Opioids such as oral codeine and morphine have been demonstrated to acutely increase exercise tolerance and alleviate dyspnea in some patients with COPD, in part by simultaneously reducing minute ventilation and decreasing the sensation of breathlessness. Nevertheless, the long-term administration of sustained-release morphine has not been shown yo be superior to placebo in reducing breathlessness in such patients, most of whom reported drowsiness, nausea, constipation, and other adverse effects. Although many patients become tolerant to these effects over time, the generally poor results of opioid administration in research studies of COPD patients suggest that these drugs probably should be used on a patients with recalcitrant dyspnea who do not respond oxygen. For similar reasons, opioids should be discontinued if they are more burdensome than beneficial. However, because their analgesic properties are undisputed, opioids may be particularly useful in patients such as those with lung cancer who have pain in addition to breathlessness.