A drug is a chemical or biological
substance that affects the body and its
functions. Drug absorption is when a drug
enters the bloodstream. The level of
absorption can affect the speed and the
quantity of the drug at the site of
action.
This is termed bioavailability. If a
tablet releases the drug quickly, blood
levels may become too high, whereas slow release may result in low levels of
absorption. Additional factors affecting
absorption and bioavailability include
the properties of the drug, and the
physiology of the person such as pH
levels in the stomach and its speed of
emptying. Therefore specific formulations
are used to release the drug at the
desired speed. Common formulations
include: tablets, capsules, suppositories,
transdermal patches, and solutions. Drug
metabolism is the process by which the
body breaks down drugs which mainly
occurs by means of enzymes located in the liver.
Many factors including intake of
specific foods and other drugs can
increase or decrease the speed at which
drugs are broken down and thus determine
their levels within the blood. A drug may
be broken down to stop it from working.
However, there are other drugs that need
to be broken down or modified before
they become active.
These are known as prodrugs. Some drugs are directly eliminated from the body by
the kidneys in urne. For this to happen
the drug must be water soluble. Drugs
that are not water-soluble may be
metabolized by the liver or enzymes in
the blood into breakdown products that
can be eliminated in urine. Diseases that
affect the liver or the kidneys can
cause blood levels of a drug to increase.
