Adverse effects In opioid-naive patients, common adverse effects at the start of therapy include Sedation and mental clouding Nausea and vomiting Constipation Itching Respiratory depression Myoclonus Because steady-state plasma levels are not approached until 4 to 5 half-lives have passed, drugs with a long half-life (particularly levorphanol and methadone) have a risk of delayed toxicity as plasma levels rise.