It is my understanding that the booster doesn't replace any product but actually boosts the effectiveness of all of the other products. Dr. G does say to put the collagen serum on first and then use the booster. I'm not quite sure why that is.
I agree with Cheesy. The booster makes everything work better. When you use the collagen serum, it draws into your skin fast and makes you feel like you are dry when in fact you have super fast absorbtion. When it absorbs that fast it draws in all moisture you have. So applying it afterward will still help it fraw into your skin. I apply the serum over a moistened face after cleansing. It also makes your skin look younger and smoother from this absorbing effect it has. I went out to dinner last night with my daughter and she ordered a glass of wine and got carded. She is 21. The waitress couldn't believe she was old enough and my daughter pointed to me and said, just ask my mom. The waitress thought I was her older sister and couldn't believe I will be 50 this year. So proof that is does help. I used the serum under my makeup before we went to dinner.