S.L.
I had this same question for my pediatrician when I nursed my son, so I know this information was accurate 3 years ago, but I can't guarantee that nothing has changed. More is known about Claritin and it is considered to be slightly safer than Allegra, mostly because not much is known about Allegra. It is summarized here http://kellymom.com/bf/can-i-breastfeed/meds/cold-remedy/ if you are interested (Claritin is L2 and Allegra is L3). Claritin-D and Allegra-D both have a decongestant component, which can dry up your milk, so don't take those.