As with any drug, there is always the possibility that you could take too much and overdose. Benadryl is a drug that blocks histamine receptors. It has side effects including urinary retention (having difficulty peeing) and sleepiness. These side effects are even more pronounced when you take the medicine every day. If you do not develop these side effects, a medication like Benadryl is generally safe.
I will say that I think that your Benadryl is probably not help much with your itching. Your itching is caused by a medications side effect. Narcotic pain killers often have this side effect and can be a very annoying thing to deal with. The itching that comes along with the narcotic pain killers is not caused by histamine release. This means that Benadryl is probably not helping you much. Your best bet is to transition (at least partially) to a different pain killer. You can talk to your doctor
about what type of pain killer is best for you to try in your particular situation. The best type of physician for you to talk to for this issue is your primary care physician
. The two of you can discuss your back pain, your pain medication side effects, and the side effects of Benadryl to watch out for.