Pentazocine
Pentazocine. Mixed κ-opioid agonist + weak μ-opioid antagonist — mid-strength analgesic with ceiling effect on respiratory depression. Dysphoric /… Oral half-life 3 h.
Mixed κ-opioid agonist + weak μ-opioid antagonist — mid-strength analgesic with ceiling effect on respiratory depression. Dysphoric / hallucinogenic at higher doses (κ-mediated). Schedule IV; less commonly used now.
PK: THE INTRAMUSCULAR ABSORPTION RATE OF 17.55 WAS A SAMPLING ARTEFACT. It implies a 2.4-MINUTE absorption half-life, faster than most intravenous bolus mixing, and the only supporting numeral in its paper is "mean peak plasma concentrations at 15 minutes" — the EARLIEST SAMPLING POINT, so the peak was censored by the schedule rather than measured. The evidence runs the other way entirely: the intramuscular half-lives, 4.6 and 5.3 h, EXCEED the intravenous value of 3.38 h, which is the signature of absorption-limited input. The intramuscular bioavailability of 1 is likewise unobtainable, since that study had no intravenous arm. Oral F 0.184 and the volume of 5.56 L/kg are verbatim and stand, the latter being intravenous-derived and therefore safe to pair with a bioavailability. The three route half-lives now carry their own measured values rather than one rounded 3 h. The subcutaneous route is removed as declared but unsupported.
Pharmacokinetics
- Oral — t½ 3 h · F 18% · Vd 389 L
- IV — t½ 3.4 h · F 100% · Vd 389 L
- Intramuscular — t½ 4.6 h · Vd 389 L
Sources: PMID:923183