Alfentanil is an opioid analgesic with a rapid onset of action. Alfentanil interacts predominately with the opioid mu-receptor. These mu-binding sites are discretely distributed in the human brain, spinal cord, and other tissues. In clinical settings, alfentanil exerts its principal pharmacologic effects on the central nervous system. Its primary actions of therapeutic value are analgesia and sedation. Alfentanil may increase the patient's tolerance for pain and decrease the perception of suffering, although the presence of the pain itself may still be recognized. In addition to analgesia, alterations in mood, euphoria and dysphoria, and drowsiness commonly occur. Alfentanil depresses the respiratory centers, depresses the cough reflex, and constricts the pupils. Alfentanil, marketed under the trade name Alfenta, Rapifen in Australia is indicated for the management of postoperative pain and the maintenance of general anesthesia.

Butorphanol is a synthetic opioid agonist-antagonist analgesic with a pharmacological and therapeutic profile that has been well established since its launch as a parenteral formulation in 1978. The introduction of a transnasal formulation of butorphanol represents a new and noninvasive presentation of an analgesic for moderate to severe pain. This route of administration bypasses the gastrointestinal tract, and this is an advantage for a drug such as butorphanol that undergoes significant first-pass metabolism after oral administration. The onset of action and systemic bioavailability of butorphanol following transnasal delivery are similar to those after parenteral administration. Butorphanol blocks pain impulses at specific sites in the brain and spinal cord. Butorphanol has agonistic activity at the κ-receptor and antagonistic activity at the μ-receptor. It also exhibits partial agonistic activity at the σ-receptor.

Fentanyl is a potent agonist of mu opioid receptor. It is used to relieve severe pain, such as after surgery or during cancer treatment, and breakthrough pain (flare-ups of intense pain despite round-the-clock narcotic treatment). Fentanyl is an extremely powerful analgesic, 50–100-times more potent than morphine. Fentanyl harbors massive risk for addiction and abuse regardless of its prescription form. Fentanyl abuse is especially dangerous to those without a tolerance to opioids. The substance’s already elevated risk of overdose is multiplied when someone without a tolerance abuses it.

Levorphanol, brand name Levo-Dromoran, is an opioid medication used to treat moderate to severe pain. Levorphanol is indicated for the management of moderate to severe pain where an opioid analgesic is appropriate. It is a potent synthetic opioid mu-receptor agonist similar in action to morphine. Like other opioid mu-receptor agonists, it is believed to act at receptors in both the brain and spinal cord to alter the transmission and perception of pain. The onset and peak analgesic effects following administration of levorphanol are similar to morphine when administered at equal analgesic doses. Levorphanol produces a degree of respiratory depression similar to that produced by morphine at equal analgesic doses, and like many opioid mu-receptor agonists, levorphanol produces euphoria or has a positive effect on mood in many individuals.

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.

Ethylmorphine is a derivative of morphine with analgesic and antitussive effect. It acts by activating the opioid receptors and thus has a direct influence on the CNS system. Ethylmorphine was approved in Europe for the treatment of dry cough (Codethyline, Dionine).

Dihydromorphine is a semi-synthetic opioid derived from morphine. dihydromorphine is a moderately strong analgesic and is used clinically in the treatment of pain and is also the active metabolite of dihydrocodeine. Dihydromorphine acts as an agonist at the μ-opioid (mu), δ-opioid (delta) and κ-opioid (kappa) receptors. Dihydromorphone is approved for clinical use in the United States, Europe, and Japan; and sold under the brand name Dilaudid. Similar to morphine, and other morphine derivatives, hydromorphone has a high potential for addiction and abuse and is listed as a Schedule II drug in the United States Controlled Substances Act of 1970 (and similarly regulated in other countries).

Eptazocine is an opioid analgesic which was introduced in 1987 by Morishita in Japan . It acts as a mixed κappa opioid receptor agonist and mu-opioid receptor antagonist.