Background: The misrepresentation and adulteration of illicit drugs is an enduring feature of illegal drug markets. The use of unregulated drugs increases the risk of overdose, adverse health events, and other unwanted effects. Objectives: We used data from DrugsData, an anonymous drug checking service, to investigate deception in the US ecstasy market between 1999-2021. We documented trends and patterns in the misrepresentation and adulteration of MDMA and compared the nature and extent of deception across different forms of ecstasy. Methods: We operationalized various metrics of misrepresentation and identified specific MDMA adulterants, including timing of market entry and exit. Analytically, we employed descriptive statistics, trend analysis, and tests of bivariate associations to investigate trends and patterns of deception. We also assessed the criterion validity of DrugsData against law enforcement seizure data collected by NFLIS-Drug. Results: Most (58%) of the alleged ecstasy in our sample was misrepresented in some manner. Patterns of misrepresentation varied across time, as exemplified by periods of decline (1999-2009, MDMA concentration decreased from 51% to 11%), rebound (2009-2018, MDMA concentration increased from 11% to 59%), and stabilization (2018-2021, MDMA concentration stabilized between 71% and 75%). We also documented 179 known substances other than MDMA in alleged ecstasy items and found that “molly” was more likely to contain novel psychoactive substances. Conclusions: Drug checking services can play an important role in monitoring these dynamics to reduce these harms. Increasing funding for drug checking services and removing bureaucratic barriers to their operations should be a US drug policy priority.