Firstly, painkillers before a shot? Americans have always puzzled me. To prevent any discomfort during a shot a local anesthetic is the only option, I hope this sounds ridiculous because it's meant to. Contrary to the popular belief Americans have painkillers are not always the answer. I have included a link to the best health service in the world. Painkillers reduce pain for stuff like teething however if you do not allow your babies natural pain relief mechanisms to develop then they will most likely always have a very low pain threshold. Medication should always be the last resort not the first. Believe it or not a hug will help. All painkillers can damage the body. Especially in infants. Please read the link. It is good advice.