Q. My female lab mix was taken to vet due to licking both front legs down to the skin she was put on prednisone and antibiotics.and she has started agaid
A. I would take her back for a follow up with your vet. It is possible that she is having a relapse if she has been off the meds for a while now, or she may need something stronger/different. I wonder if the licking was caused by an allergy or a mite (like mange)-did your vet do a skin scraping to confirm the cause (they do this if they think the hair loss is from mites).
The other thing to consider would be her food. Its unlikely that such an extreme case is food allergy related but it is still possible. I would try switching her to a high quality, grain free dry food with a different type of protein ex. Eating a chicken based food now, switch to a fish food and see if this helps in addition to new meds.