excretion
Excretion is how the body takes the rubbish out. After a drug has done its job and often been chemically altered by the liver, the body still needs a physical exit to send the leftovers away. Excretion is that exit: the routes by which a drug and its breakdown products actually leave the body.
The kidneys are the main door, removing water-soluble drugs and metabolites in the urine. The liver provides a second major door by dumping substances into bile, which travels to the gut and leaves in the feces. Minor routes include the lungs (for volatile agents such as inhaled anaesthetics), sweat, saliva, tears, and breast milk. Which route dominates depends largely on the molecule's water solubility and size.
It is worth distinguishing excretion from metabolism. Metabolism chemically changes a drug, usually making it more water-soluble; excretion physically removes the resulting compounds. The two together make up elimination. A drug can be eliminated almost entirely by excretion of the unchanged molecule, almost entirely by metabolism, or by some mix of both.
Penicillin is excreted largely unchanged in the urine, which is why kidney impairment markedly prolongs its action, whereas the volatile anaesthetic gas is exhaled almost entirely by the lungs.
Different drugs leave by different doors.
Highly lipid-soluble drugs cannot be excreted easily by the kidney because the tubule simply reabsorbs them; they usually must be metabolised to a more water-soluble form first. This is the main reason phase I and phase II metabolism exist.