Tramadol (sold under the brand name Ultram) is a narcotic analgesic proposed for moderate to severe pain. Tramadol and its O-desmethyl metabolite (M1) are selective, weak OP3-receptor agonists. Opiate receptors are coupled with G-protein receptors and function as both positive and negative regulators of synaptic transmission via G-proteins that activate effector proteins. As the effector system is adenylate cyclase and cAMP located at the inner surface of the plasma membrane, opioids decrease intracellular cAMP by inhibiting adenylate cyclase. Subsequently, the release of nociceptive neurotransmitters such as substance P, GABA, dopamine, acetylcholine, and noradrenaline is inhibited. The analgesic properties of Tramadol can be attributed to norepinephrine and serotonin reuptake blockade in the CNS, which inhibits pain transmission in the spinal cord. The (+) enantiomer has the higher affinity for the OP3 receptor and preferentially inhibits serotonin uptake and enhances serotonin release. The (-) enantiomer preferentially inhibits norepinephrine reuptake by stimulating alpha(2)-adrenergic receptors. Tramadol is used primarily to treat mild-severe pain, both acute and chronic. Its analgesic effects take about one hour to come into effect and 2 h to 4 h to peak after oral administration with an immediate-release formulation. On a dose-by-dose basis, tramadol has about one-tenth the potency of morphine and is approximately equally potent when compared to pethidine and codeine. The most common adverse effects of tramadol include nausea, dizziness, dry mouth, indigestion, abdominal pain, vertigo, vomiting, constipation, drowsiness, and headache. Compared to other opioids, respiratory depression and constipation are considered less of a problem with tramadol.

Pentazocine is a synthetically prepared prototypical mixed agonist-antagonist narcotic (opioid analgesic) drug of the benzomorphan class of opioids used to treat moderate to moderately severe pain. Pentazocine is sold under several brand names, such as Fortral, Sosegon, Talwin NX. Pentazocine acts as an agonist of κ-opioid receptors and as an antagonist of μ-opioid receptors. This compound may exist as one of two enantiomers, named (+)-pentazocine and (−)-pentazocine. Side effects are similar to those of morphine, but pentazocine, due to its action at the kappa opioid receptor is more likely to invoke psychotomimetic effects. High dose may cause high blood pressure or high heart rate.

Methadone, sold under the brand names Dolophine among others, is an synthetic opioid that is used as the hydrochloride to treat pain and as maintenance therapy or to help with detoxification in people with opioid dependence. Methadone hydrochloride is a mu-agonist; a synthetic opioid analgesic with multiple actions qualitatively similar to those of morphine. Some data also indicate that methadone acts as an antagonist at the NMDA-receptor. The contribution of NMDA receptor antagonism to methadone’s efficacy is unknown. Most common adverse reactions are: lightheadedness, dizziness, sedation, nausea, vomiting, and sweating. Avoid use mixed agonist/antagonist and partial agonist opioid analgesics with DOLOPHINE because they may reduce analgesic effect of DOLOPHINE or precipitate withdrawal symptoms.

ISOMETHADONE HYDROBROMIDE, DL- is a hydrobromide salt of isomethadone, a synthetic opioid analgesic. This is a controlled substance in the US.

Adolor Corporation and Pfizer were developing an orally available and selective opioid delta (δ) receptor agonist, ADL 5747 (also known as PF 04856881), for the treatment of pain. ADL-5747 showed efficient pain-reducing properties in the two preclinical models of chronic pain. Their effects were mediated by δ-opioid receptors, with a main contribution of receptors expressed on peripheral Nav1.8-positive neurons. ADL-5747 had been in phase II clinical trials for the treatment of pain. However, this studies on this drug candidate were discontinued in 2010.

Dimepheptanol is one of the phenylpiperidine series, an opioid receptor agonist, which is active as an opioid analgesic. It is also elaborated in a number of close analogues, derivatives or diastereoisomers with similar properties. Dimepheptanol is a synthetic narcotic analgesic that has no accepted medical use in the United States.

Clonitazene is a synthetic opioid analgesic, structurally related to etonitazene. In the USA clonitazene is a schedule I narcotic controlled substance.

Myrophine is an opiate analog and long-acting prodrug for morphine with a slow onset of effects. It is weaker than morphine as an analgesic but longer-lasting in effects and was thought to have a more local anesthetic effect than morphine, though with a somewhat greater tendency to cause histamine reactions like itching and rash. In addiction studies conducted in human subjects in the 1950s, myrophine did not substitute for morphine in withdrawal, did not produce notable morphine-like effects, and did not produce addiction or dependence regardless of dose or how it was administered.

Diampromide is a synthetic narcotic analgesic. It was designed as a methadone analog and hence with a chiral center similar to that of the parent. Diampromide approximates to the potency of pethidine and morphine in mice and rats, respectively. It has been evaluated in postoperative pain. Diampromide is not found in any pharmaceutical preparations sold in the United States. It is under international control according to the UN Single Convention 1961 and its amendments, Schedule I.

Lofentanil is a pure mu-opioid receptor agonist derived from fentanyl. It is the most potent opioid to be administered to humans, about 500-1000 times more potent than morphine. Lofentanil provides a higher affinity quotient with longer dissociation times for the mu-receptors than fentanyl. The clinical study of the compound is difficult because there is a very individual sensibility. The appropriate doses are not easy to evaluate. Reversal of the loventanil depression needs very high and repeated naloxone dose. Practical use of lofentanil is limited. Lofentanil side effects are: nausea, vomiting and sedation.