Hi, there are a lot of misconceptions surrounding diet drinks/artificial sweeteners and negative health outcomes, however pretty much all of theses have little to no basis in science. Current evidence indicates that there is little reason to worry about consuming artificial sweeteners instead of sugar, unless you fall into one of the following categories: children, pregnant women, nursing mothers, and those prone to seizures, headaches or migraines.
To summarise, having the odd glass of a diet drink is fine.