Something a treatment does to the body other than the desired effect.
Every treatment (medication, splinting, bandaging, surgery, etc.) affects the body. Most treatments have more than one effect. The effect you're looking for is the desired effect. Anything else the treatment does is a side effect. Some side effects have medical benefits and are often alternative uses for the treatment.
Also Known As: When a side effect is harmful or unwanted, it is considered an adverse effect or an adverse reaction.
Benedryl () is an antihistamine; it helps stop itching and hives. Benedryl also causes drowsiness in most people. When taking it for itching, the drowsiness is considered a side effect. On the other hand, Benedryl is also marketed as a sleep aid and in that case drowsiness is the desired effect.