side effect

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A peripheral or secondary effect, especially an undesirable secondary effect of a drug or therapy.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic An unintended consequence of any action in addition to the intended consequence of that action.
• noun idiomatic, medicine An adverse effect, an unintended consequence of a drug or therapy; usually not a beneficial effect.
• noun computing A change in state caused by a function call (typically "side-effect").

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any adverse and unwanted secondary effect
• noun a secondary and usually adverse effect of a drug or therapy