Drug use without indication. The patient is taking a medication without a valid indication or reason for use.
Adverse drug reactions. The patient has a medical problem that is the result of an adverse drug reaction or adverse effect.
Drug interactions. The patient has a medical problem that is the result of a drug-drug, drug-food, or drug-laboratory test interaction.
Some adverse drug reactions do not result in injury to a patient, but may just be a minor annoyance.
Medication error is a separate term with its own definition. The National Coordinating Council on Medication Error Reporting and Prevention has a standard definition.
Many of these terms overlap but they each have distinct meanings.