Hi bianca. I'm not a doctor, I'm not a scientist, but I know quite a lot about the effects of various chemicals on the body, and to my mind, aspartame's one of the worst. This's found in diet drinks (carbonated ones), as well as in stuff like Alka Seltzers, toothpaste and lots of other commonly-used things.
Aspartame is banned in many countries abroad, but the UK still uses it.
It's made up of 3 things, one of them being methanol. It breaks down in the body into formaldehyde - which is used as an embalming fluid - and formic acid (found in ant sting poison)!
When formaldehyde enters body cells, it binds itself to proteins and DNA. Changes in DNA've been shown to've resulted from formaldehyde exposure, and so whether it causes cancer or not, I would personally steer clear of anything containing aspartame. It's used because it's 200 or more time sweeter than sugar, so a small amount goes a long way - without the calorie content - but I wouldn't want to risk the side effects!