As you said, they're not meant to be made in advance. The way they're made up, the minerals and vitamins start to deteriorate after 15 minutes, so the drink straight away thing is there for that reason. Also they'll start to separate. The slim fast will stay drinkable because that's how it's designed. It's not design as a mix and then drink kind of shake. But it also doesn't have the minerals and vitamins that CD shakes do. If mixing them at work is difficult, you can get a shaker which is easy and quick to use, add the water, add the powder and shake for about 60 seconds. I use that if I'm out, at uni, travelling etc. I'm like Leah, I can't stand the tetras, they make me feel ill, so is the only option for me. Hope you find the best option for you.