Providing straightforward information pertaining to drugs, drug use & drug policy. The Grey Pages promotes drug-related literacy and advocates a system of viable and tolerant drug policies. This is my personal collection of commentaries, essays, tid-bits, and other such writings on everything ranging from drug use, drug policy and drug-myths, to drug-science, addiction, human behavior, and the workings of the human brain. I started this blog with a particular focus on opioids, and over the past year have found my interest gravitate toward the intriguing, ever-changing world of designer intoxicants (i.e. "research chemicals" or "designer drugs").

Sunday, November 20, 2011

Receptor Affinity & Selectivity of Common Opioid Agonists

Buprenorphine: mu partial agonist, kappa antagonist, ORL1-partial agonist

Morphine: mu agonist, delta & kappa agonist (in high doses) . 38/1 mu/kappa selectivity

Oxycodone: predominant mu agonist, delta & kappa agonist (very low affinity) . 135/1 mu/kappa selectivity

Hydrocodone: mu agonist, delta & kappa agonist (in high doses) . 68/1 mu/kappa selectivity

Hydromorphone: mu agonist, delta & kappa agonist (in high doses) . 26/1 mu/kappa selectivity

Oxymorphone: mu agonist, delta & kappa agonist (in high doses) . 82/1 mu/kappa selectivity

Levorphanol: mu agonist, kappa agonist, delta agonist, NMDA antagonist, 5HT & NE reuptake inhibitor . no selectivity

Methadone: predominant mu agonist, NMDA antagonist, nACh antagonist, delta & kappa agonist (very low affinity) . 493/1 mu/kappa selectivity

Fentanyl: predominant mu agonist, delta & kappa agonist (very low affinity) . 121/1 mu/kappa selectivity

Etorphine: mu agonist, kappa agonist, delta agonist . no selectivity

Codeine: mu agonist, delta & kappa agonist (in high doses) . 35/1 mu/kappa selectivity

Tramadol: selective mu agonist (very low affinity), 5HT & NE reuptake inhibitor . high selectivity

Ranked Mu-Receptor Affinities of Various Opioids:

A series of binding assays was performed to rank the relative MOR binding affinies of common opioids - The drugs have been ranked by 3 groups - Higher numbers indicate a weaker affinity, while lower numbers indicate strong affinity

NM= nanomolars MM= micromolars

MOR Binding Potency

100nm or greater (weak) - Tramadol, Codeine, Pethidine, Propoxyphene, Pentazocine

1 to 100nm (moderate) - hydrocododone, oxycodone, Methadone, fentanyl, morphine

1nm or lower (strong) - levorphanol, oxymorphone, hydromorphone, buprenorphine, sufentanil


  1. Tramadol is basically a prodrug. Its active metabolite: O-desmethyltramadol, is over 100 times more potent than tramadol. Therefore a MUCH higer affinity for the μ receptor.

    1. Yes, but these metabolites don't occur in high enough quantities to account for much, unless you're <1% of the population where it does. It's kinda like how some of the oxycodone metabolizes into oxymorphone, but in sufficiently small quantities as to not affect anything.

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