Factors Contributing to Drug Interactions

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Chapter: Biopharmaceutics and Pharmacokinetics : Pharmacokinetic Drug Interactions

Some of the more important risk factors that lead to drug interactions include 1. Multiple drug therapy 2. Multiple prescribers 3. Multiple pharmacological effects of drug 4. Multiple diseases/Predisposing illness 5. Poor patient compliance 6. Advancing age of patient 7. Drug related factors

Factors Contributing to Drug Interactions

Some of the more important risk factors that lead to drug interactions include –

1. Multiple drug therapy is very common in most acute and chronic care settings, for e.g., therapy in patient suffering from hypertension and congestive heart failure includes antihypertensives as well as digitalis which together may lead to abnormal heart rhythms. Concurrent use of non-prescription drugs, for e.g. aspirin as well as herbal medications also lead to drug interactions. Theoretically, the possibility for drug interactions to occur is over 50% when a patient is receiving five medications, and the probability increases to 100% when seven drugs are used.

2. Multiple prescribers - Some individuals go to more than one physician, and it is common for a patient to be treated by one or more specialists in addition to a family doctor. It is frequently difficult for one prescriber to become aware of all the medications that have been prescribed by others for a particular patient, and many difficulties arise from such situations. For example, one doctor may prescribe an anxiolytic for a patient while another prescribes an antihistamine having sedative properties with the possible consequence of an excessive depressant effect.

3. Multiple pharmacological effects of drug - Most drugs used in current therapy exhibit more than one type of pharmacological action and have the capacity to influence many physiological systems. Therefore, two concomitantly administered drugs will often affect some of the same systems, for e.g. antihistamines (secondary effect is sedation) enhance the sedative effect of tranquillizers.

4. Multiple diseases/Predisposing illness Some patients take several drugs owing to their suffering from more than one disease, for e.g. a patient with both diabetes and hypertension. Multiple therapies in such individuals generally result in drug interactions, for e.g., oral hypoglycaemics and beta-blockers can result in decreased response to antidiabetic drug resulting in elevated blood sugar levels.

5. Poor patient compliance this results when a patient does not take medication in the manner intended by the doctor; which may be due to inadequate instructions from the doctor or pharmacist, confusion regarding taking several medicines, etc. all of which may lead to either underdosing or overdosing, and a consequent drug interaction.

6. Advancing age of patient Increased tendency of drug interaction episodes in elderly is generally due to decrease in liver function in such individuals.

7. Drug related factors - Clinically significant interactions are most likely to occur between drugs that have potent effects, a narrow therapeutic index and a steep dose-response curve (e.g., cytotoxic, antihypertensive, and hypoglycemic drugs, digoxin, warfarin, etc.).

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