Learn about the side effects of codeine, from common to rare, for consumers and healthcare professionals. Find patient medical information for Codeine on WebMD including its uses, side effects and safety, interactions, pictures, warnings, and user ratings Codeine is changed to morphine in the body. Some people change codeine to morphine more quickly than others. These individuals are called ultra-rapid metabolizers of codeine. Contact your doctor immediately if you experience extreme sleepiness, confusion, or shallow breathing. Codeine is an opiate and prodrug of morphine mainly used to treat pain, coughing, and diarrhea. It is commonly used as a recreational drug. View codeine phosphate information, including dose, uses, side-effects, renal impairment, pregnancy, breast feeding, contra-indications and important safety information. Codeine is a type of strong painkiller called an opioid. It's used to treat pain, for example after an operation or an injury. It's often used when other painkillers, such as paracetamol and ibuprofen, have not worked. Codeine can also be used to treat a dry or painful cough or diarrhoea. Codeine is used to manage mild to moderate pain, where treatment with an opioid is appropriate, and when other pain treatments such as non-opioid pain medicines do not treat your pain well enough or you cannot tolerate them. Codeine is an opioid and considered a strong prescription pain medicine. What is this medication? CODEINE (KOE deen) treats pain. It is often prescribed when other medications have not worked well enough or cannot be tolerated. It works by blocking pain signals in the brain. It belongs to a group of medications called opioids. Codeine belongs to a class of medications called opiate (narcotic) analgesics and to a class of medications called antitussives. When codeine is used to treat pain, it works by changing the way the brain and nervous system respond to pain. Codeine is a narcotic pain reliever (analgesic) used to treat mild to moderately severe pain frequently combined with Tylenol or aspirin. Common side effects include itching, rash, stomach pain, constipation, shortness of breath, nausea, lightheadedness, and dizziness.