Yes I think others are right and officially they are allowed after week 2, as they can potentially delay going into ketosis.
CDCs differ in their advice though (not only on this - it seems they give different advice on so many things!)
Mine said I could have bars after week 1, and I must admit since then I've been having one a day and my losses have still been okay.
I'd definitely ask about them, they are a lifesaver for me as not only are they quite yummy, they are so convenient to have when out and about and at work etc.
Potter x