2. How Drugs Work
• drugs produce either desired or undesired effects on the body
• once in the blood, drugs are circulated throughout the body
• the properties of both the drug and the body influence where
the drug will go and what concentration it will have at its target
site
• the site where a drug causes an effect is the site of action
• the objective of drug therapy is to deliver the right drug, in
the right concentration, to the right site of action and at the
right time to produce a desired effect
3. • when a drug produces an effect, it is interacting at a molecular
level with cellular material or structure (receptor)
• the receptor is described as a lock into which the drug
molecule fits (key) and only those drugs able to bind chemically
to the receptors at the site of action can produce an effect
• this is why drugs are selective in their action – they only act
on specific targeted receptors and tissues
• receptors are located on the
surfaces of cell membranes and
inside the cells
4. • when drug molecules bind with a receptor, they can cause a
reaction that stimulates or inhibits cellular function
• the pharmacological effects of these interactions are termed
agonism or antagonism
agonists
• drugs that activate receptors and produce cellular response that
may either accelerate or slow normal cellular processes – depends
on the type of receptor involved
• for example: epinephrine-like drugs act on the heart to increase
heart rate / acetylcholine-like drugs act on the heart to slow the heart
rate
antagonists
• drugs that bind to receptors but do not activate them
• they block the receptors’ action by preventing other drugs or
substances from interacting with them
5. • the number of receptors available to interact with a drug will
directly influence the effect
• a minimum number of receptors have to be occupied by drug
molecules to produce an effect – if too few there will be little to
no effect
• once all receptors are occupied, increasing the dose will not
increase the effect
• receptors can be changed by drug use
• extended stimulation of cells with an agonist can reduce the
number or sensitivity of the receptors – the effect of the drug is
reduced
• extended inhibition of cell functions with antagonists can
increase the number or sensitivity of receptors - if the
antagonist is stopped abruptly, the cells can have an extreme
reaction – antagonist drugs are gradually discontinued to avoid
withdrawal symptoms
6. drug action at the site of action
• like a lock and key, only certain drugs are able to interact with certain
receptors
types of action
• act through physical action – the protective effects of topical agents
• react chemically – antacids that reduce excess gastric acidity
• modify metabolic activity of pathogens – antibiotics
• change osmolarity of blood and draw water out of tissue into the blood
• incorporate into cellular material to interfere with normal cell function
• join other chemicals to form a complex that is more easily excreted
• modify the biochemical or metabolic process of the body’s cell or enzyme
systems
• some drugs work by changing the ability of ions to move into or out of cells
(calcium and sodium channels allow movement of ions into nerve cells –
stimulate function / potassium channels move ions our of nerve cells –
obstruct function)
• some drugs modify the creation, release or control of nerve cell hormones
that regulate physiological responses
7. Concentration and Effect
• it is difficult to measure the amount of a drug at the site of
action and thus predict an effect
• many factors influence a drug’s movement from site of
administration to site of action – metabolism, excretion,
membrane permeability, etc…
• one way to monitor the amount of a drug in the body and its
effect at the site of action is to use a dose-response curve
• human variability: different people have different
characteristics that affect how a drug product behaves in them
8. • a better way to relate the amount of drug in the body to its
effect is to determine drug concentrations in body fluids
• blood is usually used because there is rapid equilibrium
between the site of administration and the site of action
how used…
• manufacturers use data to evaluate drug products
• pharmacy professional evaluate compounding formulations
• research measures human variability of drug products
• physicians monitor drug therapy for patients
