Once your body recognizes that it has ingested a virus, toxin or poison it will expel it by inducing vomiting. Unfortunately, it takes a while for it to tell, so it keeps vomiting even when the stomach is empty (the toxin is in the blood by then). Alcohol is a great example of a toxin that is absorbed in great quantities before inducing vomiting. But you can short cut this by "thinking" something might contain toxins by thinking its really gross and throwing it up sooner, before a lot is absorbed.