Pethidine is a narcotic and gets into breastmilk. Its okay for us, it takes a few hours to filter out our system. But in a newborn, it can take days and is not good for establising breastfeeding - makes baby sleepy and less effective sucking etc. so not an ideal situation. I only heard about how bad it was a few months ago when the leading authority in breastfeeding and drugs spoke at a seminar in Australia and urged women to consider something other than pethidine during labour (and after) as its one of the two drugs majorly contraindicated for breastfeeding.