50 enzymes, but most of the metabolism occurs in only a few of them. The
functioning of these enzymes is influenced by drugs and other chemical
A drug–drug interaction can occur if one drug inhibits an enzyme system, causing
another drug to become toxic. The PI will inform the nurse which drugs should not
be given together in order to prevent a drug–drug interaction.
Excretion refers to the removal of drugs from the body either unchanged or as
metabolites. Clearance refers to the total volume of blood, serum, or plasma from
which a drug is completely removed per unit of time to account for the excretion.
The half-life of a drug provides a measure of the expected rate of clearance. Half-
life refers to the time required for plasma concentrations of the drug to be
reduced by 50%.
Lithium and gabapentin, mood stabilizers, are notable examples of renal excretion.
Any impairment in renal function or renal disease may lead to toxic symptoms.
Dosing refers to the administration of medication over time so that therapeutic
levels may be achieved or maintained without reaching toxic levels
steady state, occurs when absorption equals excretion and the therapeutic level
RS AFFECTING DISTRIBUTION OF A DRUG
Effect on Drug Distribution
Size of the organ
Larger organs require more drug to reach a concentration level equivalent to other organs and tissues.
Blood flow to the organ
The more blood flow to and within an organ (perfusion), the greater the drug concentration. The brain has hi
The brain has high perfusion.
Solubility of the drug
The greater the solubility of a drug within a tissue, the greater its concentration.
Plasma protein binding
If a drug binds well to plasma proteins, particularly to albumin, it will stay in the body longer but have a slow
on. slower distribution.
Both the gastrointestinal tract and the brain are surrounded by layers of cells that control the passage or uptak
uptake of substances. Lipid readily absorbed and pass the blood–brain barrier.
Pharmacokinetics is significantly altered at the extremes of the life cycle. Gastric absorption
changes as individuals age. Gastric pH increases, and gastric emptying decreases.