Table 2. Pharmacological and equianalgesic characteristics of some common opioids.
| Opioids | Relative potency to morphine P.O. | Main Receptor activity | Routes of administration | O: P ratio | Onset (min) | Peak (min) | T 1/2 (h) | Duration of pain relief (h) |
|---|---|---|---|---|---|---|---|---|
| Fentanyl | 150 | µ agonist | IV, ED. Transmucosal, Transdermal | - | 5 IV/TM | - | 2 IV | 0.4- 0.5 IV 72 h TD |
| Phenazocine | 5.0 | µ agonist | PO, PR | 1:0.4 | 20 | 45-60 | ? | 6 |
| Methadone | 1.0 single 3-4 repeated | µ agonist | PO, SC, IV, IM, SL, PR | 1:2 | 30-60 | 30-120 | 15 8-80 | 6-8 |
| Morphine | 1.0 | µ agonist | PO, PR, IV, IM, SC, ID, ED, Topical | 1:3 IV 1-2 IM | 30-60 | 60-90 | 3 | 4-6 |
| Nalbuphine | 1.0 | Mixed agonist/ antagonist | SC, IV, IM | 1:4- 1:5 | 15-30 | 45-60 | 5 | 5-6 |
| Tramadol | 0.25 | µ, O, k agonist + non-opioids | PO, PR, IV, IM, SC | 1:4 | 20-60 | 30-60 | 4-6 | 6 |
| Pethidine | 0.125 | µ, O, k agonist | PO, SC, IV, IM | 1:3 | 30-60 | 60-120 | 2.5 | 2-4 |
| Codeine | 0.1 | Prodrug | PO, IM | 1:1.5 | 30 | 45-60 | 3 | 4 |
| Pentazocine | 0.06 | Mixed agonist/ antagonist | PO, SC, IV, IM | 1:4 | 40-60 | 60-180 | 2 | 2-4 |
Reproduced from Stannard CF, Booth S. Churchill's Pocketbook of Pain, 2e (Oct 21, 2004) Elsevier Churchill Livingstone, New York, 2004. with permission.
ED: Epidural, Oral (PO), Buccal and Sublingual (SL), Rectal (PR), Intravenous (IV), Subcutaneous (SC), Intramuscular (IM), Intradermal (ID), Transmucosal(TM), Topical or Transdermal (TD), Inhalation, Oral/Parenteral Ratio (O:P), Half-Life (T 1/2),