C H A P T E R 2
Drugs and the body
25
KEY POINTS
■■
Pharmacokinetics is the study of how the body deals
with a drug.
■■
The concentration of a drug in the body is
determined by the balance of absorption,
distribution, metabolism and excretion of the drug.
■■
In determining the amount, route and appropriate
timing of a drug dose, the pharmacokinetics of that
drug has to be considered.
FACTORS INFLUENCING DRUG EFFECTS
When administering a drug to a person, the professional
must be aware that the human factor has a tremendous
influence on what actually happens to a drug when it
enters the body. No two people react in exactly the
same way to any given drug. Even though textbooks
and drug guides explain the pharmacodynamics and
pharmacokinetics of a drug, it must be remembered that
such information is usually based on studies of healthy
adult males. Things may be very different in the clinical
setting. Consequently, before administering any drug,
it is vital to consider a number of factors. These are
discussed in detail in the following sections and summa
rised in Box 2.2.
KEY POINTS
Weight
The recommended dose of a drug is based on drug
evaluation studies and is targeted at a 70 kg person.
People who are much heavier may require larger doses
to get a therapeutic effect from a drug because they have
increased tissues to perfuse and increased receptor sites
Critical concentration
4
2
1
0
1
2
3
4
5
6
(Drug half-life)
Serum drug concentration (Units/mL)
Serum drug concentration
Time in hours
Time of day
Critical concentration
O
r
a
l
A
B
Toxic level
10
AM
10
AM
2
PM
6
PM
10
PM
2
PM
6
PM
10
PM
Day 1
Day 2
I
n
t
r
a
v
e
n
o
u
s
FIGURE 2.4
Influence of biological half-life,
route of administration, and dosing regimen
on serum drug levels.
A.
Influence of route of
administration on the time course of drug levels
after administration of a single dose of a drug.
The dashed lines indicate how the biological
half-life of the drug may be determined from the
curve of drug concentration after an intravenous
dose. At time 0, immediately after the injection,
there were 4 units of the drug in each millilitre
of serum. The drug concentration fell to half of
this amount, 2 units/mL, after 1 hour, the drug’s
biological half-life.
B.
Influence of dosing regimen
on serum drug levels (drug given four times
daily, at 10
am
and at 2, 6, and 10
pm
). The drug
accumulates as successive doses are given
throughout each day; the drug is being given at a
rate greater than the person’s body can eliminate
it. This dosing regimen has been chosen so that
the person will have a therapeutic level of the drug
for a significant portion of the day yet never have
a toxic level of the drug.
Weight
Age
Gender
Physiological factors—diurnal rhythm, electrolyte
balance, acid–base balance, hydration
Pathological factors—disease, hepatic dysfunction, renal
dysfunction, gastrointestinal dysfunction, vascular
disorders, low blood pressure
Genetic factors
Immunological factors—allergy
Psychological factors—placebo effect, health beliefs,
compliance
Environmental factors—temperature, light, noise
Drug tolerance
Cumulation effects
Interactions
■■
BOX 2.2
Factors affecting the body’s response to
a drug
