The DEA classification system is a matter of legal restriction that does not align very well with actual addiction risk. LSD is Schedule I (illegal) but is essentially non-addictive and appears to be effective for PTSD and depression. Long-acting formulations of Schedule II stimulants (Vyvanse, Concerta) have relative low abuse potential. The benzodiazepine alprazolam (Xanax) is highly addictive but is only a Schedule IV.
Schedule I - High potential for abuse, no accepted medical use (illegal)
Schedule II - High potential for abuse, with accepted medical use; refills not allowed
Schedule III - Potential for severe mental addiction or moderate physical addiction
Schedule IV - Abusing the drug may lead to mild addiction
Schedule V - Abusing the drug may lead to mild addiction, but lowest potential for abuse
Non-scheduled - Some of the examples shown may have abuse potential, but are not restricted by DEA