Nausea is a symptom, not a disease in itself, but a sign that something is wrong in the body. It is experienced as an unpleasant sensation of wanting to vomit and a lack of appetite. It is often accompanied by sweating, paleness, heart palpitations, and discomfort in the stomach.

Nausea occurs when a specific area in the brainstem - the vomiting center - is activated, typically because neurotransmitters or toxins accumulate in the brain. The toxins can come from within the body, for example if the kidneys or liver fail, or from external sources, such as bacteria in food, medications, or other harmful substances.

Vomiting is an automatic reaction in which the stomach forcefully empties its contents through the mouth to protect the body from harmful substances. Chronic nausea or repeated vomiting, however, is an abnormal condition that can have many different and serious causes.

A wide range of conditions can cause nausea and vomiting - for example motion sickness, dizziness, pregnancy, migraine, poisoning, kidney or liver failure, as well as brain injuries. Nausea can also be a side effect of treatments such as morphine, chemotherapy, or radiation therapy.