(-)3-((S)-((2S,5R)-4-Allyl-2,5-dimethyl-l-piperazinyl)(3-thienyl)methyl)phenol and pharmaceutically acceptable esters or salts thereof, in essentially enantiomerically pure form have utility as receptor-binding species, e.g., as therapeutic agents for mediating analgesia; as co-administered agents with various other bioactive compositions, including anesthetics, barbiturates, analgesics, etc., for reducing, treating, reversing or preventing drug-mediated respiratory depression that may be directly or indirectly caused by use of such various bioactive compositions; as a conjugate in agonist/antagonist pairs for verifying/assaying receptor and neurotransmitter function; and as a therapeutic agent having utility in combating drug addiction, cardiac disorders, alcohol addiction, drug overdose, cough, lung edema, diarrhea, respiratory, and gastro-intestinal disorders.