Tonazocine is an opioid

Opioid

An opioid is a psychoactive chemical that works by binding to opioid receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract...

 analgesic

Analgesic

An analgesic is any member of the group of drugs used to relieve pain . The word analgesic derives from Greek an- and algos ....

 of the benzomorphan

Benzomorphan

Benzomorphan is a chemical compound.It is the base for a series of drugs which variably act on the opioid and sigma receptors, including, but not limited to, the following compounds:...

 family which made it to phase II clinical trial

Clinical trial

Clinical trials are a set of procedures in medical research and drug development that are conducted to allow safety and efficacy data to be collected for health interventions...

s for the treatment of postoperative pain, but development was apparently ceased and ultimately it was never marketed. Tonazocine is a partial agonist

Partial agonist

Partial agonists bind and activate a given receptor, but have only partial efficacy at the receptor relative to a full agonist...

 at both the mu-opioid and delta-opioid receptors, but acting more like an antagonist

Receptor antagonist

A receptor antagonist is a type of receptor ligand or drug that does not provoke a biological response itself upon binding to a receptor, but blocks or dampens agonist-mediated responses...

 at the former and more like an agonist at the latter. It lacks most of the side effect

Side effect

In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequences of the use of a drug.Occasionally, drugs are...

s of other opioids such as adverse effect

Adverse effect

In medicine, an adverse effect is a harmful and undesired effect resulting from a medication or other intervention such as surgery.An adverse effect may be termed a "side effect", when judged to be secondary to a main or therapeutic effect. If it results from an unsuitable or incorrect dosage or...

s on the cardiovascular system and respiratory depression, but it can cause sedation

Sedation

Sedation is the reduction of irritability or agitation by administration of sedative drugs, generally to facilitate a medical procedure or diagnostic procedure...

 (although to a lesser degree of typical opioids), and in some patients it may induce hallucination

Hallucination

A hallucination, in the broadest sense of the word, is a perception in the absence of a stimulus. In a stricter sense, hallucinations are defined as perceptions in a conscious and awake state in the absence of external stimuli which have qualities of real perception, in that they are vivid,...

s.