An adverse reaction (also known as an adverse effect) is an undesired or harmful effect occurring after taking a medication or receiving surgery. Onset may be sudden or develop over time. If this reaction is minor, it may be referred to as a side effect.
Adverse effects are sometimes referred to as "iatrogenic" because they are generated by a physician or treatment. Occasionally adverse effects occur when starting, increasing or discontinuing a treatment. Using a drug or other medical intervention which is contraindicated
may also increase the possibility of adverse effects. Adverse effects commonly cause medical complications of a disease or procedure and negatively affect its prognosis.