I ask this question now because I am 5 mos pregnant and scared to death of what to do when the itch returns I want to be able to breastfeed soley and successfully. If not, it may not be a problem this time.
Some women do notice a slight decrease in milk supply while taking antihistimines.
If this was taken only during the pregnancy I don't see how it would have any affect at all on your milk supply. I would suspect other problems as the cause for not being able to breastfeed your first child. Seek professional help from a Lactation Consultant in your area at the first sign of trouble with this baby. You may consider a prenatal consultation to discover what might have been the problem last time as I would doubt that the benedryl could have caused you to have so much difficulty. ACCE, has been a childbirth educator and breastfeeding counselor for over 18 years.