Background:  Castor oil is one of the most popular drugs for induction of labour in a non‐medical setting; however, published data on safety and effectiveness of this compound to induce labour remain sparse.Aim:  To assess the safety and effectiveness of castor oil for induction of labour in pregnancies with an ultrasound estimated gestational at birth of more than 40 weeks.Methods:  Data were extracted from hospital‐based records of all pregnant women who attended antenatal clinics on the Thai–Burmese border and who were more than 40 weeks pregnant. The effectiveness of castor oil to induce labour was expressed as time to birth and analysed with a Cox proportional hazards regression model. Measures associated with safety were fetal distress, meconium‐stained amniotic fluid, tachysystole of the uterus, uterine rupture, abnormal maternal blood pressure during labour, Apgar scores, neonatal resuscitation, stillbirth, post‐partum haemorrhage, severe diarrhoea and maternal death. Proportions were compared using Fisher's exact test.Results:  Of 612 women with a gestation of more than 40 weeks, 205 received castor oil for induction and 407 did not. The time to birth was not significantly different between the two groups (hazard ratio 0.99 (95% confidence interval: 0.81 to 1.20; n = 509)). Castor oil use was not associated with any harmful effects on the mother or fetus.Conclusions:  Castor oil for induction of labour had no effect on time to birth nor were there any harmful effects observed in this large series. Our findings leave no justification for recommending castor oil for this purpose.