Getting in is step one
A tablet on the kitchen counter does nothing. To help, a drug has to dissolve, cross a barrier, and reach the bloodstream so it can be carried to where it is needed. That first journey — from the site where you put the drug into the body, into the circulating blood — is called absorption. It is the opening move of pharmacokinetics, the study of what the body does to a drug.
Pharmacologists summarise the whole journey with the letters ADME: Absorption, Distribution, Metabolism, Excretion. Absorption is the front door. If a drug never gets through that door — or only a sliver gets through — then nothing else in the chain matters, however clever the molecule is.
The route shapes the response
The same molecule can behave very differently depending on how it enters. Swallow a painkiller and relief comes in half an hour; inject it and relief can come in minutes. The route decides how fast the drug appears, how much appears, and even how reliably it does so from person to person. That is why choosing a route is a real clinical decision, not a detail.
The fraction of a dose that actually reaches the bloodstream intact, by a given route, has its own name: bioavailability. For an IV dose it is 100% by definition. For a swallowed tablet it is often much less, because the drug has to survive the gut and the liver first. We will spend a whole guide on bioavailability later — for now just hold the idea that *the dose you give is not the dose that arrives*.