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Nausea (Latin nausea, from Greek ναυσία - nausia, "ναυτία" - nautia, motion sickness", "feeling sick or queasy") is a sensation of unease and discomfort in the upper stomach with an involuntary urge to vomit. It occasionally precedes vomiting. A person can suffer nausea without vomiting. (Greek ναῦς - naus, "ship"; ναυσία started as meaning "seasickness".)
Nausea is a non-specific symptom, which means that it has many possible causes. Some common causes of nausea are motion sickness, dizziness, migraine, fainting, gastroenteritis (stomach infection) or food poisoning. Side effects of many medications including cancer chemotherapy, nauseants or morning sickness in early pregnancy. Nausea may also be caused by anxiety, disgust and depression.
There are many causes of nausea. One organization listed 700 in 2009. Gastrointestinal infections (37%) and food poisoning are the two most common causes. Side effects from medications (3%) and pregnancy are also relatively frequent. In 10% of people the cause remains unknown.
Food poisoning usually causes an abrupt onset of nausea and vomiting one to six hours after ingestion of contaminated food and lasts for one to two days. It is due to toxins produced by bacteria in food.
Nausea or "morning sickness" is common during early pregnancy but may occasionally continue into the second and third trimesters. In the first trimester nearly 80% of women have some degree of nausea. Pregnancy should therefore be considered as a possible cause of nausea in any women of child bearing age. While usually it is mild and self-limiting severe cases known as hyperemesis gravidarum may require treatment.
Stress and depression
While most causes of nausea are not serious, some serious causes do occur. These include: Intercranial Pressure secondary to head trauma or hemorrhagic stroke, diabetic ketoacidosis, brain tumor, surgical problems, heart attack, pancreatitis, small bowel obstruction, meningitis, appendicitis, cholecystitis, Addisonian crisis, Choledocholithiasis (from gallstones), hepatitis, as a sign of carbon monoxide poison and many others.
Often, a good oral history of the symptom will lead to appropriate treatment where few investigations are needed; however, basic lab tests may be appropriate. If a bowel obstruction is possible, abdominal x-rays may be useful. For instance, if severe anxiety appears to be the cause of nausea, then a short course of an anti-anxiety medication such as lorazepam (Ativan) may be all that is needed. If pain is present the nausea can be caused by pain or by opioids used to treat the pain. These may include codeine, hydrocodone, meperidine, morphine, oxycodone, hydromorphone, or fentanyl. These drugs cause nausea in several ways. They may stimulate the eighth cranial nerve which causes vestibular effects such as dizziness or they may stimulate what is called the Chemo Receptor Trigger zone (CTZ) which in turn stimulates the Vestibular Apparatus in the brain which in turn causes nausea and perhaps vomiting. In addition, opioids may slow gastric emptying, causing constipation, which is often accompanied by nausea. If changing opioids does not abate the symptom then addition of antiemetics becomes necessary? Choice of treatment then is indicated by causation. If pain and anxiety are present then drugs or doses need to be increased; if nausea is caused by movement then adding an over the counter medication such as meclizine is indicated; if constipation is the cause then a laxative is in order; if these fail then often addition of metacloperamide (to stimulate gastric emptying) and haloperidol in very low doses (to block stimulation of the CTZ) will often give relief. If the nausea is very acute, followed by vomiting, in an otherwise healthy person, it is often best to with hold food and then give clear liquids at least in the first 24 hours may be best as the nausea/vomiting my be a defense mechanism where the body is ridding itself of a toxin. If this is done, however, the age and general condition of the person has to be considered. An infant or small child can become dangerously dehydrated in 2-3 hours, while a healthy 21year old can withstand the fluid loss for somewhat longer. In no case should the person be left alone, they should be watched and medical help called for if symptoms of dehydration such as very concentrated (very yellow) urine, poor skin turgor (pull gently on a pinch of skin and if it stays pinched for even a few seconds), confusion, dry eyes and mouth, occur. Nausea can also be caused by poor kidney or liver function. Acutely this is a medical emergency. If the person has a chronic condition then often judicious use of metacloperamide and haloperidol can be very helpful as these conditions cause toxins to accumulate in the blood which stimulate the CTZ, like opioids, and the nausea/vomiting usually respond to the same treatment. Nausea/vomiting caused by chemotherapy is caused in yet a different way and therefore needs medications such as ondansetron for control. Note that there is no all purpose antiemetic. Successful treatment depends on identification of cause.
If dehydration is present due to loss of fluids from severe vomiting, rehydration with oral electrolyte solutions is preferred. If this is not effective or possible, intravenous rehydration may be required. NIH Medline recommends drinking clear fluids, sitting quietly and eating bland foods. Medline recommends you call a doctor if you:
- Cannot keep any food or liquid down
- Vomit 3 or more times in 1 day
- Are nauseated for more than 48 hours
- Feel weakness
- Have fever
- Have stomach pain
- Do not have to urinate for 8 hours or more 
Dimenhydrinate (Gravol) is an inexpensive and effective over the counter medication for preventing postoperative nausea and vomiting. Meclizine is another antihistamine antiemetic. In certain people, cannabinoids may be effective in reducing chemotherapy associated nausea and vomiting. Ondansetron (Zofran) is effective for nausea and vomiting. Pyridoxine or metoclopramide are the first line treatments for pregnancy related nausea and vomiting. Many consider medical marijuana to be an effective herbal remedy for nausea. Several studies have demonstrated the therapeutic effects of cannabinoids for nausea and vomiting in the advanced stages of illnesses such as cancer and AIDS. 
In hospital settings topical anti-nausea gels are not indicated because of lack of research backing their efficacy. Topical gels containing lorazepam, diphenhydramine, and haloperidol are sometimes used for nausea but are not equivalent to more established therapies.
While short-term nausea and vomiting are generally harmless, they may sometimes indicate a more serious condition. When associated with prolonged vomiting, it may lead to dehydration and/or dangerous electrolyte imbalances. Repeated intentional vomiting, characteristic of bulimia, can cause stomach acid to wear away at the enamel in teeth.
Nausea and or vomiting is the main complaint in 1.6% of visits to family physicians in Australia. However only 25% of people with nausea visit their family physician. It is most common in those 15–24 years old and less common in other ages.
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