Cancer has gradually become a common and frequent disease, seriously threatening human life and health. Cancer pain is the most common symptom of cancer patients and the main reason affecting the quality of life of cancer patients. Modern medicine has developed a variety of painkillers that can relieve or reduce pain. But as a result, some patients have questions about whether the use of painkillers will produce any side effects. What are the possible adverse effects of cancer painkillers? Drug resistance: With repeated use of the drug, its efficacy decreases, and it is necessary to increase the dosage of the drug or shorten the time of administration in order to maintain the effect of pain relief. Physical dependence: Withdrawal symptoms such as irritability, yawning, runny nose, chills, and insomnia occur after the drug is rapidly stopped. Gradual dose reductions can prevent this from occurring. Most patients need to gradually increase the dose; only 20% of patients have the medication process requires dose reduction. Addiction: In medicine, it is called mental dependence, which refers to a feeling of euphoria after using the drug and a desire to use the drug. It is a form of behavioral expression that accompanies drug abuse. Domestic and international data show that the incidence of mental dependence due to the treatment of pain is less than 1%. Resistance and physical dependence arising from the application of narcotic drugs can also be completely avoided by rational use of medication. Constipation: Almost all patients using opioid analgesics suffer from constipation, and when starting opioid analgesics, a program of timed laxation should be initiated, including the use of laxatives and stool looseners: senna, phenolphthalein, paraffin oil, etc. Respiratory depression: the most potentially serious side effect, usually occurs in patients using opioids for the first time and the dose is too high, accompanied by central nervous system depression in the analgesic, with repeated use of the drug, the risk of occurrence of a gradual decrease, when respiratory depression occurs, with 1:10 naloxone dilution of the slow intravenous drip; for coma should be done tracheotomy. Sedation and drowsiness: can occur after the first or repeated combination of opioid analgesics. Reduce the dose of individual drugs, prolong the time interval between administrations, and choose drugs with a short plasma half-life. Nausea and vomiting: may be treated with mephenchlorpromazine, mephentermine, and vitamin B6. Acute poisoning: respiratory depression, coma, pupil constriction and gastrointestinal tract spasm, etc. Naloxone is used to treat opioid poisoning.