Drug interactions
Drug interactions can occur between drugs or
between drugs and foods. They can interfere with the results of a laboratory
test or produce physical or chemical incompatibilities. The more drugs a
patient receives, the greater the chances that a drug interaction will occur.
Potential drug interactions include:
·
additive effects
·
potentiation
·
antagonistic effects
·
decreased or increased absorption
·
decreased or increased metabolism and excretion.
Additive effects can occur when two drugs with similar
actionsare administered to a patient. The effects are equivalent to the sum of
either drug’s effects if it were administered alone in higher doses.
Giving two drugs together, such as two analgesics
(pain reliev-ers), has several potential advantages: lower doses of each drug,
decreased probability of adverse reactions, and greater pain con-trol than from
one drug given alone (most likely because of differ-ent mechanisms of action).
There’s a decreased risk of adverse ef-fects when giving two drugs for the same
condition because the patient is given lower doses of each drug—the higher the
dose, the greater the risk of adverse effects.
A synergistic effect, also called potentiation, occurs when two drugs that
produce the same effect are given together and one drug potentiates (enhances
the effect of) the other drug. This pro-duces greater effects than when each
drug is taken alone.
An antagonistic
effect occurs when the combined response of two drugs is less than the
response produced by either drug alone.
Two drugs given together can change the absorption
of one or both of the drugs:
Drugs that change the acidity of the stomach can
affect the abil-ity of another drug to dissolve in the stomach.
Some drugs can interact and form an insoluble
compound that can’t be absorbed.
Sometimes, an absorption-related drug interaction
can be avoided by administering the drugs at least 2 hours apart.
After a drug is absorbed, the blood distributes it
throughout the body as a free drug or one that’s bound to plasma protein.
When two drugs are given together, they can compete
for protein-binding sites, leading to an increase in the effects of one drug as
that drug is displaced from the protein and becomes a free, unbound drug.
Toxic drug levels can occur when a drug’s
metabolism and excre-tion are inhibited by another drug. Some drug interactions
affect excretion only.
Drug interactions can also alter laboratory tests
and can produce changes seen on a patient’s electrocardiogram.
Interactions between drugs and food can alter the therapeutic ef-fects
of the drug. Food can also alter the rate and amount of drug absorbed from the
GI tract, affecting bioavailability—the amount of a drug dose that’s made
available to the systemic circulation. Drugs can also impair vitamin and
mineral absorption.
Some drugs stimulate enzyme production, increasing metabol-ic rates and the
demand for vitamins that are enzyme cofactors (which must unite with the enzyme
in order for the enzyme to function). Dangerous interactions can also occur.
For instance, when food that contains Vitamin K (such as green, leafy
vegeta-bles) is eaten by a person taking warfarin, the drug’s anticoagula-tion
properties are decreased and blood clots may form.
Grapefruit can inhibit the metabolism of certain medications, resulting
in toxic blood levels; examples include fexofenadine, albendazole, and atorvastatin.
Because of all the interactions food can have with drug metabolism, being aware
of drug interactions is essential.
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