I recently got an email from reader Karen Kosel asking exactly that; she followed up by posting me a website that said some really unpleasant things about Splenda - to be specific, that it may cause shrinkage of the thymus gland, which is part of the immune system, and may also cause enlarged liver and kidneys. For balance, they also concluded that sucralose (the chemical that makes Splenda sweet) was probably "not as toxic as aspartame".