absorption
Absorption is the journey a drug takes from where you put it in to where it can actually do something — the bloodstream. Swallow a tablet and the active molecule must dissolve in the gut fluids, cross the wall of the intestine, and slip into the blood vessels on the other side. Until it makes that crossing, the drug is just sitting in your gut doing nothing useful.
Whether and how fast a molecule is absorbed depends on its physical properties and the barrier it must cross. A drug needs to be soluble enough to dissolve, yet greasy enough to pass through the fatty cell membranes lining the gut, and the right size and shape to permeate. These competing demands are why solubility, lipophilicity and permeability are central concerns in drug design.
Absorption is the first letter of ADME — absorption, distribution, metabolism, excretion — the four processes that together decide what the body does to a drug. It applies to every route except direct intravenous injection, where the drug is placed straight into the blood and absorption is, by definition, complete and instantaneous.
A weakly soluble tablet may pass through the gut only partly dissolved, so a large fraction is never absorbed and is excreted in the feces; reformulating it as fine particles or an amorphous dispersion can dramatically raise the absorbed amount.
Poor solubility can throttle absorption before chemistry even matters.
Absorption sets how much drug gets in and how fast, but a well-absorbed drug can still be wasted by first-pass metabolism in the gut wall and liver before it ever reaches the rest of the body.