Drug-drug interactions represent a significant consideration in clinical research and medical practice, as concomitant medication use is common and can substantially affect the safety and efficacy of investigational products. These interactions may be pharmacokinetic, affecting how drugs are absorbed, distributed, metabolized, or excreted, or pharmacodynamic, affecting the physiological response to drugs through additive, synergistic, or antagonistic effects at target receptors or systems. Understanding potential interactions is essential for appropriate product labeling and safe clinical use.

Pharmacokinetic interactions frequently involve the cytochrome P450 enzyme system, which metabolizes many drugs in the liver. An investigational product that inhibits a CYP enzyme may increase blood levels of co-administered drugs metabolized by that enzyme, potentially causing toxicity. Conversely, enzyme induction may decrease blood levels of co-administered drugs, potentially reducing efficacy. Other pharmacokinetic interactions involve competition for plasma protein binding, alterations in gastrointestinal absorption, or effects on renal excretion through interactions with drug transporters.

Clinical trial protocols typically specify prohibited medications and establish monitoring requirements for interactions with commonly used concomitant medications. Exclusion criteria may preclude participants taking drugs known to interact significantly with the investigational product, while the protocol or Investigator's Brochure should guide investigators on managing participants who require interacting medications. Dedicated drug-drug interaction studies are often required by regulatory authorities to characterize interaction potential and inform prescribing recommendations that will appear in product labeling.