Vicodin is a powerful prescription painkiller that is used to treat moderate to severe pain. It is a combination of hydrocodone and acetaminophen. Hydrocodone is an opioid that works by binding to opioid receptors in the brain and spinal cord, which blocks the transmission of pain signals. Acetaminophen is a non-opioid pain reliever that works by inhibiting the production of prostaglandins, which are chemicals that cause pain and inflammation.