(For this reason, many toxic and easily digestible products such as rat poison contain an emetic. This presents no problem for the effectiveness of the rodenticide as rodents are unable to vomit.) Inducing vomiting can remove the substance before it is absorbed into the body. Emetics can be divided into two categories, those which produce their effect by acting on the vomiting center in the medulla, and those which act directly on the stomach itself. Some emetics, such as ipecac, fall into both categories; they initially act directly on the stomach, while their further and more vigorous effect occurs by stimulation of the medullary center.Salt water and mustard water, which act directly on the stomach, have been used since ancient times as emetics. Care must be taken with salt, as excessive intake can potentially be harmful.Copper sulfate was also used in the past as an emetic. It is now considered too toxic for this use.Hydrogen peroxide is used as an emetic in veterinary practice. Self-induced
Eating disorders (anorexia nervosa or bulimia nervosa)
To eliminate an ingested poison (some poisons should not be vomited as they may be more toxic when inhaled or aspirated; it is better to ask for help before inducing vomiting)
Some people who engage in binge drinking induce vomiting to make room in their stomachs for more alcohol consumption.
The area postrema is a circumventricular organ and as such lies outside the blood–brain barrier; it can therefore be stimulated by blood-borne drugs that can stimulate vomiting or inhibit it.There are various sources of input to the vomiting center:

The chemoreceptor trigger zone at the base of the fourth ventricle has numerous dopamine D2 receptors, serotonin 5-HT3 receptors, opioid receptors, acetylcholine receptors, and receptors for substance P. Stimulation of different receptors are involved in different pathways leading to emesis, in the final common pathway substance P appears involved. The vestibular system, which sends information to the brain via cranial nerve VIII (vestibulocochlear nerve), plays a major role in motion sickness, and is rich in muscarinic receptors and histamine H1 receptors. The cranial nerve X (vagus nerve) is activated when the pharynx is irritated, leading to a gag reflex. The vagal and enteric nervous system inputs transmit information regarding the state of the gastrointestinal system. Irritation of the GI mucosa by chemotherapy, radiation, distention, or acute infectious gastroenteritis activates the 5-HT3 receptors of these inputs. The CNS mediates vomiting that arises from psychiatric disorders and stress from higher brain centers. The medulla plays an important role for triggering the vomiting act.The vomiting act encompasses three types of outputs initiated by the chemoreceptor trigger zone: Motor, parasympathetic nervous system (PNS), and sympathetic nervous system (SNS). They are as follows:

Increased salivation to protect tooth enamel from stomach acids. (Excessive vomiting leads to dental erosion.) This is part of the PNS output.