a) Parenteral vs Oral

Why are drugs given parenterally instead of orally? (10 marks)

Note the mark allocation is 10 marks, so you can either do it as an essay, or list out at least ten points.

‘Parenteral’ means any route other than via oral route, but in practice is generally accepted to mean administration via injection

  • Intravenous (IV) into the vein
  • Subcutaneous (SC) into fatty tissue under the skin
  • Intramuscular (IM) into muscle

Note the mark allocation is high, you may want to brain storm ideas to warm up, and give examples. 

Reasons why drugs are given parenterally are:

  • For drugs which have low oral bioavailability eg. biopharmaceuticals (vaccines, heparin, insulin, monoclonal antibodies, etc), some antibiotics
    • Physiological barriers of the GI including acid and enzymes eg. insulin
    • Unable to pass through the GI wall eg. vancomycin
    • Hepatic first pass metabolism eg. fentanyl
  • For more intensive therapy in conditions requiring hospitalisation eg antibiotics, opioids (high percent of PO morphine is metabolised in the first pass effect)
    • Complete bioavailability - more predictable pharmacokinetics
    • Rapid onset of action (IV)
  • Local action - eg lignocaine as a local anaesthetic (SC route)
  • Patients who are unconscious 
  • Patients who are vomiting eg anti-emetics 
  • Patients who are non-compliant eg haloperidol
  • IM depot drugs can be delivered over extended periods of time which is useful for non-compliant patients, or those who prefer the convenience (eg depot contraception, antipsychotics)
  • Total parenteral nutrition (TPN) for GI patients (eg stomach resection, Crohn's disease), severe burns patients and those in shock and during chemo- and radiotherapy.




The advantages of PO route are:

  • convenient for the patient
  • tablets/capsules are more straightforward to keep/store/carry around
  • no specially trained staff required for administration
  • not associated with risk of infection
  • not associated with pain

The disadvantages of PO route:

  • Whilst some drugs have very good oral bioavailability, some drugs have lower or very poor oral bioavailability

Advantages of parenteral administration (I/V, S/C, I/M):

  • High bioavailability
  • Rapid onset of action. IV - entire dose delivered into the systemic circulation immediately. S/C and I/M - absorption process from injection site, more delayed response.
  • Bypasses the first pass metabolism

Disadvantages:

  • Pain from injection
  • Infection risk for IV administration
  • Trained staff required for IV and IM administration
  • More expensive



https://www.nursingtimes.net/archive/administration-of-drugs-3-parenteral-10-09-2011/

https://www.ncbi.nlm.nih.gov/books/NBK568677/


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