Overview
The Pharmacology Behind the Risk
Opioids produce analgesia primarily by activating mu opioid receptors in the central and peripheral nervous systems.
Opioids produce analgesia primarily by activating mu-opioid receptors in the central and peripheral nervous systems. That same receptor activity reduces arousal and ventilatory drive, constricts the pupils, slows gastrointestinal motility, and can produce euphoria. Analgesia and life-threatening toxicity are therefore not unrelated effects; they are different expressions of the same pharmacology. A patient may become progressively sedated while still reporting pain. That report does not make dose escalation safe. Sedation is a warning that the opioid effect is already exceeding the patient’s physiologic reserve, particularly when alcohol, benzodiazepines, sedating antihistamines, muscle relaxants, or gabapentinoids are also present. Buprenorphine is a high-affinity partial mu agonist. It binds tightly, displacing many full agonists, but activates the receptor less completely. This produces a ceiling effect on respiratory depression, not immunity from overdose. If it is administered while a full agonist still occupies the receptors, buprenorphine can displace that agonist without providing equivalent activation; acute precipitated withdrawal can follow. Methadone is a long-acting full mu agonist with additional NMDA-receptor antagonist activity. Its elimination half-life is long and highly variable, while its analgesic duration is...