Opiate: A medication or illegal drug that is  either derived from the opium poppy, or that mimics the  effect of an opiate (a synthetic opiate). Opiate drugs are  narcotic sedatives that depress activity of the central  nervous system, reduce pain, and induce sleep. Long- term use of opiates can produce addiction, and overuse can  cause overdose and potentially death.

Side Effects: Constipation, sedation, euphoria, dizziness, fatigue, depression, tremors, sleeplessness, anxiousness, flu-like symptoms, upset stomach, dry mouth, pupil constriction, itching, hallucination, delirium, sweating, muscle   and bone pain, confusion, extreme irritability and muscle spasms. Severe side effects can include severe respiratory depression, confusion or stupor, coma, clammy skin, circulatory collapse and cardiac arrest.

Opiate drugs include: heroin, morphine, codeine, Oxycontin, Dilaudid, methadone,  hydrocodone, vicoden, opana and others

Resources: MedicineNet.com and http://www.opiates.com/opiates/opiate-side-effects.html