Demerol is one of the brand names of pethidine which is also called meperidine. It is an opioid pain medication which often comes in injectable form. It is short-acting and is prescribed for moderate to severe pain. Since Demerol also makes the users experience euphoria, it is known to be addictive. Its side effects include confusion, dizziness, chest pain, cardiac and respiratory issues, hives, and itching. Its known street names are Demmies and Smack.