Side effects: Problems that occur when treatment goes beyond the desired effect. Or problems that occur in addition to the desired therapeutic effect.
Example -- The common side effects of cancer treatment including fatigue, nausea, vomiting, decreased blood cell counts, hair loss, and mouth sores are instances of side effects that occur in addition to the desired therapeutic effect.
Drug manufacturers are required to list all known side effects of their products. When side effects of necessary medication are severe, sometimes a second medication, lifestyle change, dietary change, or other measure may help to minimize them.