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.

  2. Stopping opiates is actually a difficult battle, nevertheless it could be gained using the proper resources. Today we are likely to assessment the top methods to relaxed withdrawal signs due to prolonged opiate habit in order to proceed to an existence located in restoration.
    loperamide for opiate withdrawal

  3. We provide a safe and secure way for you to get TOP SHELF GRADE AAA+ MEDICAL MARIJUANA/CANNABIS AND PAIN PILLS.call or text 857-288-8354. email-medicaoutlet1@gmail.com and please only serious sms or callsHydros Yellows: 10/325-$4.50 each
    > Hydros Blues 10/500-$3.50 Each.
    > 16/32 OZ Actavis prometh codeine cough syrup
    > 30mg IR adderall-$3.00 each
    > 20 mg IR adderall-$2.00 each
    > Percocet 5mg-round white- $3.00 each
    > Morphine-$11 each
    > Dilaudid 8mg-$6 each
    > Dilaudid 4mg-$3 each
    > Xannies (White and yellow Bars)-$6.00 each
    > Roxicet 30 blues-$9.00 each-sale!!
    > OC 80's-$15.00 each
    > Fent pops Atiq 1200mcg-$22.00 each
    > Fent pops Atiq 1600mcg-$27.00 each
    > Fent Patches Gel 100mcg-$12 each