How long until hives go away after stoppping penicillin?

It depends: Hives that areresult of taking penicillin usually resolve in less than a week. However, if you have developed a more severe allergic reaction, lesions may stay longer and may require treatment with anti-histamines and sometimes corticosteroids ("cortisone"). If otc antihstamines such as Cetirizine or Loratadine haven't helped, see a physician.