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Chapter: Clinical Pharmacology: Fundamentals of clinical pharmacology

Drug interactions

Drug interactions can occur between drugs or between drugs and foods.

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.


Adding it all up


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 situation


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.


Fighting it out


An antagonistic effect occurs when the combined response of two drugs is less than the response produced by either drug alone.


An absorbing problem


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.


Bound and determined


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 waste

Toxic drug levels can occur when a drug’s metabolism and excre-tion are inhibited by another drug. Some drug interactions affect excretion only.


Back to the lab


Drug interactions can also alter laboratory tests and can produce changes seen on a patient’s electrocardiogram.


Menu planning


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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