What is the term for an unintended reaction to a drug?

The term for an unintended reaction to a drug is "adverse effect." An adverse effect refers to any harmful, unintended, or undesirable outcome that occurs as a result of taking a medication. These effects can vary in severity and may affect different individuals in different ways, highlighting the importance of monitoring and reporting any adverse reactions during treatment.

While "side effect" often refers to a secondary, sometimes beneficial effect that accompanies the main therapeutic effect of a drug, adverse effects are typically viewed as negative outcomes and can lead to changes in medication or dosage. "Overdose" describes a situation where an excessive amount of a drug is taken, leading to harmful effects, but it is specific to the amount rather than the nature of the reaction itself. An "allergic reaction" is a specific immune response to a drug that can also be classified as an adverse effect but is more narrowly defined compared to the broader category of adverse effects.