According to the passage, a drug

For a drug to work, it has to get to the place in the
body where the problem lies, and that's why the
science of pharmacokinetics is important. Enough
of the drug has to stay at the site of action until
the drug does its job, but not so much that it
produces severe side effects or toxic reactions.
Every doctor knows that selecting the right dose is
a tricky balancing act. Many drugs get to their site
of action through the bloodstream. How much
time these drugs need to work and how long their
effects last, often depend on how fast they get into
the bloodstream, how much of them gets into the
bloodstream, how fast they leave the
bloodstream, how efficiently they're broken down
by the liver, and how quickly they're eliminated by
the kidneys and intestines.