A 2-year-old female with normal birth and developmental history presents with increased agitation and decreased arousability. Her father suffers from chronic pain secondary to a back injury, and her mother found an open container of pills on the bed. Vitals reflect bradycardia, bradypnea, hypotension, and slight hypothermia. On physical exam, she exhibits somnolence, constricted pupils, hypoactive bowel sounds, and hyporeflexia. What substance was most likely ingested