Opiates are a widely researched class of drugs whose effects are well understood. In general, these effects include:
  • Pain relief and sedation
  • Constipation
  • Nausea
  • Respiratory depression, including shallow breathing, lowered blood pressure and reduced heart rate.
Fentanyl is one of the most potent opiate-based painkillers approved for medical use, intended for chronic pain patients who have developed a tolerance to less potent opiates such as oxycodone or morphine and are no longer achieving therapeutic pain relief. 
Fentanyl’s effects are active at much lower doses than heroin, so careful control over dosage and use frequency is important. However, fentanyl’s effects are shorter lasting, and are often not considered as euphoric as heroin among those that have used it.
Understanding of the effects of other synthetic opioids that have not been as extensively studied is limited. Like fentanyl, other synthetic opioids like W-18 and U-47700 are extremely potent; that is, active at very low doses. People who have used these substances also report effects similar to other opiates, like relaxation and pain relief.