The goal of this clinical trial is to evaluate the efficacy and safety of olanzapine plus metoclopramide in preventing opioid-induced nausea and vomiting (OINV) in adult patients with advanced cancer who are initiating strong opioid therapy. The main questions it aims to answer are: (1) Does the combination of olanzapine and metoclopramide reduce the incidence of OINV? (2)What adverse events do participants experience when taking the combination of olanzapine and metoclopramide? Researchers will compare the olanzapine-metoclopramide combination to a no prophylactic treatment control group to determine whether the combination is effective in preventing OINV.

Participants will: Take olanzapine (2.5 mg/day ) and metoclopramide (10 mg three times daily) or receive no prophylaxis for 7 days; Through follow-up, nausea, vomiting, the time of the first attack of nausea and vomiting, the duration of nausea and vomiting, the use of strong opioids and adverse events were evaluated and recorded, as well as the pain score (using NRS) and quality of life (EQ-5D-5L) of the patients were evaluated at baseline and on day 7.

Patients were randomly assigned in a 1:1 ratio to: The study group received olanzapine 2.5mg qn+ metoclopramide 10mg tid po for a total of 7 days for preventive treatment; The control group did not receive preventive treatment. Both groups could receive other treatments recommended by doctors after the appearance of OINV.

Inclusion criteria

1. Patients with malignant tumors diagnosed by pathology or histology;
2. Patients diagnosed with locally advanced or advanced stages by imaging;
3. Age ≥ 18 years old;
4. The eastern cooperative oncology group (ECOG) performance status of 0-3;
5. The expected survival period shall be no less than 4 weeks;
6. Moderate or severe cancer pain with a Numerical Rating Scale (NRS) score of ≥ 4 points;
7. Be able to take oral medication;
8. Initial treatment with potent opioid painkillers (such as morphine, oxycodone, fentanyl, etc.);
9. No systemic chemotherapy or radiotherapy was received within one month prior to selection, and no drugs that may induce nausea and vomiting were used.
10. There were no gastrointestinal discomforts such as nausea or vomiting at the time of selection, and no intestinal obstruction.
11. Possess normal comprehension and communication skills, be capable of completing research evaluations and following research procedures.

Exclusion criteria

1. Diabetic patients with a clear diagnosis and poorly controlled blood sugar levels;
2. There are symptoms of nausea or vomiting;
3. Symptomatic intracranial diseases, such as brain metastases or leptomeningeal metastasis;
4. Received chemotherapy drug treatment within one week before the trial medication or during the trial period;
5. Receive radiotherapy for the head, abdomen or pelvic cavity within one week before the trial or during the trial;
6. New drugs with emetic or antiemetic effects have been used within 48 hours before the start of the trial;
7. Patients with severe electrolyte imbalance, abnormal kidney or liver function;
8. Patients with gastrointestinal bleeding;
9. Pregnant or lactating women;
10. Patients diagnosed with breast cancer;
11. Those whose electrocardiogram examination indicates heart disease or prolonged QTc interval;
12. There is a history of allergy or contraindications to olanzapine or metoclopramide.