A secondary and usually adverse effect of a drug or therapy WordNet 3.0 & Open English WordNet

In medicine, a side effect is an effect, whether therapeutic or adverse, that is unintended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequences of the use of a drug. Wikipedia

Effect that is secondary to the one intended. Wikipedia Disambiguation

An unintended effect of the use of a drug Wikipedia Disambiguation

Medicinal effect, whether therapeutic or adverse, that is secondary to the one intended Wikidata

Any secondary effect, especially an undesirable one. OmegaWiki

An adverse effect, an unintended consequence of a drug or therapy; usually not a beneficial effect. Wiktionary

Medicine: an adverse effect. Wiktionary (translation)