3 Clinical psychopharmacology3.3 Pharmacodynamics
3.3.6.3 Analgesics opioid and non opioid
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Written by Resident Doctor Committee · Published
Assessing pain
Assess pain fully rather than attributing it to psychiatric illness, which is diagnostic overshadowing.
Pain type | Cause | Treatment |
|---|---|---|
Nociceptive | Tissue injury or inflammation | Paracetamol, non-steroidal anti-inflammatory drugs (NSAIDs), opioids |
Neuropathic | Somatosensory lesion, with burning pain and allodynia | Gabapentinoids, amitriptyline, duloxetine |
Nociplastic | Altered central processing, as in fibromyalgia | Exercise, psychological treatment, some antidepressants |
Opioids work poorly in neuropathic and nociplastic pain.
Opioids
Opioids depress breathing and consciousness, so treat overdose by supporting ventilation and giving naloxone, repeated or infused if toxicity recurs.
Respiratory risk rises with other central depressants, at initiation or escalation, after lost tolerance, and with organ impairment.
Constipation shows little tolerance; other effects are nausea, itch, urinary retention and endocrine suppression.
In chronic non-cancer pain, benefit is limited while dependence, falls and overdose are real risks.
Opioid-induced hyperalgesia is suspected when pain worsens or spreads as the dose rises.
Tolerance is needing more drug for the same effect, dependence is withdrawal on stopping, and addiction is impaired control with use despite harm.
Serotonergic opioids
Tramadol adds serotonin and noradrenaline reuptake inhibition to its mu-opioid effect, so it can cause serotonin toxicity with antidepressants and raises seizure risk.
Pethidine also carries high risk and fentanyl and methadone need caution; morphine, codeine, buprenorphine, hydromorphone and oxycodone carry lower risk.
Combinations with monoamine oxidase inhibitors are particularly hazardous.
Non-opioid analgesics
Paracetamol overdose can injure the liver despite few early symptoms.
NSAIDs carry gastrointestinal, renal, fluid-retention and cardiovascular risks.
NSAIDs reduce lithium clearance, and new tremor, ataxia, vomiting or confusion needs urgent assessment for lithium toxicity.
Selective serotonin reuptake inhibitors impair platelet serotonin uptake and raise gastrointestinal bleeding risk, especially with NSAIDs or anticoagulants.
Gabapentinoids
Gabapentinoids bind the alpha-2-delta subunit of calcium channels, reducing excitatory transmitter release, and help selected neuropathic pain.
They cause dizziness, sedation, ataxia, oedema, weight gain and falls, and add respiratory risk with other depressants.
They cause dependence, are Class C drugs in the UK, and need dose adjustment in renal impairment; abrupt stopping can cause withdrawal or seizures.
Prescribing in psychiatric practice
Review overdose and suicide risk and quantities supplied, and taper rather than stop abruptly when dependence exists.