Vomiting has many and varied causes. It can be triggered by ingesting a toxin or poison (e.g., alcohol, a prescription or recreational drug, or contaminated food), by an inflammation of the stomach lining, or by a bowel obstruction. It may also occur in diseases that delay the emptying of the stomach, such as diabetes. Injuries to the head, such as concussions, often lead to vomiting. Psychological factors may provoke vomiting in situations that are frightening or in some way horrifying or repulsive. People with motion sickness or other conditions of the vestibular (balance) system experience nausea and vomiting as a result of certain types of movement (e.g., travelling in a car or airplane).
Vomiting on purpose may play an important role in eating disorders such as anorexia and bulimia. When this is the case, it's hard to detect, even after weeks or months of constant vomiting. Although people who vomit like this may do so without losing weight or becoming dehydrated, induced vomiting can lead to malnutrition and metabolic abnormalities.
Nausea during pregnancy is common, especially during the first three months. For pregnant women and some other people, certain smells and tastes can cause nausea. Medications and medical therapies can also produce nausea as a side effect. For example, chemotherapy treatment for cancer often causes nausea and vomiting. If you feel nauseous because of medication you're taking, you should decide with your doctor whether to put up with it (the nausea may be temporary), or to stop taking the medication. You may also be prescribed an anti-nausea medication to take with the medication causing the nausea.
If you vomit or feel the urge to vomit for a prolonged period of time, you should seek medical attention. If you vomit anything other than the contents of your stomach (fluids and partially digested food), or produce vomit that contains blood, substances that look like coffee grounds, or something you think is odd or strange, see a doctor.