I think it depends on the thickener, some you give prior to the EBM so it starts thickening any contents before the EBM hits and some you mix into the EBM. I was told to give it prior to the EBM. If the thickener isn't working you can also give 1/4 of a capsule of slippery elm mixed with 5mls of EBM 10 minutes prior to feeding. I used to do that with Jovie to help her reflux. With Matilda I did the medicated, thickened formula route & with Jovie I did the homeopathic route. I have to say, Jovie's been heaps easier.