This year, both Coca-Cola and Pepsi have had to review their recipes after one of their ingredients was deemed to be carcinogenic. This is a caramel flavouring called MEI, and the company has to use less of it or put a cancer warning on the bottle or tin.
However, even without MEI there are clear health issues associated with drinking Coca-Cola or indeed a lot of other soft drinks. Basically they contain large amounts of sugar and very little nutrition, which is bad news both for your weight and your teeth. Diet Coke is less harmful in that way, but most so-called diet products will not really help you to lose weight in the long run - only a balanced, varied diet high in fresh, natural foods will do that. Coke also contains caffeine, so it can give you a false energy boost which makes it hard to relax but doesn't give real energy.