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Routes Into the Body

Mouth, vein, skin, lung, under the tongue. The route of administration decides how much drug arrives and how fast — sometimes more than the dose itself.

The same dose, very different fates

The route of administration is the path a drug takes to enter the body. The big division is between the oral route — swallowing — and the parenteral route, which means injection, bypassing the gut altogether. There are many others: under the tongue (sublingual), through the skin (transdermal), into the lungs, the nose, the eye. The route is not a detail. It can change how much active drug reaches the bloodstream by a factor of ten or more.

The fraction of the dose that reaches the systemic circulation unchanged is called bioavailability. By definition an intravenous injection has 100% bioavailability — it goes straight into the blood. Other routes must first survive a journey, and they always lose something to incomplete absorption along the way.

Why swallowing is the hardest route to win at

Oral is the route patients love — no needles, take it at home — but it is a gauntlet for the drug. After being absorbed from the gut, blood from the intestine goes first to the liver, which may chew up much of the drug before it ever reaches the rest of the body. This is the first-pass effect, and for some drugs it destroys most of the dose. That is exactly why some medicines are placed under the tongue: the sublingual route drains into veins that skip the liver on the first lap.

Worked example — why route changes the effective dose

Suppose a patient needs 10 mg of active drug to reach the bloodstream.

Intravenous (IV) route:
  bioavailability F = 1.00 (100%)
  dose needed = 10 mg / 1.00 = 10 mg

Oral route, drug heavily cleared by the liver on first pass:
  bioavailability F = 0.20 (only 20% survives)
  dose needed = 10 mg / 0.20 = 50 mg

Same target in the blood (10 mg), but the oral tablet must contain
5x more drug to make up for what the gut and liver take away.

General relation:
  amount reaching blood = oral dose x F
  oral dose = target amount / F
Bioavailability turns a target blood level into an actual dose — the lower the F, the bigger the dose the route demands.

Choosing a route is a balancing act

Each route trades convenience, speed, and reliability against one another. Injection gives full bioavailability and instant action but needs a trained hand and a sterile product. Transdermal patches deliver a drug slowly and steadily for days through the skin, sparing the patient repeated dosing — but only a few drugs can cross skin fast enough. Inhalation puts a drug straight into the lungs, ideal for asthma. The formulator picks the route, then designs the dose and the form to match it.