Provided is a composition for preventing nausea or vomiting, including a 5-HT3 receptor antagonist and a corticosteroid, and having a pH adjusted to 3.0-6.0. Provided also is a method for preparing a composition for preventing nausea or vomiting, including: mixing a 5-HT3 receptor antagonist with a corticosteroid; and adjusting pH of the mixture obtained from the preceding operation to 3.0-6.0.