Clinical Psychopharmacology · Paper A
Opioid pharmacology
Opioids act on several distinct receptor types in the body, and knowing which receptor a drug targets predicts both its clinical use and its risks. E.g. two patients given an opioid painkiller: one becomes drowsy but breathes normally, the other's breathing slows dangerously. The difference often comes down to whether the drug is a full agonistA drug or chemical that activates a receptor to produce an effect.Read more → or a partial agonistA drug that activates a receptor but produces a smaller effect than a full agonist, even at a high dose.Read more → at the mu receptor. Clinically, the choice of opioid, not just the dose, sets the ceiling on respiratory-depression risk.
Opioids act at G-protein-coupled receptorsMU receptors give analgesia (pain relief), euphoria, respiratory depression, pinpoint pupils (miosis) and constipation; KAPPA receptors give spinal-level analgesia but dysphoria (an unpleasant mood state) rather than euphoria. Morphine is a prototypical full mu agonistA drug or chemical that activates a receptor to produce an effect.Read more →. BUPRENORPHINE is a high-affinityHow strongly a drug binds to its receptor.Read more → PARTIAL mu agonist, meaning it activates the receptor less fully than morphine. E.g. a patient switched to buprenorphine too early, while a full agonist such as heroin still occupies their receptors, can go into precipitated withdrawal, since buprenorphine's weaker binding displaces the fuller agonist. In practice, buprenorphine is only started once early withdrawal symptoms appear, and NALOXONE (a short-acting antagonistA drug that blocks a receptor, stopping it from being activated.Read more →) is kept available to reverse overdose.
Every opioid exam question is secretly a receptor question. Opioids are a large family of drugs, from morphine to methadone to tramadol, and the fastest way to predict what any one of them does, and how dangerous it is, is to ask which receptor it acts on and HOW STRONGLY. Hold that one idea through every section below.
Opioids are drugs that act on opioid receptors, a family of G-protein-coupled receptors (cell-surface receptors that trigger an internal signalling cascade when activated) found throughout the brain, spinal cord, and gut. The two most exam-relevant receptor types are MU and KAPPA. MU receptor activation produces analgesia (pain relief), euphoria, respiratory depression, pinpoint pupils (miosis), and constipation. KAPPA receptor activation produces spinal-level analgesia but dysphoria (an unpleasant, unsettled mood state) rather than euphoria.
A common mistake is treating 'opioid' as one uniform drug class with one uniform risk. It is not. The single most exam-relevant distinction is between a FULL (activates the receptor maximally, e.g. morphine or heroin) and a (activates the receptor only partly, however much drug is given, e.g. buprenorphine). This distinction changes both the ceiling on danger and how the drug behaves when combined with another opioid.
Imagine turning a tap. A full like morphine is a tap that can be opened all the way: the more you give, the more the effect grows, including the effect that kills in overdose: respiratory depression (breathing becomes dangerously slow or stops). A like buprenorphine is a tap that only opens partway, no matter how hard you turn it: there is a CEILING effect, so beyond a certain dose, giving more does not further depress breathing. That ceiling is exactly why buprenorphine is used in opioid substitution treatment: it is safer in overdose than a full agonist.
Methadone: a full mu , used in opioid substitution treatment. Because it is a full agonist, it carries the same theoretical overdose risk pattern as other full agonists, and dosing must be titrated carefully, especially in the first two weeks of treatment when is still building.
Buprenorphine: a high- PARTIAL mu . Because it binds tightly but activates only partially, giving buprenorphine to a patient who still has a full agonist (like heroin) occupying their receptors can displace that full agonist and PRECIPITATE acute withdrawal: for example, a patient started on buprenorphine too soon after their last heroin use may suddenly develop severe withdrawal symptoms. In practice, buprenorphine is only started once early withdrawal symptoms have already begun to appear, precisely to avoid this.
NALOXONE is a short-acting opioid : it binds mu receptors without activating them, displacing and rapidly reversing overdose, including respiratory depression. Because naloxone's action is shorter than many opioids' duration of action, a reversed patient can relapse back into overdose once the naloxone wears off, so ongoing monitoring after reversal matters. Naltrexone is a related, longer-acting antagonist used for relapse-prevention treatment rather than emergency reversal.
Tramadol (and, to a lesser extent, pethidine) is not a 'pure' opioid. Alongside weak mu-receptor activity, it also blocks reuptake of serotonin and noradrenaline: the same broad mechanism as some antidepressants. This adds two extra risks pure opioids like morphine do not carry: a lowered seizure threshold, and serotonin toxicity () when combined with another drug. For example, a patient already taking an SSRI (selective serotonin reuptake inhibitor, a common first-line antidepressant class) who is then prescribed tramadol for pain is at meaningfully raised risk of serotonin syndrome, a risk that would not apply if plain morphine were used instead.
Opioid toxicity is easy to confuse with other causes of a reduced conscious level, and telling them apart matters because the antidote, naloxone, only works on one of them. Classic opioid toxicity presents as a triad: pinpoint pupils (miosis), respiratory depression, and a reduced conscious level. Benzodiazepine overdose can look similar (sedated, reduced consciousness) but typically spares the pupils and causes less severe respiratory depression, and it has its own , flumazenil, which does not touch opioid effects and vice versa. Alcohol intoxication also causes reduced consciousness, but usually WITHOUT pinpoint pupils, often with a smell of alcohol and a measurable blood alcohol level. , discussed above with tramadol, is a further look-alike worth separating out precisely because it can be triggered BY an opioid (tramadol) yet needs a completely different response, stopping the drug and supportive care, rather than naloxone, which will not reverse it.
Opioid dependence is diagnosed against a defined set of features, not a gut feeling. Under ICD-11, dependence syndrome is recognised when a cluster of physiological, behavioural and cognitive features is present, classically including impaired control over use, increasing priority given to using over other activities, tolerance (needing more of the drug for the same effect), and a physiological withdrawal state when the drug is stopped or reduced: with continued use despite clear evidence of harm. Acute opioid toxicity is a clinical diagnosis made at the bedside from the triad above (pinpoint pupils, respiratory depression, reduced consciousness) plus a history or strong suspicion of opioid exposure, and is effectively confirmed in practice by the patient's response to a trial dose of naloxone reversing the picture.
Opioid dependence management follows a structured pathway. UK practice, guided by the Department of Health's drug misuse and dependence guidelines (the so-called 'Orange Book') alongside NICE, favours substitution treatment with methadone or buprenorphine, dose titration under supervision, and take-home naloxone provision for patients at overdose risk. Any new prescription of tramadol should prompt a medication review for interacting drugs before it is prescribed.
Questions often disguise the mu-vs-kappa or full-vs-partial distinction inside a clinical vignette rather than naming it directly: a patient who becomes agitated and unwell rather than sedated after starting buprenorphine is describing precipitated withdrawal, not a paradoxical drug reaction. A patient on tramadol who develops confusion, sweating, and tremor after starting an antidepressant is describing , not simple opioid toxicity: the treatment, and the whole way of thinking about the case, is completely different.
Opioids act at mu (analgesia, euphoria, unlimited respiratory depression risk in full ) and kappa (dysphoria) receptors; buprenorphine's ceiling effect makes it safer but risks precipitated withdrawal, naloxone reverses toxicity by antagonism, and tramadol adds a risk pure opioids do not share.