Opioids, in the broad sense used throughout this article, are a class of natural (endogenous and exogenous), synthetic, and semisynthetic substances that act on μ-, κ-, and δ-opioid receptors, i.e., antagonists as well as agonists. In the more narrow sense, opioids are distinguished from opiates, with the former including only synthetic, semisynthetic, or endogenous substances with opium-like pharmacological effects and the latter strictly referring to exogenous alkaloids derived from opium, the dried latex of the opium poppy (Papavum somniferum). Morphine, the original opiate, was first extracted from opium in 1804 and revolutionized medicine as the first drug to provide effective analgesia. Today, opioids are still most commonly used to treat severe acute or chronic pain. In addition to their analgesic effects, opioids induce sedation, constipation, and respiratory depression, which represent potentially life-threatening adverse effects but also have clinical uses (e.g., as anesthetic, antidiarrheal, or antitussive drugs). Opioid-receptor agonists induce a strong sense of euphoria and their recreational use, both in the form of illicit drugs (e.g., heroin) and prescription drugs (e.g., oxycodone, hydrocodone), is widespread, with severe effects on public health and other aspects of society. Continued use of opioids can lead to physical dependence (the physical adaptation to the substance associated with symptoms of tolerance and withdrawal) and psychological dependence (substance-seeking behavior in response to biochemical changes in the brain from continued exposure to the substance; often referred to as “addiction”). Acute opioid intoxication is a life-threatening condition typically characterized by altered mental status, severe respiratory depression, and miosis. Treatment of acute opioid intoxication requires emergency measures and administration of a fast-acting opioid receptor antagonist (e.g., naloxone) to counter the symptoms of acute intoxication. Since the duration of action of naloxone is shorter than that of many opioid receptor agonists, a long-acting opioid receptor antagonist (e.g., naltrexone) should be administered subsequently to detoxification to prevent opioid dependence relapse.
Pharmacology of opioids
- Classically used to describe only synthetic and semisynthetic substances with opium-like pharmacological properties (e.g., heroin)
- Today used in the broader sense to describe any (i.e., natural, synthetic, or semisynthetic) substance that binds to opioid receptors (agonists as well as antagonists).
- Opiates: alkaloids derived from the opium poppy (e.g., morphine)
According to effect on opioid receptors
- Full agonists
- Partial agonist: buprenorphine
- Mixed agonist/antagonists
- Full antagonists
According to origin
Endogenous opioids 
- Semisynthetic opioids
- Synthetic opioids
Opioid receptors 
μ (mu), δ (delta), κ (kappa)
- Heptahelical transmembrane G-protein-coupled receptors (GPCRs)
Binding agonists causes reduction of synaptic transmission through the following mechanisms:
- Presynaptic inhibition: closing of presynaptic Ca2+ channels → hyperpolarization→ ↓ release of acetylcholine, noradrenaline, serotonin, glutamate, nitric oxide, and substance P
- Postsynaptic inhibition: opening of postsynaptic K+ channels → hyperpolarization
- Effects of opioids depend on relative binding affinity of different opioid receptors.
- Mainly used as analgesics, but also used as sedatives, antidiarrheals, and antitussives 
- Pain relief primarily via the two following mechanisms:
|Overview of opioid effects |
|Site of action||Clinical uses||Side effects|
|μ-opioid receptor|| || |
|δ-opioid receptor|| |
|κ-opioid receptor|| || |
|Nonspecific/other sites of action|| |
At correct dosage, clinically relevant respiratory depression is unlikely in the treatment of chronic pain.
While the sedative, orthostatic, and emetic effects of opioids go down with tolerance, miosis and constipation remain unaffected.
Receptor affinity, intrinsic activity, and ceiling effect 
Receptor affinity describes the extent to which a ligand binds to a target receptor.
- Receptor affinity does not always correspond to potency.
- In some cases, less potent opioids with higher receptor affinity inhibit more potent opioids with lower receptor affinity competitively, rendering them ineffective.
Opioids of different potency should not be combined!
Intrinsic activity (efficacy) 
Intrinsic activity is defined as the extent to which a drug activates a receptor after binding to it.
- Governs the potency of the functional response (e.g., analgesic effect)
- If substances with no intrinsic activity have higher receptor affinity than agonists, they may act antagonistically.
The ceiling effect describes the pharmacological phenomenon that once the therapeutic limit is reached, an increase in dose will no longer increase the functional response, but only the side effects.
Full opioid receptor agonists (e.g., morphine)
- No ceiling effect
- Increase in dose always leads to increased functional response and there is no cut-off point.
Partial opioid receptor agonists (e.g., buprenorphine)
- Ceiling effect
- At a certain point, an increase in dose does not increase the functional response, but only the side effects.
Relative analgesic potency 
- The analgesic potency of opioids is described in relation to morphine, which, accordingly, has the analgesic potency of 1.
- Higher relative analgesic potency allows for lower doses achieving the same analgesic effect.
