Adverse Effect

In medicine, an adverse effect is a harmful and undesired effect resulting from a medication or other intervention such as chemotherapy or surgery. An adverse effect may be termed a "side-effect" (when judged to be secondary to a main or therapeutic effect) and may result from an unsuitable or incorrect dosage or procedure (which could be due to medical error). 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 medical complications of a disease or procedure and negatively affect its prognosis. They may also lead to non-compliance with a treatment regimen.
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Adverse effect Pharmacotherapy An undesirable and unintended, although not necessarily unexpected, result of therapy or other intervention–eg, [more]Adverse effect - Community and Resources

An unwanted side effect of treatment. 

EFFETTO CONTRARIO. EFFETTO DANNOSO. EFFETTO NOCIVO