Codeine is a medication used to treat mild to moderately severe pain. It belongs to a class of drugs known as opioid analgesics and works by changing how your body feels and responds to pain. It is also sometimes used to suppress coughs. However, it's important to note that codeine can be habit-forming and may cause side effects such as drowsiness, constipation, and nausea. It should only be used as directed by a healthcare professional.