How do pharmacodynamic interactions occur?

Pharmacodynamic drug-drug interactions (DDIs) occur when the pharmacological effect of one drug is altered by that of another drug in a combination regimen. DDIs often are classified as synergistic, additive, or antagonistic in nature, albeit these terms are frequently misused.

Are drug-drug interactions universal and always predictable?

Clinically significant interactions are often predictable and usually undesired (see Some Drugs With Potentially Serious Drug-Drug Interactions Some Drugs With Potentially Serious Drug-Drug Interactions* Drug interactions are changes in a drug's effects due to recent or concurrent use of another drug or drugs (drug- ...