Common side effects include vomiting, constipation, sedation, confusion, hallucinations, and injuries related to poor coordination. Serious side effects may include decreased breathing (respiratory depression), serotonin syndrome, low blood pressure, addiction, or coma. In 2016, more than 20,000 deaths occurred in the United States due to overdoses of fentanyl and analogues, half of all reported opioid-related deaths. Fentanyl works primarily by activating μ-opioid receptors. It is around 100 times stronger than morphine, and some analogues such as carfentanil are around 10,000 times stronger.