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.

Nalbuphine is a semi-synthetic opioid agonist-antagonist used commercially as an analgesic under a variety of trade names, including Nubain and Manfine. Nalbuphine is an agonist at kappa opioid receptors and an antagonist at mu opioid receptors. Nalbuphine analgesic potency is essentially equivalent to that of morphine on a milligram basis up to a dosage of approximately 30 mg. The opioid antagonist activity of Nalbuphine is one-fourth as potent as nalorphine and 10 times that of pentazocine. Nalbuphine is indicated for the management of pain severe enough to require an opioid analgesic and for which alternative treatments are inadequate. Nalbuphine can also be used as a supplement to balanced anesthesia, for preoperative and postoperative analgesia, and for obstetrical analgesia during labor and delivery. The onset of action of Nalbuphine occurs within 2 to 3 minutes after intravenous administration, and in less than 15 minutes following subcutaneous or intramuscular injection. The plasma half-life of nalbuphine is 5 hours, and in clinical studies, the duration of analgesic activity has been reported to range from 3 to 6 hours. Like pure µ-opioids, the mixed agonist-antagonist opioid class of drugs can cause side effects with initial administration of the drug but which lessen over time (“tolerance”). This is particularly true for the side effects of nausea, sedation and cognitive symptoms. These side effects can in many instances be ameliorated or avoided at the time of drug initiation by titrating the drug from a tolerable starting dose up to the desired therapeutic dose. An important difference between nalbuphine and the pure mu-opioid analgesic drugs is the “ceiling effect” on respiration. Respiratory depression is a potentially fatal side effect from the use of pure mu opioids. Nalbuphine has limited ability to depress respiratory function.

Oxymorphone is an analgesic that is FDA approved for the treatment of moderate to severe pain. It is also indicated for relief of anxiety in patients with dyspnea associated with pulmonary edema secondary to acute left ventricular dysfunction. Oxymorphone (brand names Opana, Numorphan, Numorphone) is a full opioid agonist and is relatively selective for the mu-opioid receptor, although it can bind to other opioid receptors at higher doses. Adverse reactions (≥ 2% of patients): seen with the immediate release tablet formulation Nausea, pyrexia, somnolence, vomiting, pruritus, headache, dizziness, constipation, and confusion. Concomitant use with serotonergic drugs may result in serotonin syndrome. Avoid use of mixed agonist/antagonist and partial agonist opioid analgesics with Opana because they may reduce analgesic effect of Opana or precipitate withdrawal symptoms.

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.

Oxycodone is a semisynthetic opioid used for the management of acute and chronic pain severe enough to require an opioid analgesic and for which alternative treatments are inadequate. Oxycodone is a highly selective full agonist of the μ-opioid receptor (MOR), with low affinity for the δ-opioid receptor (DOR) and κ-opioid receptor (KOR). After oxycodone binds to the MOR, a G protein-complex is released, which inhibits the release of neurotransmitters by the cell by reducing the amount of cAMP produced, closing calcium channels, and opening potassium channels. After a dose of conventional (instant-release) oral oxycodone, the onset of action is 10–30 minutes, and peak plasma levels of the drug are attained within roughly 30–60 minutes in contrast, after a dose of OxyContin (an oral controlled-release formulation), peak plasma levels of oxycodone occur in about three hours. The duration of instant-release oxycodone is 3 to 6 hours, although this can be variable depending on the individual. Oxycodone in the blood is distributed to skeletal muscle, liver, intestinal tract, lungs, spleen, and brain. Serious side effects of oxycodone include reduced sensitivity to pain (beyond the pain the drug is taken to reduce), euphoria, anxiolysis, feelings of relaxation, and respiratory depression. Common side effects of oxycodone include constipation (23%), nausea (23%), vomiting (12%), somnolence (23%), dizziness (13%), itching (13%), dry mouth (6%), and sweating (5%).

Benzhydrocodone is a prodrug of hydrocodone. Benzhydrocodone is formed by covalently bonding hydrocodone to benzoic acid. Benzhydrocodone itself is not pharmacologically active, but must be metabolized to hydrocodone by enzymes in the intestinal tract to optimally deliver its pharmacologic effects. Hydrocodone is a full agonist of the opioid receptors with a higher affinity for the mu-opioid receptor. Upon binding, hydrocodone produces an analgesic effect with no ceiling. APADAZ a combination of benzhydrocodone and acetaminophen is FDA approved and indicated for the short-term (no more than 14 days) management of acute pain severe enough to require an opioid analgesic and for which alternative treatments are inadequate. APADAZ, even when taken as recommended, can result in addiction, abuse, and misuse, which can lead to overdose and death.

Dihydrocodeine is an opioid analgesic used as an alternative or adjunct to codeine to treat moderate to severe pain, severe dyspnea, and cough. It is semi-synthetic, and was developed in Germany in 1908 during an international search to find a more effective antitussive agent to help reduce the spread of airborne infectious diseases such as tuburculosis. It was marketed in 1911. Dihydrocodeine is metabolized to dihydromorphine -- a highly active metabolite with a high affinity for mu opioid receptors. Dihydrocodeine is used for the treatment of moderate to severe pain, including post-operative and dental pain. It can also be used to treat chronic pain, breathlessness and coughing. In heroin addicts, dihydrocodeine has been used as a substitute drug, in doses up to 2500mg/day to treat addiction.

Meptazinol is a unique opioid analgesic. Binding studies suggest a relative selectivity for mu-1 opioid receptor sites. Meptid is indicated for the treatment of moderate to severe pain, including post-operative pain, obstetric pain and the pain of renal colic. The most commonly reported adverse reactions after treatment with meptazinol are nausea, vomiting, dizziness, diarrhoea and increased sweating, constipation, abdominal pain, rash, vertigo, headache, drowsiness, somnolence and dyspepsia.

Codeine is an opiate used to manage mild to moderate pain severe enough to require an opioid. Codeine is a selective agonist for the mu opioid receptor and has an affinity to delta and kappa-opioid receptors. In some countries, this drug is regulated under various narcotic control laws, because its chronic use can cause physical dependence. In others, it is available without a medical prescription in combination with paracetamol.