Serum creatinine level 2.56 to 1.59 in 2 weeks after starting on prednisone and discontinuing benicar (olmesartan). is this artificial due to 10% weight gain?

No: Creatinine is muscle breakdown product that is excreted through kidneys. It is affected by muscle mass or renal function decline but weight gain does not affect it. Dehydration OR Hydration can make BUN go up or down but creatinine stays the same. Therefore both the numbers are important to make right assessment.

Yes but not artifact: Without knowing your full history we can only speculate but the drop in creatinine is good news all around. Severe dehydration can lead to increased creatinine but the effect isn't all from concentration. Your 10% weight gain is probably mostly water weight and to the extent it represents increased intravascular volume means greater blood flow to kidneys and thus improved function.