An italian study has recently concluded that aspartame, even at doses of 20mg/kg of body weight can have carcinogenic effects
Now, I don't know how realistic the risk is since I don't yet have data on how much aspartame a can or 500ml bottle of diet coke contains. Some googling gives me some numbers with no units (0.06% [by volume?] or 15 [no unit at all, mg?]), so I won't link to those as the lack of units indicates that the informants are too dumb to be worth listening to or has no interest in accuracy. In any case, I don't drink diet coke much anymore.
What I do drink is iced tea (just steeped green tea) with Summit Clear, for the sweetness, since the green tea doesn't have any sugar.
Summit Clear has Sucralose which isn't aspartame. Good news, since I have to make it to 62 or so, by which time Timmy will be 20+ and independent.