It might do a little harm, but everyone’s bodies are different, and therefore handle medication differently. It may not have any effect on you at all, good or bad. Meaning, it might make you groggy, or it might not improve your allergies at all beyond what you experience with the 1 pill. However, that being said, Claritin does nothing for my allergies—you might consider a change of medication. Benadryl is the only one that seems to work for me, but once I took a pill too many and knocked myself out cold for 9 hours.