Streamlining Research on Controlled Substances Act of 2021 This bill makes changes to the registration process for conducting research on controlled substances that are included in schedule I of the Controlled Substances Act. Among the changes, the bill does the following: exempts from the registration requirements an agent or employee of a registered researcher, permits a single registration for contiguous research sites, and requires the Drug Enforcement Administration and the Department of Health and Human Services to review and issue guidance on the processes to obtain or modify federal authorization to conduct research with controlled substances.