Codeine is an opium derivative that is rather similar to morphine in actions, uses, contraindications, precautions and adverse reactions. When given in the same dosage as morphine, the respiratory depression is the same, but orally administered codeine is about 60% of the potency of the parenteral, or not by mouth, version unlike morphine, which is potent in both methods of delivery. It causes more histamine release than morphine which may result in low blood pressure, flushing and sometimes constriction of the bronchi of the lungs.
The analgesic potency of codeine is about one-sixth of that of morphine and its antitussive, or cough suppressing, activity is less than half of morphine. Because of these reasons, codeine is only used for mild to moderately severe pain in addition to being used for the suppression of coughing, especially nonproductive coughs.
It is readily absorbed by the digestive tract and takes about fifteen to thirty minutes to begin working.
Codeine is contraindicated for patients with asthma, head injury, liver dysfunction and hypothyroidism, amongst others. Common side effects include dizziness, drowsiness, nausea, constipation and pruritus, or itching. There are three especially dangerous possible side effects: severe respiratory depression, circulatory collapse and anaphylactoid reactions.