Expansion
Enteral routes are convenient; parenteral routes bypass first pass metabolism
Expansion
Enteral
- Oral: convenient and safe, but subject to first pass metabolism, variable absorption and a slow onset. Useless in vomiting or unconsciousness
- Sublingual and buccal: rapid, bypasses first pass (glyceryl trinitrate)
- Rectal: partially bypasses first pass; useful in vomiting and in children
Parenteral
- Intravenous: 100 per cent bioavailability, immediate, titratable; but invasive and irreversible once given
- Intramuscular: faster than oral, allows depot preparations; avoid in coagulopathy
- Subcutaneous: slow steady absorption (insulin, heparin)
- Intrathecal: for drugs that cannot cross the blood-brain barrier
Other
- Inhaled: high local concentration with low systemic exposure (salbutamol, inhaled steroids)
- Topical and transdermal: local effect or slow systemic delivery
The choice is driven by the clinical urgency, the drug’s bioavailability, whether the gut is working, and whether local rather than systemic effect is wanted. Inhaled therapy in asthma is the clearest example of route determining the therapeutic ratio.