A drug interaction is a reaction between two (or more) drugs or between a drug and a food, beverage, or supplement. Drug interactions can be classified as drug-drug, drug-food/beverage, drug-condition/disease, and drug-laboratory interactions. Drug interactions may increase or decrease the effect of the drug(s) or may cause unwanted side effects, including severe and lethal effects. Serious effects of drug interactions include bleeding, bone marrow suppression, arrhythmias, liver failure, seizures, and renal failure. Certain classes of medications are at high risk of drug interactions. Reading drug labels is one way to decrease drug interactions.
| Drug class | Example |
| Antiarrhythmics | Digoxin |
| Anticoagulants | Warfarin |
| Antibiotics | Erythromycin |
| Platelet inhibitors | Clopidogrel, aspirin |
| Anti-epileptics | Phenytoin |
| Immunosuppressants | Cyclosporine |
| MAO inhibitors | Phenelzine |
| Antipsychotics | Lithium |
| Supplements | St.John’s wort |
Drug-drug interactions: Drug-drug interactions can occur inside (pharmacological) or outside (pharmaceutical) the body.
Pharmaceutical interactions may be physical or chemical.
| Type of interaction | Examples |
| Physical | Exposure to light humidity can degrade drugs faster; crushing a tablet may cause it to be absorbed faster than expected |
| Chemical | Potassium phosphate and calcium chloride mixed together in intravenous lines precipitate as calcium phosphate; Phenytoin (Dilantin) and lorazepam (Ativan) become ineffective if mixed together |
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