A drug interaction occurs when one substance changes the effect of another drug in the body — either increasing its potency (sometimes to toxic levels), decreasing its effectiveness, or producing entirely new effects. Interactions can occur between two prescription drugs, between a drug and an over-the-counter medication, or between a drug and food, alcohol, or a supplement.
The consequences range from minor inconveniences to life-threatening emergencies. Some interactions — like opioids combined with benzodiazepines — have killed thousands of people. Others require only modest monitoring adjustments. Understanding the severity of a specific interaction is the difference between a conversation with your doctor and an emergency room visit.
PillScope's interaction data is sourced from FDA-approved prescribing information (the "package insert") for each medication. Severity ratings reflect FDA classification, clinical severity in published case reports, and whether the interaction appears in the contraindication versus warning-and-precaution sections of the label.
Metabolic (CYP Enzyme) Interactions
Most drug metabolism happens in the liver via CYP450 enzymes. When one drug blocks these enzymes, another drug accumulates to toxic levels. Warfarin, fentanyl, statins, and benzodiazepines are all processed by CYP enzymes — a common source of dangerous interactions.
Additive CNS Depression
Two drugs that each slow the central nervous system — opioids, benzodiazepines, alcohol, sleep aids, muscle relaxants — stack synergistically. The combination can stop breathing entirely at doses that would individually be survivable. This is the mechanism behind most prescription overdose deaths.
QT Interval Prolongation
Some drugs extend the heart's electrical recovery time (QT interval). When multiple QT-prolonging drugs combine — methadone, antipsychotics, fluoroquinolones, certain antidepressants — the risk of fatal ventricular arrhythmia (torsades de pointes) escalates sharply.