Think some may react a little diff. than others. I have always used Test E. and never had a prob., shared a dose from same vial for my buddy and he had some bad pip and reddish itchy rash about 4 hours afterwards. He never had a problem with Test C. But for some reason the E give him trouble. He got throught it for awhile until he got his Cypionate back in by warming up the Test E. before inj.