Most drugs slip through, they don't get carried
A cell membrane is a thin sheet of fat. The main way most drugs cross it is passive diffusion: molecules drift from where they are crowded (high concentration, e.g. in the gut) to where they are sparse (the blood), needing no energy and no helper. The driving force is simply the concentration difference across the membrane.
Because the barrier is fatty, a drug crosses easily only if it is itself somewhat fat-loving — its lipophilicity. A greasy, uncharged molecule dissolves into the membrane and passes; a water-loving, electrically charged one is repelled and stays behind. A few drugs that resemble natural nutrients hitch a ride on carrier-mediated transport proteins instead, but passive diffusion governs the great majority.
The pH-partition rule
Here is the elegant part. Most drugs are weak acids or weak bases, meaning they flip between two forms: an uncharged form (fat-soluble, crosses membranes) and an ionized, charged form (water-soluble, trapped). Which form dominates depends on the surrounding pH. This dependence is ionization, and the rule that ties it to absorption is the pH partition hypothesis.
The simple version: a drug is best absorbed where it is least ionized. A weak acid (like aspirin) stays mostly uncharged in the acidic stomach, so it can begin absorption there. A weak base flips to its uncharged form in a more alkaline environment like the small intestine. As a slogan: *acids like acid, bases like base* — each is unionized, and therefore absorbable, in the matching pH.
Weak ACID in the stomach (pH ~ 2): HA <--> H+ + A- acidic surroundings push this LEFT -> mostly HA (uncharged, fat-soluble) -> ABSORBED Weak BASE in the small intestine (pH ~ 6-7): BH+ <--> H+ + B less acidic surroundings push this RIGHT -> mostly B (uncharged, fat-soluble) -> ABSORBED Flip the pH and the drug becomes charged = trapped on that side.