Like many synthetic drugs, such as bath salts, it’s impossible to know what exactly is in K2/Spice—the ingredients and quantities are a complete mystery without a lab to test them. We do know that these man-made chemicals bind more powerfully to receptors in your brain than marijuana, making their side effects far more dramatic.
You will never know what batches or blends could put you in the emergency room or prove to be fatal, so they should be avoided regardless of how legitimate the label appears.