The enteral and injectable workhorses
The oral route is the default for most chronic medicines: cheap, painless, self-managed. The cost is speed and certainty — onset is typically 30–60 minutes, and absorption can be derailed by food, stomach acid, and the first-pass effect. The intravenous route is the opposite: instant, complete, fully controllable, ideal for emergencies and titration — but invasive, and an overdose injected has nowhere to go but forward.
Between them sit two injection routes into tissue. The intramuscular route puts the drug into well-perfused muscle for fairly quick, reliable uptake — good for vaccines and depot preparations. The subcutaneous route places it in the fat layer just under the skin, where blood flow is slower, giving a gentler, more gradual rise; insulin and many self-injected biologics use it because patients can do it themselves.
The specialists
The sublingual route dissolves a tablet under the tongue; the rich blood supply there gives fast onset *and* bypasses the first-pass effect — the classic example is nitroglycerin for angina. The transdermal route uses a skin patch as a slow drug reservoir, releasing drug steadily over hours to days (nicotine, fentanyl, hormones) — convenient and steady, but only for potent, fat-soluble molecules.
The inhalation route sends drug as a gas or aerosol into the lungs. The lung's huge, thin surface gives near-instant uptake for systemic drugs (general anaesthetics), and for asthma inhalers it delivers the drug *straight to the target tissue* with tiny systemic doses. The rectal route (suppositories, enemas) is the backup for patients who are vomiting, unconscious, or very young; it absorbs reasonably and partly avoids first-pass, though absorption is uneven.
Route Speed of onset First-pass? Typical use ---------------------------------------------------------------- IV seconds bypassed emergencies, titration Inhaled seconds bypassed anaesthesia, asthma Sublingual 1-3 min bypassed angina (nitroglycerin) IM minutes bypassed vaccines, depots SC ~15-30 min bypassed insulin, biologics Rectal variable partial vomiting / unconscious Oral 30-60 min YES (full) chronic everyday meds Transdermal hours -> days bypassed patches: nicotine, HRT