Morcet is a brand name for a medication containing the active ingredient codeine phosphate. It belongs to a group of medications called opioid analgesics and is used to relieve moderate to severe pain. Codeine works by binding to certain receptors in the brain and nervous system to decrease the sensation of pain. It can be habit-forming and should be used only under the guidance of a healthcare professional. Side effects of Morcet may include drowsiness, dizziness, constipation, and nausea.