Cancer pain is the most common symptom of cancer patients, especially in advanced cancer patients, and is also a factor that directly affects patients’ quality of life. The control of cancer pain follows the principles stipulated in WHO and NCCN guidelines, and non-invasive drug delivery is the preferred route, and the opioid drugs currently used in the third-step pain treatment for cancer mainly include morphine-based immediate release, controlled (slow) release preparations, oxycodone, fentanyl, etc. The adverse effects of opioid analgesics are related to a variety of factors, such as individual differences, age factors, liver and kidney function, drug dosage, drug interactions, etc., and have little to do with the type of opioid drug and the route of administration. In order to alleviate the pain of cancer pain patients, we should not only give full play to the analgesic effect of opioid drugs, but also pay attention to the common adverse drug reactions and take active preventive measures to improve patients’ compliance in treatment. Constipation The affinity and pharmacological effects of opioid receptor agonists on receptors show a dose-effect relationship, that is, the efficacy increases with increasing dose, but at the same time the side effects also increase. The opioid mu receptors, which act on the central nervous system, mainly produce analgesic effects, while activation in the gastrointestinal tract mainly inhibits the peristalsis of the gastrointestinal tract and reduces the secretion of bile and pancreas. Long-term oral administration of opioid analgesics can cause severe constipation. Clinicians should carefully distinguish the true cause and degree of constipation, instruct patients to adjust their dietary habits and lifestyle, and use laxative medications (mainly laxatives) to minimize the patient’s constipation symptoms. Nausea and vomiting Opioid analgesics directly excite the vomiting chemoreceptors located in the medulla oblongata and cause nausea and vomiting, and this effect can be enhanced by vestibular excitation. Because opioid analgesics increase vestibular sensitivity, clinically effective μ-receptor agonists cause some degree of nausea and vomiting. For example, nausea and vomiting occur in 2/3 of patients at the start of morphine and last for approximately 7 days. All patients on opioid analgesics should be treated with antiemetics. After the opioid analgesic dosage has stabilized, the patient’s symptoms can be reduced or disappear, at which point if the patient still has nausea, other causes should be sought. Hypersedation A small number of patients may experience hypersedation, such as drowsiness, during the first few days of opioid pain medication, and the symptoms tend to resolve on their own after a few days. If a patient experiences significant symptoms of over-sedation, the dose of opioid analgesics should be reduced and then gradually adjusted to satisfactory analgesia after symptoms have subsided. In a few cases, the patient’s symptoms of excessive sedation continue to worsen, and the patient should be alerted to serious adverse reactions such as drug overdose poisoning and respiratory depression. Patients with symptoms of excessive sedation such as drowsiness should pay attention to exclude other causes of drowsiness and impaired consciousness, such as the use of other central sedative drugs, hypercalcemia, etc. Urinary retention The incidence of morphine induced bladder sphincter spasm leading to urinary retention is <5% . However, the incidence of urinary retention may be as high as 20% in patients who are also sedated. Therefore, patients are advised to avoid overfilling of the bladder and to give good time and space for urination; to avoid concomitant sedation; and to induce spontaneous urination by running water, hot water flushing of the perineum, and/or massage of the bladder area.