drug–drug interaction

A drug–drug interaction happens when taking two drugs together changes how one or both behave, so the combined effect differs from either drug alone. It is like two workers sharing one tool: if one keeps the tool busy, the other cannot do its job on schedule. Many patients take several medicines at once, so these interactions are a real-world safety concern.

Interactions split into two broad types. Pharmacokinetic interactions change how much drug reaches the bloodstream or tissues, most often by one drug inhibiting or inducing the enzymes (such as cytochrome P450) or transporters that handle another. Pharmacodynamic interactions instead change the drug's effect at its target, for example two drugs that both depress breathing adding up to a dangerous degree.

When one drug blocks the metabolism of another, the second drug can accumulate to toxic levels; conversely, an enzyme-inducing drug can speed clearance and make a partner drug stop working. Discovery teams screen candidates for their potential to inhibit or induce major metabolic enzymes precisely to anticipate such problems before they reach patients.