Decay at tooth margin causing an abscess. dentist wants to decrease infection with antibiotic before possible root canal retreatment. good idea?

Of Course: Always a great idea to decrease an infection. Did the decay cause a gingival abscess or has the decay entered the canal itself? If into the canal, it's not a possible root canal, it's a necessary root canal. Let your dentist evaluate and provide appropriate care.

Yes: If your dentist said so I would agree.

Rt Canal Retreatment: Good day; Decay around the margin would not effect a need for endodontic retreatment of the tooth. Typical retreatment and antibiotic medication are usually initiated simultaneously causing the swelling and discomfort to diminish.