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Pentazocine  - is a kappa partial agonist; a mu partial agonist, and a delta full agonist.  It is used to treat moderate to moderately severe pain. It is sold under several brand names, such as Fortral, Talwin NX, Talwin , Talwin PX, Fortwin and Talacen (with acetaminophen). This compound may exist as one of two enantiomers, named (+)-pentazocine and (-)-pentazocine. (-)-pentazocine is a  mu opioid receptor agonist, while (+)-pentazocine is not, instead displaying a ten-fold greater affinity for the the σ receptor. 

Side effects are similar to those of morphine, but pentazocine may be more likely to cause hallucinations and other psychotomimetic effects; side effects include cardiovascular effects and  respiratory depression . 

Most common Pentazocine side effects

• Drug abuse and dependence - (32 reports)
• Chest pain - (14 reports)
• Fever - (11 reports)
• Hypotension - (11 reports)
• Death - (10 reports)
• Cardiac arrest - (9 reports)
• Hyperkalemia - (9 reports)
• Fatigue - (8 reports)
• Breathing - slowed or stopped - (7 reports)
• Thrombocytopenia - (7 reports)

Pentazocine is classified under the Controlled Substances Act in the United States. Internationally, pentazocine is a Schedule III drug under the Convention on Psychotropic Substances.

Related and derivative drugs include 

• Cyclazocine is a delta partial agonist, a  ?-opioid partial agonist and a kappa partial agonist [Source:  Toll, L., et al.   (1998)  - Standard binding and functional assays related to medications development division testing for potential cocaine and opiate narcotic treatment medications. NIDA Res Monogr, 178: 440-466. [PMID: 9686407 ] .
• Dezocine (Dalgan) is an analgesic that can produce  euphoria at low doses,  but produces dysphoria and hallucinations at high doses, “most likely due to action at -opioid receptors”.  It has antagonistic action which was unspecified.  Side effects also include dizziness and respiratory depression.
• Metazocine has a mixed agonist-antagonist action at the mu opioid receptor.  It  clinical use is limited by dysphoric and hallucinogenic effects which are hypothesised as being caused by activity at kappa opioid receptors and sigma receptors
• Phenazocine (brandnames Prinadol, Narphen) is an analgesic with a similar profile of effects to Pentazocine, except that it produces euphoria or  dysphoria as well as hallucinations at high doses.  The receptor activity is speculated and is “most likely due to action at opioid  receptors”