Pharmacology Demystified

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Drug Actions
Drug action is the physiochemical interaction between the drug molecule and molecules
in the body that alters a physiological process of the body in one of three ways.
• Replacement: The drug replaces an existing physiological process such as
estrogen replacement.
• Interruption: The drug interferes with a physiological process. This occurs
when an antihypertensive (high blood pressure) drug interferes with the
process that constricts blood vessels and may cause blood pressure to rise.
The blood vessels remain dilated and pressures remain normal or drop.
• Potentiation: The drug stimulates a physiological process as in the case of furosemide
(Lasix) which is a diuretic and stimulates the kidneys to excrete urine.
A drug action begins when the drug enters the body and is absorbed into the
bloodstream where the drug is transported to receptor sites throughout the body
(see Pharmacokinetics, in this chapter). Once the drug hooks onto a receptor site,
the drug’s pharmacological response initiates. The pharmacological response
is the therapeutic effect that makes the patient well.
Drugs have multiple actions. These are the desired effect and effects other
than the desirable effect. The desirable effect is what makes the patient well or
prevents the disease or disorder. An effect other than the desirable effect is known
as a side effect. Some side effects are desirable and others are undesirable (see Side
Effects, in this chapter).
The strength of a drug action is determined by how much of the drug is given,
(the dose) and how often the drug is given (the frequency). For example, a patient
who has a sore throat can be given a large dose of an antibiotic—a loading dose—
on the first day of treatment and a normal or maintenance dose for the next five days.
Drug activity is divided into three phases. These are:
• Pharmaceutic Phase: This phase occurs after the drug is given and involves
disintegration and dissolution of the dosage form.
• Pharmacokinetic Phase: This is the way the drug is absorbed, distributed,
and eliminated.
• Pharmacodynamic Phase: This is the effect the drug has on the body.
PHARMACEUTIC
CHAPTER 2 Drug Action and Drug Interactions
The pharmaceutic phase is the form of the drug such as a tablet, capsule, liquid,
elixirs, or syrups. The drug in solid form must disintegrate before dissolution,
which is the process by which a drug goes into solution before it becomes avail-