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How and Why
Drugs Work
Chapter 5
Intended and Unintended
Effects of Drugs
• Intended responses:
- Reason for using the drug
• Unintended responses:
- Side effects
• The main distinction between intended
responses and side effects depends on the
therapeutic objective.
Common Side Effects of Drugs
• Nausea or vomiting
• Changes in mental alertness
• Dependence
- Withdrawal
• Allergic reactions
• Changes in
cardiovascular activity
Common Side Effects of Drug
Figure 5-1: Common side effects with drugs of abuse. Almost every organ or system
in the body can be negatively affected by the substances of abuse.
Dose-Response
• Many factors can affect the way an
individual responds to a drug, including the
following:
- Dose
- Tolerance
- Potency
Dose-Response (continued)
• Additional factors
- Pharmacokinetic properties:
• Rate of absorption
• Manner distributed throughout the body
• Rate metabolized and eliminated
- Form of the drug
- Manner in which the drug is administered
Dose-Response Curve
Figure 5-2: Dose-response curve for relieving a headache with aspirin in three users. User
A is the most sensitive and has 100% headache relief at a dose of 60 mg. User B is the next
most sensitive and experiences 50% headache relief with a 600-mg dose. The least
sensitive is user C: with a 600-mg dose, user C has no relief from a headache.
Margin of Safety
• The range in dose between the amount of
drug necessary to cause a therapeutic effect
and a toxic effect.
Potency vs. Toxicity
• Potency: The amount of drug necessary to
cause an effect
• Toxicity: The capacity of a drug to do damage
or cause adverse effects in the body
Drug Interaction
• Additive effects
- Summation of effects of drugs taken
concurrently
• Antagonistic (inhibitory) effects
- One drug cancels or blocks effects of
another
• Potentiative (synergistic) effects
- Effect of a drug is enhanced by another
drug or substance
Pharmacokinetic Factors
That Influence Drug Effects
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Administration
Absorption
Distribution
Inactivation
Biotransformation and elimination
Forms and Methods
of Taking Drugs
• Oral ingestion
• Inhalation
• Injection
• Topical
application
Drugs can be introduced into the body using various
methods, such as pills, capsules, oral liquids, topicals, or
injections.
© NADKI/ShutterStock, Inc.
Distribution
• Most drugs are distributed throughout the
body in the blood.
• It takes approximately 1 minute for a drug
to circulate throughout the body after it
enters the bloodstream.
• Drugs have different patterns of distribution
depending on their chemical properties.
Required Doses for Effects
• Threshold dose: The minimum amount of a
drug necessary to have an effect
• Plateau effect: The maximum effect a drug
can have regardless of the dose
• Cumulative effect: The buildup of drug
concentration in the body due to multiple
doses taken within short intervals
Time-Response Factors
• The closer a drug is placed to the target
area, the faster the onset of action.
• Acute drug response:
- Immediate or short-term effects after a
single drug dose
• Chronic drug response:
- Long-term effects after a single dose
Biotransformation
• Biotransformation: The process of
changing the chemical or pharmacological
properties of a drug by metabolism.
• The liver is the major organ that
metabolizes drugs in the body.
• The kidney is the next most important organ
for drug elimination.
Physiological Variables That
Modify Drug Effects
• Age
• Gender
• Pregnancy
© Pixtal/SuperStock
Adaptive Processes
• Tolerance: Changes causing decreased
response to a set dose of a drug
• Dependence: The physiological and
psychological changes or adaptations that
occur in response to the frequent
administration of a drug
• Withdrawal
Adaptive Processes
The relationship and consequences of adaptive
processes to drug abuse. The processes
discussed in the text are highlighted in the
figure.
Tolerance
• Reverse tolerance (sensitization): Enhanced
response to a given drug dose; opposite of
tolerance
• Cross-tolerance: Development of tolerance
to one drug causes tolerance to related drugs
Drug Dependence
Physical
Dependence
(e.g.,withdrawal
and rebound)
Psychological
Dependence
(e.g., craving)
Psychological Factors Affecting
Drug Effect
• Individual’s mental set
• Placebo effects
Addiction and Abuse
• The use of the term addiction is sometimes
confusing. It is often used interchangeably
with dependence, either physiological or
psychological in nature; other times, it is
used synonymously with the term drug
abuse. A more accurate definition is the
compulsive drug use despite negative
consequences.
Addiction and Abuse (continued)
• Factors affecting variability in
dependence/addiction:
- Hereditary factors (genetic variants);
responsible for 40–60% vulnerability
- Drug craving
Addiction and Abuse (continued)
• Other factors contributing to drug use
patterns:
- Positive versus negative effects of drug
- Peer pressure
- Home, school, and work environment
- Mental state