Sarahlou - the correct 100% answer is no. I still kind of think the first time you do a VLCD you shoudl try to 100% it to give yourself the best chance. However, some people have found that drinking Coke Zero doesn't take them out of ketosis or cause them any issues. It's not approved by CD, it's not part of the diet, but like I said above - it is a tool you can consider if you are really really struggling) or you're an old hand at the diet and are happy with all the chemically rubbish in diet drinks!