“Diet soda is worse for you than regular soda.”
Moderate confidence because randomized trials consistently show that replacing sugary drinks with diet drinks lowers sugar and calorie intake and can modestly help weight. There is no good evidence that diet soda is worse than regular soda. Long-term disease associations are mostly observational and may be affected by confounding and reverse causation.
Diet soda is not worse for you than regular soda. When it replaces sugary soda, it removes most of the sugar and calories and may modestly help with weight management. You do not need to drink it, but you also do not need to fear it.
The useful question is what it replaces. Water is a good everyday default, but diet soda is a reasonable step down from regular soda and can still fit into a healthy eating pattern.
Replacing regular soda with diet soda is different from replacing water with diet soda. The strongest benefits appear when it displaces a sugary drink.
Can diet soda help reduce sugar?
Yes. It can help someone reduce added sugar and calories when used instead of regular soda. It is a tool, not a requirement or a magic weight-loss drink.
Safety note: If you have phenylketonuria (PKU), avoid or restrict aspartame according to your care plan. Check the drink label.
If you enjoy diet soda, normal use does not need to be a source of fear. Pay more attention to your overall eating pattern than to whether every drink is perfectly clean.
If you regularly drink sugary soda, try replacing one with diet soda, sparkling water or water. Choose the option you can realistically maintain.