How can drug interactions change the effects or safety of medicines?

Drug interactions occur when one substance alters the pharmacological effect of another, potentially increasing or decreasing effectiveness, or causing unanticipated adverse effects. These interactions generally arise through two primary mechanisms:1

Pharmacokinetic Interactions

These affect how the body processes a drug, including its absorption, distribution, metabolism, and excretion.1
* Absorption: One drug may interfere with the uptake of another in the gastrointestinal tract. For example, antacids can bind to tetracycline, forming insoluble complexes that prevent the body from absorbing the antibiotic.21
* Distribution: Drugs often bind to proteins in the blood. If two drugs compete for the same binding sites, they can alter each other's distribution throughout the body.1
* Metabolism: Drugs are often broken down by liver enzymes. Some medications inhibit these enzymes, causing another drug to accumulate to toxic levels, while others may increase enzyme activity, causing a drug to be cleared too quickly and lose its therapeutic effect.21
* Excretion: One drug may alter kidney function, either accelerating or slowing the removal of another medication from the body.1

Pharmacodynamic Interactions

These occur when drugs interact at their receptors. For instance, an agonist and an antagonist may compete for the same receptor, diminishing each other's effects, while two agonists may intensify a response. Additionally, some drugs produce biochemical changes that increase sensitivity to the toxic effects of others, such as how certain diuretics can increase the toxicity of digitalis by lowering potassium levels.1

While many interactions are hazardous, some are therapeutically beneficial, such as combining chemotherapeutic agents to improve cancer treatment or using multiple antihypertensive drugs to mitigate side effects. Patients should always inform their physicians of all medications and supplements they are taking to avoid dangerous, unintended consequences.123