No, all antihistamines are NOT alike. Benadryl is in a class of antihistamines called first generation. While effective at controlling allergy symptoms, the first generation drugs also cause some quite bothersome side effects in many who use them, including severe drowsiness. Not everyone has these side effects, but many do.
So, eventually, a new class of allergy drugs was developed called second generation antihistamines, which includes drugs like Claritin, Clarinex, Zyrtec and Allegra. These drugs are just as effective at controlling allergy symptoms, but they are much less likely to cause side effects, especially the drowsiness side effect.
One problem with the newer medicines, though, is that because they were "new", they were only available by prescription. But in the past couple of years, both Claritin and Zyrtec have become available over the counter.
So there may be a viable alternative to Benadryl that you can use. However, I would encourage you to speak with your doctor, as he/she knows your condition and other treatment you are getting.
To your health,