- Values for relative potency differ depending on route of administration.
Acute pain management 
- General approach: lowest effective dose should be prescribed/administered for the shortest duration possible
- Acute severe pain (e.g., ureteric colic, fractures)
- Emergency pain management and sedation (e.g., acute coronary syndrome, pulmonary edema) 
- Analgosedation during surgery
Chronic pain management 
Pain management outside emergency medicine or anesthesiology should follow the WHO analgesic ladder algorithm.
- Only consider opioids if other pharmacological and nonpharmacological measures have not achieved sufficient pain relief.
- Evaluate patients for risk factors for opioid dependency (e.g., history of substance use).
- Opioid-naive patients should receive immediate-release/short-acting formulations.
- Avoid concomitant use of benzodiazepines, which can lead to severe CNS and respiratory depression.
- Initiate treatment on a trial basis with regular monitoring and adjustments.
Urine drug testing: recommended for all patients receiving chronic opioid therapy ; 
- For opioids and other controlled substances that may increase the risk of opioid overdose (e.g., benzodiazepines)
- Unexpected results may be due to treatment nonadherence, recreational substance use, or cross-reactivity with other substances (e.g., poppy seeds).
- An unexpected test result should be discussed with the patient in an open and nonjudgmental manner before any conclusions are made. Treatment for pain should never be withheld on the basis of an unexpected test result.
- Confirmatory testing (e.g., mass spectrometry) should be used in the event of an unexpected test result.
- See “Management of substance use disorder” in “Substance-related and addictive disorders” for more information.
- Acute exacerbations of chronic pain (e.g., breakthrough cancer pain)
- Chronic pain syndromes (e.g., cancer pain, compression fractures of the spine)
Avoid long-term IV opioid administration, since this can rapidly lead to opioid tolerance and, ultimately, dependence.
Opioids for pain management
|Overview of opioids used for pain management |
|Route of administration and corresponding analgesic potency||Duration of analgesic action||Receptor interaction||Indications||Side effects and other features|
|Morphine|| || || |
|Hydromorphone|| || || || |
| || || || || |
|Oxycodone|| || || || |
|Codeine|| || || |
|Tramadol|| || || || |
| || || |
|Pentazocine|| || || || || |
|Methadone|| || || || |
|Buprenorphine || || || || || |
|Fentanyl|| || || |
| || || || || |
Antagonization of buprenorphine requires high doses of naloxone or naltrexone due to its very high receptor affinity.
Dextromethorphan (DXM): synthetic codeine analog used for cough suppression
Weak opioid receptor agonist and NMDA receptor antagonist
- Opioid effects are weak and mostly seen with overdose (can be treated with naloxone).
- Addictive potential is low.
- Inhibition of serotonin and noradrenaline reuptake increases the risk of serotonin syndrome when used with other serotonergic drugs.
- Weak opioid receptor agonist and NMDA receptor antagonist
- Codeine: taken PO
Loperamide: μ-receptor agonist
- Can not pass the blood-brain barrier (low abuse potential due to lack of central opioid effects)
- Inhibits propulsive peristalsis, increases sphincter tone, and inhibits intestinal fluid secretion
- Adverse effects include constipation, vomiting, and nausea.
- Inhibits propulsive peristalsis
- Only available as a combination drug with atropine to prevent misuse
- May produce central effects and toxicity at high doses.
Treatment of opioid use disorder
- Morphine: used in opioid substitution therapy
- Treatment of acute opioid withdrawal
- Long-term maintenance therapy
- Buprenorphine: used for relapse prevention
Opioid receptor antagonists
Opioid receptor antagonists bind to opioid receptors without activating them. Antagonists with high affinity to the opioid receptors can be used as antidotes in acute opioid intoxication due to their ability to displace opioids from the receptors.
Centrally acting opioid-receptor antagonists
|Overview of centrally acting opioid-receptor antagonists|
|Routes of administration|| || |
|Pharmacology|| || |
|Indication|| || |
“Use nalTRACKsone to get back on TRACK:” Naltrexone is used to prevent opioid relapse.
Peripherally acting μ-opioid receptor antagonists
- Mechanism of action: antagonization of μ-opioid receptors outside the CNS (e.g., in the gastrointestinal tract)
- Indication: reversal of opioid side effects, such as opioid-induced constipation or pruritus
- Methylnaltrexone: quaternary ammonium derivative of naltrexone with limited ability to penetrate the blood-brain barrier
Absolute contraindications 
- Acute or severe bronchial asthma
- Chronic bronchitis
- Gastrointestinal conditions
- Concomitant substance use that could lead to life-threatening drug interactions
- Hypersensitivity: true allergic reaction to opioids and/or compounds
Relative contraindications 
- Pregnancy and breastfeeding
- Renal failure
- Liver failure
- Concurrent SSRI use
- Acute pancreatitis, biliary tract impairment
We list the most important contraindications. The selection is not exhaustive.