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Drug dosage
Dose-response relationship
• Depends on multiple
factors
• A drug usually has
one desired effect
that causes a change
in a target organ or
structure
• It will also have
secondary effects
Main effects and side effects
• Main effect – the
effect you want the
drug to have
• Side effects –
secondary effects that
may or may not be
desirable or helpful
• Goal is to use a dose
of a drug that is
Dosage-response curve
• Making dosage
decision
• Compare dosage to
the percentage of
people showing
different effects
• ED10- effective dose
where 10% of people
show response of
interest
Therapeutic Index
• A measure of drug safety
• The ratio of the dosage at
which 50% of patients
would be expected to die
(LD50) to the dosage
where 50% would show
the desired effect (ED50)
• The larger the ratio the
safer the drug
• The safest drug is one
Age as a factor that influences the
drug response curve
• Infants take longer to
metabolize most
drugs because of an
immature liver
• Older adults also do
not metabolize drugs
quickly because their
system less efficient
Other factors influencing drugresponse curve
• Gender specific drug
response – males and
females
physiologically
different
• Psychological factors
– expectations most
notably
History of Usage - Tolerance
• Metabolic tolerance –
long term use of a drug
can result in chronic
elevation of enzymes that
metabolizes the drug
• Neuronal tolerance –
prolonged use of a drug
causes physical and
chemical changes in
neurons effected by the
drug
• If the drug is an
antagonist to a
Behavioral and learned tolerance
• Behavioral tolerance
– the subject learns to
overcome the effects
of the drug – drug
levels remain high,
but you learn to
function normally
“acting sober”
• Learned tolerance –
the body becomes
classically
conditioned
Drug Interactions on dose
response curve
• Drug interaction – the
effects of taking 2 drugs
are not just additive; the
response to 1 drug is
greatly increased or
decreased by the
administration of a
second
• Can be useful or it can be
a problem
• Example of a useful
interaction:
– Drug A not effective at low
dosages
Undesirable Interactions
• Interaction increases
undesirable side
effects
• Mixing alcohol with
anti-depressants such
as prozac increases
the risk of liver
damage – a possible
side effect of both