PlainMeds guide, sources read October 8, 2026
Understanding drug interactions
A drug interaction is a change in how a medicine works when it meets another medicine, a food or drink, or a health condition. 1,455 of the FDA labels quoted on PlainMeds have a drug interactions section, and 14,122 pairs of medicines are named in each other's labels.
What kinds of interactions are there?
FDA, Drug Interactions: What You Should KnowDrug-drug interactions occur when two or more drugs react with each other.
Drug-food/beverage interactions result from drugs reacting with foods or beverages.
Drug-condition interactions may occur when an existing medical condition makes certain drugs potentially harmful.
Source: FDA, Drug Interactions: What You Should Know
Where does a label list interactions?
A prescription label lists them in its Drug Interactions section and, for combinations that must be avoided, in Contraindications or the boxed warning. Many entries name an enzyme or a class (“strong CYP3A4 inhibitors”) and give examples. A label lists what its studies and reports found; it is not a complete list of every combination. An over-the-counter Drug Facts label lists them under “Do not use” and “Ask a doctor or pharmacist before use”.
Source: FDA drug labels via openFDA
Which medicines do other labels name most?
| Medicine | Brand names | Generic labelers (all forms) | Pharmacy cost | Boxed warning | Shortage | Recalls since 2012 |
|---|---|---|---|---|---|---|
| Phenytoin | Dilantin, Phenytek | 29 labelers | $0.145 per capsule 100 mg capsule | No | No current entry | 11 |
| Rifampin | none listed | 13 labelers | $0.679 per capsule 300 mg capsule | No | In shortage | 14 |
| Warfarin | Jantoven | 19 labelers | $0.093 per tablet 2 mg tablet | Yes | No current entry | 7 |
| Ketoconazole | Nizoral, Ketodan | 30 labelers | $0.249 per gram 2% cream | No | No current entry | 3 |
| Carbamazepine | Tegretol, Carbatrol, Equetro | 42 labelers | $0.119 per tablet 200 mg tablet | Yes | A presentation being discontinued | 11 |
| Digoxin | Lanoxin | 26 labelers | $0.133 per tablet 125 mcg tablet | No | No current entry | 4 |
Source: FDA drug labels via openFDA
How to check your own medicines
The interaction checker reads each label for the others and quotes what it says, both ways. PlainMeds does not grade interactions; a pharmacist can say what an entry means for one person.