Opioid analgesic

Opioid analgesics, also called narcotics, are drugs usually used for treating pain. Opiod analgesics are defined as "all of the natural and semisynthetic alkaloid derivatives from opium, their pharmacologically similar synthetic surrogates, as well as all other compounds whose opioid-like actions are blocked by the nonselective opioid receptor antagonist naloxone.

Pharmacology
There a several opioid receptors. All are are G-protein-coupled cell surface receptors.
 * Mu receptors are responsible for analgesia.
 * Delta
 * Kappa

Available opioid analgesics
Current opioid analgesics are below Tables of morphine equivalent daily dose and IV to PO conversion are available to help dosing.
 * 18,19-dihydroetorphine
 * 1) Alfentanil
 * 2) Alphaprodine
 * 3) beta-casomorphins
 * 4) Buprenorphine
 * 5) Butorphanol
 * 6) carfentanil
 * 7) Codeine
 * 8) deltorphin I, Ala(2)-
 * 9) dermorphin
 * 10) Dextromoramide
 * 11) Dextropropoxyphene
 * 12) dezocine
 * 13) dihydrocodeine
 * 14) Dihydromorphine
 * 15) Diphenoxylate
 * 16) dynorphin (1-13)
 * 17) endomorphin 1
 * 18) endomorphin 2
 * 19) Enkephalin, Ala(2)-MePhe(4)-Gly(5)-
 * 20) Enkephalin, D-Penicillamine (2,5)-
 * 21) enkephalin-Met, Ala(2)-
 * 22) eseroline
 * 23) Ethylketocyclazocine
 * 24) Ethylmorphine
 * 25) Etorphine
 * 26) Fentanyl
 * 27) Heroin
 * 28) Hydrocodone
 * 29) Hydromorphone
 * 30) ketobemidone
 * 31) Levorphanol
 * 32) lofentanil
 * 33) Meperidine
 * 34) Meptazinol
 * 35) Methadone
 * 36) Methadyl Acetate
 * 37) Morphine
 * 38) Nalbuphine
 * 39) nocistatin
 * 40) Opiate Alkaloids
 * 41) Opium
 * 42) Oxycodone
 * 43) Oxymorphone
 * 44) paracymethadol
 * 45) Pentazocine
 * 46) Phenazocine
 * 47) Phenoperidine
 * 48) Pirinitramide
 * 49) Promedol
 * 50) protopine
 * 51) remifentanil
 * 52) Sufentanil
 * 53) Tilidine
 * 54) tyrosyl-1,2,3,4-tetrahydro-3-isoquinolinecarbonyl-phenylalanyl-phenylalanine

Drugs that are both mu-opioid receptor agonists and norepinephrine reuptake inhibitors.
 * 1) Tapentadol
 * 2) Tramadol

Effectiveness
Narcotics are commonly prescribed for pain, and their usage may be increasing. In emergency rooms, non-Hispanic white patients are more likely to receive narcotics than patients of other ethnicities.

Narcotics are effective for both short (1-16 weeks) and long-term (6-24 months) use.

Narcotics, with long-term use, 80% of patients may have drug toxicity, most commonly gastrointestinal. In addition, substrance abuse and "aberrant medication-taking behaviors" may occur. Advice for using administering chronic narcotics and for treating acute pain among patients on chronic methadone is available.

Constipation
Constipation may be reduced by methylnaltrexone, a mu-opioid receptor antagonist. In a randomized controlled trial, 48% of patients receiving methylnaltrexone had a bowel movement compared to 15% of patients received placebo (number needed to treat = 3.0. Click here to adjust these results for patients at higher or lower risk.) Although mu-receptors provide analgesia, methylnaltrexone is a charged quaternary amine so that it does not well cross the blood-brain barrier.

Tolerance
N-methyl-d-aspartate receptor activation may lead to neuropathic pain and tolerance. Methadone, which is a N-methyl-d-aspartate receptor antagonist, may reduce tolerance.