Multiple Substance Impaired Driving Prevention Act of 2021 This bill includes substances other than alcohol within the scope of federal programs for preventing impaired driving. It also addresses reporting of impaired driving arrests, citations, convictions, and related matters to federal and other databases. Under current law, if a state does not have (or enforce) certain laws to address repeat intoxicated drivers, a portion of that state's Federal-aid highway funding must be reserved for alcohol-related countermeasures for impaired driving. This bill provides that reserved funds may also be used for countermeasures for single or multiple substance impaired driving. Additionally, states may use grants for impaired driving countermeasures to improve (1) reporting of information about crashes and impaired driving to federal, state, and local databases; and (2) research using such information. The Government Accountability Office must also study the extent to which states and localities report information about impaired driving to relevant federal databases and use the databases to identify repeat impaired driving offenders.