Not to defend diet coke (any kind of soda is not healthy for you, regardless), but I would generally assume that drinking 144oz (assuming 18x8oz cans/day) of any type of beverage that isn't plain old water would tend to cause some level of serious health effects, given that's more than your entire general recommended daily fluid intake from all sources. I feel like the general takeaway is that most food and drink is bad for you in excess, and companies constantly slapping "diet/low fat/low carb/etc." labels on junk food products that are marginally healthier than their peers gives a false impression that you can have your cake and eat it too in terms of negative health effects from these foods/drinks.