Cancer and nausea are associated in about fifty percent of people affected by cancer. This may be as a result of the cancer itself, or as an effect of the treatment such as chemotherapy, radiation therapy, or other medication such as opiates used for pain relief. About 70–80% of people undergoing chemotherapy experience nausea or vomiting. Nausea and vomiting may also occur in people not receiving treatment, often as a result of the disease involving the gastrointestinal tract, electrolyte imbalance, or as a result of anxiety. Nausea and vomiting may be experienced as the most unpleasant side effects of cytotoxic drugs and may result in patients delaying or refusing further radiotherapy or chemotherapy.

The strategies of management or therapy of nausea and vomiting depend on the underlying causes. Medical treatments or conditions associated with a high risk of nausea and/or vomiting include chemotherapy, radiotherapy, and malignant bowel obstruction. Anticipatory nausea and vomiting may also occur. Nausea and vomiting may lead to further medical conditions and complications including: dehydration, electrolyte imbalance, malnutrition, and a decrease in quality of life.

Nausea may be defined as an unpleasant sensation of the need to vomit. It may be accompanied by symptoms such as salivation, feeling faint, and a fast heart rate. Vomiting is the forceful ejection of stomach contents through the mouth. Although nausea and vomiting are closely related, some patients experience one symptom without the other and it may be easier to eliminate vomiting than nausea. The vomiting reflex (also called emesis) is thought to have evolved in many animal species as a protective mechanism against ingested toxins. In humans, the vomiting response may be preceded by an unpleasant sensation termed nausea, but nausea may also occur without vomiting. The central nervous system is the primary site where a number of emetic stimuli (input) are received, processed and efferent signals (output) are generated as a response and sent to various effector organs or tissues, leading to processes that eventually end in vomiting. The detection of emetic stimuli, the central processing by the brain and the resulting response by organs and tissues that lead to nausea and vomiting are referred to as the emetic pathway or emetic arch.

Some medical conditions that arise as a result of cancer or as a complication of its treatment are known to be associated with a high risk of nausea and/or vomiting. These include malignant bowel obstruction (MBO), chemotherapy-induced nausea and vomiting (CINV), anticipatory nausea and vomiting (ANV), and radiotherapy-induced nausea and vomiting (RINV).^{[citation needed]}

Malignant bowel obstruction (MBO) of the gastrointestinal tract is a common complication of advanced cancer, especially in patients with bowel or gynaecological cancer. These include colorectal cancer, ovarian cancer, breast cancer, and melanoma. Three percent of all advanced cancers lead to malignant bowel obstruction, and 25 to 50 percent of patients with ovarian cancer experience at least one episode of malignant bowel obstruction. The mechanisms of action that may lead to nausea in MBO include mechanical compression of the gut, motility disorders, gastrointestinal secretion accumulation, decreased gastrointestinal absorption, and inflammation. Bowel obstruction and the resulting nausea may also occur as a result of anti-cancer therapy such as radiation, or adhesion after surgery. Impaired gastric emptying as a result of bowel obstruction may not respond to drugs alone, and surgical intervention is sometimes the only means of symptom relief. Some constipating drugs used in cancer therapy such as opioids may cause a slowing of peristalsis of the gut, which may lead to a functional bowel obstruction.

Chemotherapy-induced nausea and vomiting (CINV) is one of the most feared side effects of chemotherapy and is associated with a significant deterioration in quality of life. CINV is classified into three categories:

Risk factors that predict the occurrence and severity of CINV include sex and age, with females, younger people and people who have a high pretreatment expectation of nausea being at a higher risk, while people with a history of high alcohol consumption being at a lower risk. Other person-related variables, such as chemotherapy dose, rate and route of administration, hydration status, prior history of CINV, emesis during pregnancy or motion sickness, tumour burden, concomitant medication and medical conditions also play a role in the degree of CINV experienced by a person. By far the most important factor which determines the degree of CINV is the emetogenic potential of the chemotherapeutic agents used. Chemotherapeutic agents are classified into four groups according to their degree of emetogenicity: high, moderate, low and minimal.

The European Society of Medical Oncology (ESMO) and the Multinational Association of Supportive Care in Cancer (MASCC) in 2010 as well as the American Society of Clinical Oncology (ASCO) (2011) recommend a prophylaxis to prevent acute vomiting and nausea following chemotherapy with high emetic risk drugs by using a three-drug regimen including a 5-HT3 receptor antagonist, dexamethasone and aprepitant (a neurokinin-1 antagonist) given before chemotherapy.