Olanzapine is an effective antiemetic agent for preventing highly emetogenic regimens-induced nausea and vomiting (HER-INV) in patients receiving highly emetogenic regimens (HER). The optimal dose remains debated, with the standard 10 mg dose often causing significant daytime sedation. Recent evidence suggests that lower doses (2.5 mg and 5 mg) may offer comparable efficacy with improved tolerability. However, no head-to-head randomized controlled trials (RCTs) directly compare all three doses.

To compare the relative efficacy and safety of olanzapine at 2.5 mg, 5 mg, and 10 mg, in combination with standard triple antiemetic prophylaxis, for the prevention of HER-INV in adult patients undergoing HER, using a network meta-analysis (NMA) of RCTs. RCTs comparing any two doses of olanzapine (2.5 mg, 5 mg, 10 mg) in adults with solid tumors.

Inclusion Criteria:

1. Olanzapine was used to prevent nausea and vomiting (HER-INV) in solid tumors patients receiving highly emetogenic regimens
2. Randomized controlled trials (RCTs), including conference abstracts if sufficient data are provided.
3. Adult patients (≥18 years) with solid tumors receiving highly emetogenic chemotherapy (HEC).

3. Olanzapine at 2.5 mg, 5 mg, or 10 mg, added to a standard triple antiemetic regimen (NK1 receptor antagonist + 5-HT3 receptor antagonist + dexamethasone).

4. Any of the other three olanzapine doses or placebo (2.5 mg vs. 5 mg vs. 10 mg, or vs. placebo).

5. At least one of the pre-specified efficacy or safety outcomes must be reported.

Exclusion Criteria:

1. Non-solid tumors patients, non-randomized studies, observational studies, case reports, reviews.
2. Studies involving pediatric populations, non-HEC regimens.
3. Studies where olanzapine is used as rescue medication only.
4. Studies with overlapping patient populations (the most recent or complete publication will be selected).

Information Sources: Electronic databases: PubMed, Embase, Cochrane Central Register of Controlled Trials (CENTRAL), Web of Science Core Collection. We will also search clinical trial registries (ClinicalTrials.gov, WHO ICTRP) and manually review reference lists of relevant systematic reviews and included studies.

Risk of Bias Assessment: The risk of bias for individual RCTs will be assessed using the revised Cochrane Risk of Bias tool for randomized trials (RoB 2.0) by two independent reviewers.

Subgroup Analyses: Subgroup analyses are planned by treatment regimens (such as: cisplatin-based vs. AC-based et al.).

Inclusion Criteria:

1. Olanzapine was used to prevent nausea and vomiting (HER-INV) in solid tumors patients receiving highly emetogenic regimens.
2. Randomized controlled trials (RCTs), including conference abstracts if sufficient data are provided.
3. Adult patients (≥18 years) with solid tumors receiving highly emetogenic chemotherapy (HEC).

3. Olanzapine at 2.5 mg, 5 mg, or 10 mg, added to a standard triple antiemetic regimen (NK1 receptor antagonist + 5-HT3 receptor antagonist + dexamethasone).

4. Any of the other three olanzapine doses or placebo (2.5 mg vs. 5 mg vs. 10 mg, or vs. placebo).

5. At least one of the pre-specified efficacy or safety outcomes must be reported.

Exclusion Criteria:

1. Non-solid tumors patients, non-randomized studies, observational studies, case reports, reviews.
2. Studies involving pediatric populations, non-HEC regimens.
3. Studies where olanzapine is used as rescue medication only. Studies with overlapping patient populations (the most recent or complete publication will be selected).

All patients with solid tumors received different doses of olanzapine (2.5 mg, 5 mg, 10 mg) as an add-on to standard antiemetic prophylaxis for the prevention of HER-induced nausea and vomiting in adult (≥18 years) patients with solid tumors receiving highly emetogenic regimens.