severe itching after prolonged zocor use?

My husband has been taking Zocor for about a year. He has recently developed severe all-over itching. No hives. His doctor has given him Prednisone and told him to use Benadryl. Neither of which are working very well, if at all. I'm wondering if this is a long-term use allergy developing, or if it might be unrelated. He had to quit taking Tricor and Lipitor due to itching as well. Although those reactions were apparent with days of starting. Any suggestions??