The present invention provides methods of treating or preventing a substance-related disorder using selective delta opioid receptor-1 (DOP-R1) agonists, delta opioid receptor-2 (DOP-R2) antagonists, and/or mu opioid receptor (MOP-R) antagonists, or a pharmaceutically acceptable salt, solvate, hydrate, or prodrug thereof. The methods provided herein further comprise administering a therapeutically effective amount of a combination of a DOP-R1 agonist and a DOP-R2 antagonist. The methods also comprise administering a therapeutically effective amount of a combination of a DOP-R1 agonist and an MOP-R antagonist. The methods provided herein further comprise administering a therapeutically effective amount of a combination of a DOP-R1 agonist and a DOP-R2 antagonist and a MOP-R antagonist. The invention also relates to compositions containing the same. The invention also relates to methods of determining delta opioid receptor specificity of candidate agents.