It is unlikely that enough Benadryl will get into the milk to cause the baby to be sleepy, but it is possible. However, many antihistamines, such as Benadryl, seem to decrease milk production. In young babies (say, younger than about six weeks), getting slow flow of milk from the breast often results in their falling asleep at the breast before they have had a full feeding. This may be interpreted as the baby being sleepy from the drug when in fact it is due to the baby falling asleep because feeding is taking longer.
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