Pharmacological drug-drug interactions

Pharmacological interactions: These may be pharmacokinetic or pharmacodynamic interactions.

Pharmacokinetics: It is “what the body does to the drug”. This includes absorption, distribution, metabolism, excretion, and drug elimination.

Pharmacodynamics: It is “what the drug does to the body”. This includes the action of drugs on receptors and their effects, dose-response curves, etc.

Drugs can influence the pharmacodynamics of other drugs by directly affecting each other’s effects. Drugs can also influence the pharmacokinetics of other drugs by forming complexes, activating metabolic enzymes like Cytochromes P450, competing for transporters, etc.

Types of pharmacodynamic drug-drug interactions

Additive: The effect of two or more drugs given in combination is “equal to” the sum of the effects of them acting alone (1+1=2)

Synergistic: The effect of two or more drugs given in combination is “more than” the sum of the effects of them acting alone (1+1 >2)

Antagonistic: The effect of two or more drugs given in combination is “less than” the sum of the effects of them acting alone (1+1<2), as their actions are opposite to each other

Not all drug interactions are harmful; some can be beneficial. For example, combining medications like paracetamol with aspirin is used in combination therapies to offer increased pain relief in cases of migraine headaches.

When two or more drugs are antagonistic, the result is a reduced effect of the drugs. For example, a patient taking antihypertensive medication may have reduced blood pressure control if they take certain herbal supplements. Antagonistic drug interactions can treat drug poisoning by using antidotes that block the drug receptor.