Long-Term Effects of OxyContin. Opiates have severe effects on the brain, and OxyContin is no different. Long-term effects include persistent vomiting and diarrhea caused by its action on the stomach.The stomach is unable to regulate itself properly, leading to these issues.piates have severe effects on the brain, and OxyContin is no different. Long-term effects include persistent vomiting and diarrhea caused by its action on the stomach.