With increasing severity of pain, higher doses are required for reduction of pain perception. However, these higher doses are accompanied by the dulling of all senses, leading to altered consciousness, and a reduced urge to breathe. In worst cases these drugs can dangerously impair breathing in the mother and, more commonly, in her newborn.
The chance of seeing side effects in the newborn depend on the dose and time the medication was given prior to delivery. Therefore, these medications are not usually given when delivery is imminent.