FRCA Notes

Routes of Administration of Analgesia

The curriculum asks specifically for knowledge about oral, sublingual, intravenous, intramuscular, subcutaneous and inhaled routes.

Other routes or modes of delivery such as transdermal, epidural and PCAs have their own separate pages.

Resources

Oral

  • The most commonly used means of providing analgesia

    • Benefits of the oral route
    Minimally invasive
    Simple
    Economical
    Well tolerated
    Suitable for self-administration
    Reasonable efficacy

  • Drug absorption from the oral route is affected by:
    • Drug factors: formulation, degree of ionisation, interactions with other drugs
    • Patient factors: gut motility/absorption, pH

  • The degree of hepatic first pass metabolism will also affect systemic drug concentrations
  • Morphine, for example, undergoes significantly more 1st pass metabolism than either codeine or oxycodone
Drug Oral bioavailability
Paracetamol 80 - 90%
Ibuprofen >80%
Diclofenac 50 - 60%
Codeine 50%
Morphine 30%
Oxycodone 60 - 85%


  • Trans-mucosal routes include sublingual, buccal, intranasal and rectal

  • The sublingual route is rapid, painless, non-invasive and avoids hepatic 1st pass metabolism
  • For drugs to be administered sublingually they must be lipophilic, unionised and have a low MW
  • Although a portion of the drug administered SL will be absorbed, an appreciable proportion will be swallowed and undergo conventional enteral absorption

  • There is a degree of nasal first pass metabolism which occurs due the presence of medication-metabolising enzymes in the nasal mucosa
  • The extent and clinical significance of this is unknown

  • Examples include:
    • Fentanyl: SL, buccal or IN
    • Sufentanil: SL
    • Buprenorphine: SL
    • Ketamine: SL or IN
    • Dexmedetomidine: IN

Rectal

  • The rectal route can provide effective analgesia
  • Examples include diclofenac, paracetamol, ketamine (paediatrics) and tramadol
  • Issues include:
    • Unreliable absorption
    • Specific patient consent required
    • In the case of NSAIDs, no proven increase in effectiveness or reduction in side-effects vs. oral administration for post-operative pain

  • IV administration provides reliable, rapid-onset analgesia
  • The rapid rise in plasma concentration may lead to adverse effects, particularly for IV opioids
  • Intermittent bolus dosing (or PCAs) are common, although infusions may also be used e.g. on intensive care, for cancer pain


Advantages Disadvantages
More rapid absorption than SC route Pain of injection
Avoids initial high plasma concentration of IV route Variable speed of onset and duration
Bioavailability approaches 100% Risk of nerve injury if improper site used
Risk of direct IV injection
Risk of haematoma or abscess formation at injection site


  • The subcutaneous route is useful for patients in whom oral or IV administration is impractical or inappropriate
  • Absorption is slow and depends on loco-regional blood flow
  • Most commonly used for opioids, via intermittent bolus, infusion or PCA
  • Examples include morphine and diamorphine

  • Examples include:
    • Entonox
    • Methoxyflurane (3ml methoxyflurane is apparently equi-analgesic to 25μg fentanyl (BJA, 2021))
    • Nebulised opioids (insufficient evidence for routine use)