What are the negative effects of drinking carbonated soft drinks on the body?

It depends on what all is in them and how much you drink, of course. But if you're interested in what the "fizz" does, and that fizz is produced by carbon dioxide, there are a couple of results possible. But let's look at CO2 in water first. The CO2 in water creates carbonic acid. It's a weak acid, but this acid can upset the stomach a bit, depending on what else you have in there. And because you ingest CO2 in water, it may slightly alter your blood chemistry, but not too much.