An agonist is a chemical substance that binds to specific receptors in the body and activates them to produce a biological response. For example, fentanyl and methadone act as opioid receptor agonists. A full agonist produces the maximum possible effect at a receptor, while a partial agonist activates the receptor but produces a weaker or limited response compared to a full agonist.
Sources: SAMHSA_NCBI; Recovery Research Institute; UNODC