I had coke zero all the way through the first time I did CD and had fab losses. I still have it now, but my losses haven't been as good as the very first time, but I don't blame the coke. I guess it depends on the individual.
My CDC is quite anti coke zero etc, as she says once you start introducing things that's in the "grey" area of the diet, it's hard to resist other temptations...so for now I won't have anything but water fizzy water and black tea/coffee (boring!!).
However she was not horrified when I told her I was going to be eating out for my birthday in a few weeks time, she said by then my body might do with the relief of giving myself a "head" break...so ling as I get straight back to it the following day that is...
No, I hate coke zero, so I drink diet coke (and diet lemonade, on occasion). And I know it has citric acid in and could knock me out of ketosis but I couldn't drink enough water to stay hydrated. I was dizzy constantly. My losses (so far this week, started drinking in on Tuesday) are what I'd expect: 2 pounds in three days (by my scales). I'm still in ketosis, according to the sticks, and feel much happier and more content all round. To each their own, I say, as long as it works for you
But be careful if you're having other diet drinks. I think Dr Pepper Zero is also okay because there's no citirc acid in it - but citric acid can interfere with ketosis - in other words, your body may not be able to break down fat and use ketones for energy while there's citric acid knocking about. Your body will have to burn something though, and as we're not eating many carbs, it'll probably burn muscle instead. Not good. You'll still lose weight - but it'll be muscle you're losing, not fat.
The odd can here and there when you're desperate and can't get your hands on something 'safe' won't do you too much harm, I don't think. But it's probably wise to restrict consumption of citric acid as much as possible.
Just to throw something into the mix, before CD I drank diet coke like tap water totally addicted (2ltr bottle a day) so much so had to change to caffine free. Coca cola then costing me loads so changed to Tesco own brand (third of the cost). My point is coca cola caffine free diet coke has citric acid in it, Tesco's doesn't so maybe ok to drink. However I have gone cold turkey and come off it completely.