In medicine, an adverse effect is a harmful and undesired effect resulting from a medication or other intervention such as surgery.
An adverse effect may be termed a “side effect”, when judged to be secondary to a main or therapeutic effect. If it results from an unsuitable or incorrect dosage or procedure, this is called a medical error and not a complication. Adverse effects are sometimes referred to as “iatrogenic” because they are generated by a physician/treatment. Some adverse effects only occur only when starting, increasing or discontinuing a treatment.
Using a drug or other medical intervention which is contraindicated may increase the risk of adverse effects. Adverse effects may cause complications of a disease or procedure and negatively affect its prognosis. They may also lead to non-compliance with a treatment regimen.