TAK-951

Feeling sick in the stomach (nausea) or throwing up (vomiting) are among the most common symptoms during treatment with medicines. It is hoped that a medicine called TAK-951 may help people to not feel sick in the stomach or throw up. The main aim of this study is to learn about side effects of TAK-951 when given as a single or multiple doses to healthy adults. Side effects are medical problems thought to be caused by the study treatment. Another aim is to learn how a healthy adult's body processes TAK-951 (this is called pharmacokinetics or PK). In this study, participants will receive either TAK-951 or placebo. The placebo looks like TAK-951 but does not have any medicine in it. Both TAK-951 and placebo will be given as an injection directly under the skin. This is called subcutaneous or subcutaneous (SC).
The study will be conducted in 3 parts:
* In Part 1, participants will be given one SC injection of either TAK-951 or placebo.
* In Part 2, participants will receive up to three daily SC injections of either TAK-951 or placebo of the same dose
* In Part 3, participants will receive one SC injection of either TAK-951 or placebo and another SC injection up to 1 week later.
Participants will be checked for their health either 28 days after the last injection (Parts 1 and 2) or 14 days after the last injection (Part 3).

Some adults are at a higher risk of feeling sick (nausea) or being sick (vomiting) after they have surgery. In this study, these adults will have planned surgery. The main aim of this study is to learn if TAK-951 stops these adults from getting nausea or vomiting after surgery. This will be compared with another medicine called ondansetron. Another aim is to check for side effects from the study medicines.
Before surgery, the study doctor will check who can take part in this study. Those who can take part will be picked for either Treatment Group A or Treatment Group B by chance.
Treatment Group A: Just before surgery, participants will receive a placebo slowly through a vein (infusion). Just before the end of the surgery, they will receive TAK-951 as an injection under the skin.
Treatment Group B: Just before surgery, participants will receive ondansetron slowly through a vein (infusion). Just before the end of the surgery, they will receive a placebo as an injection under the skin. In this study, a placebo will look like TAK-951 but will not have any medicine in it.
Participants will not know which study medicines they received, or in which order, nor will their study doctors or surgeons. This is to help make sure the results are more reliable.
Participants will stay in the hospital for 24 hours after their surgery so that the study doctors can check for nausea and vomiting. The study doctors will also check for side effects from the study medicines.
Participants will visit the hospital for a check-up 14 days later.