The Family Smoking Prevention and Tobacco Control Act (FSPTCA) of 2009 creates the first national system of premarket regulation of tobacco products in American history. The FDA must now review and give marketing authorization to all new tobacco products, based on a public health standard, before they can be legally marketed. Yet the law also contains an alternative pathway for market entry—the substantial equivalence (SE) clause—by which novel and altered tobacco products can be marketed by demonstrating their substantial equivalence to existing products. Over 99 percent of tobacco product applications sent to the FDA under the new law have used this mechanism, and loose application of the SE mechanism carries the risk of undoing the FDA's gatekeeping power under the law. We review the statutory and regulatory precedent for SE, examining the FSPTCA itself as well as regulatory precedent from drug and device regulation (from which the term substantial equivalence and much of the associated statutory language was derived). Our review of standards and scientific precedent demonstrates that exacting scrutiny under the public health standard should govern all SE reviews and that clinical data incorporating social scientific evidence should be routinely required for SE claims by tobacco product sponsors.