--- - |2+ Background The neuropeptide and hormone oxytocin is known to have a significant impact on social cognition and behaviour in humans. There is growing concern regarding the influence of exogenous oxytocin (OT) administration in early life on later social and emotional development, including autism spectrum disorder (ASD). No study has examined offspring development in relation to the dose of exogenous oxytocin administered during labour. Methods Between 1989 and 1992, 2,900 mothers were recruited prior to the 18th week of pregnancy, delivering 2,868 live offspring. The Child Behaviour Checklist was used to measure offspring behavioural difficulties at ages 5, 8, 10, 14 and 17 years. Autism spectrum disorder was formally diagnosed by consensus of a team of specialists. At 20 years, offspring completed a measure of autistic‐like traits, the Autism Spectrum Quotient (AQ). Oxytocin exposure prior to birth was analysed using categorical and continuous approaches (maternal oxytocin dose) with univariate and multivariate statistical techniques. Results Categorical analyses of oxytocin exposure prior to birth demonstrated no group differences in any measures of child behaviour. A small in magnitude dose–response association was observed for clinically significant total behaviour symptoms (adjusted odds ratio 1.03; 95% CI: 1.01–1.06, p < .01). Exogenous oxytocin administration prior to birth was not associated with ASD (OR: 0.64; 95% CI: 0.15–2.12, p = .46) or high levels of autistic‐like traits (p = .93), as assessed by the AQ. Conclusions This study is the first to investigate longitudinal mental health outcomes associated with the use of oxytocin‐based medications during labour. The results do not provide evidence to support the theory that exogenous OT has a clinically significant negative impact on the long‐term mental health of children. - Journal of Child Psychology and Psychiatry, EarlyView.