Hcg stays in your system for 3 days. I also gained a lot of weight back after Hcg, it seems like it caused food sensitivies I didn't have before, but the old timers say we've always had these sensitivies but didn't know it. It also seemed like the weight came back faster than I have ever gained weight in my entire life. Again, the old timers will say that is due to food sensitivities we've always had. I have basically lived on Atkins for a year now as a result and can maintain my weight where I'm at, and will lose a little here and there only to gain it all back if I eat gluten or sugar.