Opioid Pharmacology

Sedation is one of the most common central nervous system adverse effects, resulting from mu receptor activation in the thalamus, locus coeruleus, and reticular activating system. Sedation is most pronounced at initiation of therapy and after dose increases, and patients should be advised against driving or operating machinery until stable on their current dose. Partial tolerance to sedation develops over days to weeks with continued use.

Miosis — constriction of the pupils — results from mu receptor activation at the Edinger-Westphal nucleus of the oculomotor complex, producing parasympathetic stimulation of the pupillary sphincter. Pinpoint pupils (1 to 2 mm) are a reliable clinical sign of opioid effect and, unlike most other opioid central nervous system effects, tolerance to miosis does not substantially develop with chronic use. A patient on long-term opioid therapy will retain constricted pupils indefinitely.

Euphoria is produced by mu receptor activation in mesolimbic circuits, particularly the nucleus accumbens. The pleasurable effect drives reinforced drug-seeking and is the primary neurobiological basis of opioid misuse. Tolerance to euphoria develops relatively rapidly, which contributes to dose escalation in opioid use disorder.

Respiratory depression is the most dangerous adverse effect of opioids and the primary cause of death in overdose. Mu receptor activation in brainstem respiratory control centers — particularly the pre-Botzinger complex in the ventrolateral medulla — reduces the respiratory rate and blunts the normal drive to breathe in response to rising carbon dioxide levels. The carbon dioxide response curve is shifted rightward, meaning the body tolerates higher carbon dioxide levels before triggering a breath.

Partial tolerance to respiratory depression does develop with chronic opioid use, which is why patients on stable long-term opioid therapy can tolerate doses that would be dangerous in an opioid-naive individual. However, this tolerance is incomplete and can be overwhelmed by dose increases, by combination with other central nervous system depressants (particularly benzodiazepines and alcohol), or by sleep, which independently reduces the arousal response to hypercapnia.

Constipation is the most common persistent adverse effect of chronic opioid therapy. It is mediated by mu receptor activation in enteric neurons of the gastrointestinal tract, which reduces propulsive motility, increases tonic segmental contraction, and prolongs intestinal transit time. Unlike most other opioid adverse effects, tolerance to constipation does not develop with continued use — a patient on long-term opioid therapy will experience ongoing constipation regardless of how long they have been on the medication. Prophylactic bowel regimens are standard of care for any patient starting regular opioid therapy.

Nausea and vomiting result from mu receptor activation in the area postrema — the chemoreceptor trigger zone in the brainstem that lies outside the blood-brain barrier. Opioids stimulate this area directly, producing nausea particularly in ambulatory patients. Partial tolerance to nausea develops within days to weeks.

Delayed gastric emptying occurs through peripheral mu receptor effects on gastric smooth muscle, prolonging the time for oral medications and food to leave the stomach. This can affect the absorption of other oral drugs taken concurrently.

Urinary retention results from increased tone of the urethral sphincter and detrusor relaxation through peripheral mu receptor activation. It is more common with neuraxial (epidural and intrathecal) opioid administration than with systemic use and is more pronounced in men.

Pruritus (itching) is a common adverse effect, particularly with neuraxial opioids. It is mediated centrally through mu receptor activation in the spinal cord rather than through histamine release, which is why antihistamines are only partially effective. Low-dose naloxone infusions or the mu antagonist nalbuphine can provide relief without fully reversing analgesia.

Biliary spasm results from contraction of the sphincter of Oddi, increasing biliary duct pressure. This is relevant in patients with biliary colic, where opioid-induced sphincter spasm can worsen pain despite the systemic analgesic effect.

The classic presentation of opioid overdose consists of three findings that reflect extreme mu receptor activation at brainstem and midbrain sites:

The absence of miosis in an obtunded patient should prompt consideration of co-ingestion with agents that cause pupillary dilation — such as cocaine, amphetamines, or anticholinergic drugs — which can mask the opioid-induced constriction. Mixed overdoses are common in the current illicit drug supply, where fentanyl is frequently found in combination with stimulants and sedatives.

The immediate priority in opioid overdose is airway management and ventilatory support. Bag-mask ventilation with supplemental oxygen should be initiated immediately for any patient with inadequate respiratory effort. Positioning to prevent aspiration and maintenance of airway patency take precedence even before pharmacological reversal. In the prehospital setting, naloxone administration by bystanders or first responders using naloxone nasal spray has become a critical public health intervention, with widespread distribution programs aimed at reducing overdose mortality in communities with high rates of illicit opioid use.