Do not use codeine in children under 12 as it is associated with a risk of respiratory side effects. Codeine is not recommended for adolescents (12 to 18) who have problems with breathing. Codeine is an opioid medicine that is authorised for pain relief and to treat the symptoms of cough and cold. Codeine is converted into morphine by an enzyme called CYP2D6. Some people (known as ultra-rapid metabolisers) convert codeine into morphine faster than others.