Yes, it is perfectly safe to take Benadryl while on birth control. It is also safe to take Sudafed and other decongestants. It is only antibiotics that can interfere with birth control, as well as some MOAI inhibitors (for those who are on anti-psychotics or anti-depressants). So go end your allergies!
Yes, you can take benadryl while on birth control, but like with any medication make certain that you use an alternative form of birth control during this time as it can lesson the effects of the birth control. Make it not as effective as it is if you are not taking medication.