Final answer:
Various factors such as barriers in the body, the route of administration, plasma protein binding, and the blood-brain barrier can prevent the effective absorption of systemically administered medications.
Step-by-step explanation:
The effective absorption of systemically administered medications can be prevented by various factors. One of the main factors is the presence of barriers in the body, such as cell membranes, metabolic enzymes, efflux transporters, and binding proteins. These barriers can limit the absorption of medications in the gastrointestinal (GI) tract.
In addition, the route of administration plays a crucial role. Some medications are not easily absorbed from the GI tract into the bloodstream, which can necessitate alternative administration methods like intravenous or intramuscular injection.
Other barriers to absorption include plasma protein binding and the blood-brain barrier (BBB). High plasma protein binding can reduce the availability of drugs in the tissues, while the BBB restricts the passage of many molecules from the circulating blood into the central nervous system.