I doubt they put morphine or any "real" drug in it, but the chemicals (Aspartame and Acesulfame) in it are
addictive and are horrible for one's health. For one thing, they contain carcinogens. They can also cause headaches, trigger migraines, exacerbate depression and mental confusion... the list goes on. I am also sure I read a study in which aspartame was linked to increased risk of seizures for epileptics but with some Googling I have not found any references. I think it goes without saying you'd be much better off without drinking it at all. It's up to you obviously but if you think it'll kill you, I don't see why you'd want to have it!
Here's a brief article about non-sugar sweeteners (the link goes straight to the page briefly addressing the ones in Coke and Pepsi): Acesulfame K: What are the pros and cons? on MedicineNet
PS - Acesulfame K is not just in drinks. It comes in many other sugarfree products like gum and even pickles.
I even limit my intake of stevia... I just think things pretending to be something else (in this case a sugar substitute) should not be consumed in excess, even if it's "natural".