Is it safe to take an antihistamine 5 days post op tummy tuck and breast lift?

There should really be no problem taking an antihistamine even the day after the procedure. After several weeks there certainly is no issue.

In my opinion the use of antihistamines prior surgery is okay.  I would discuss this with your operating surgeon.

In some cases, I will prescribe antihistamines for itching related to narcotics after surgery.  I will always caution the patient that antihistamines may make them more drowsy in addition to the narcotics, therefore, they must have someone with them to assist them if they become too drowsy.  

Benadyl can make you drowsy and is great to take at night. A topical spray can help, but you should talk to your surgeon before using the spray.