What's the most and fastest weight gain-inducing drug ?

Hello, I'm going to try and keep it short. I need to gain as much weight as humanly possible in around 12 days and it's for something extremely important to me. I'm a physician and I know a fair bit about drugs in general and I was planning to take prednisone, although I'm not sure if it's gonna make much of a difference in just 12 days, so I was wondering if there's something, anything, more potent in this short of a time period while being relatively safe.