My consultant advices not to have them for the first 2 weeks. Can't remember if she said they can slow down hitting kitosis or they contain a few more cals? Anyway my 2 weeks are up by the time I get my next lot of products so will have a few cos they are so much easier to have at work or out & about.
It used to be a common misconception that tetras were akin to bars and should be avoided in the first 2 weeks. But this isn't the case at all. They have pretty much exactly the same amount of calories, carbs, protein and fat as the packet shakes.
The only difference is that with a packet shake, you're likely to make it up to around 300ml (personally it's nearer 400ml) whereas tetras only contain 200ml. That's why it's recommended you have an additional glass of water with a tetra.
There is absolutely no reason why you shouldn't have tetras in the first 2 weeks and personally, I think it's madness not to encourage their use. Most people don't want to advertise to the world that they're on a diet by taking a blender to work - a tetra makes life so much easier!