Yes, Claritin (loratadine) can dry up a person's milk supply, particularly if they take it alongside a decongestant such as pseudoephedrine.
Claritin can also pass through into the milk, so a person should exercise caution if they take loratadine while nursing. They should take low doses over short periods.
However, researchers consider Claritin safe, and it does not appear to cause adverse effects in infants.