. In these studies, 23,485 women were exposed to antihistamines, 14,624 of which were exposed to doxylamine. The summary relative risk (cohort studies) and odds ratio (case-control studies) for congenital malformations from antihistamine exposure were 1.09 (95% CI: 1.01-1.18) and 1.04 (95% CI: 0.91-1.19), and for doxylamine exposure, the summary relative risk and odds ratio were 0.94 (95% CI: 0.80-1.10) and 1.07 (95% CI: 0.93-1.23), respectively. Although not a new systematic review, our re-analysis demonstrates that the safety data for antihistamines, and doxylamine in particular, are based on many fewer than 200,000 participating women and exposures, and that doxylamine use is not associated with a decreased risk of malformations as previously reported.