side effect 
noun
A secondary, typically undesirable effect of a drug or medical treatment: many anti-cancer drugs now in use have toxic side effects...
- Most of those who had had a stroke were aware of their own drug treatment and were concerned about the side effect of drugs.
- Tell your doctor if you have had a particular side effect from a pain medication in the past.
- If priapism has occurred as a side effect of medication then a different medication can usually be prescribed.