Copied directly from the website below. Good summary.
So, is Splenda safe? The truth is we just don’t know yet. There are no long-term studies of the side effects of Splenda in humans. The manufacturer’s own short-term studies showed that sucralose caused shrunken thymus glands and enlarged livers and kidneys in rodents. But in this case, the FDA decided that because these studies weren’t based on human test animals, they were not conclusive. Of course, there are countless examples of foods and drugs that have proved dangerous to humans that were first found to be dangerous to laboratory rats, and then again, countless others that have not. So the reality is that we are the guinea pigs for Splenda.