The stimulation of vomiting outlined above is summarized in Figure 9.1.

Before the act of vomiting occurs, stimulation of the vomiting centre produces a sensation known as nausea, which is often associated with increased secretion by the salivary and bronchial glands. Drugs that provoke vomiting are called emetics.

Emetics are rarely used in medical practice except in cases of poisoning. They may be divided into two types:

This group of drugs produces vomiting by irritating the stomach. The only one in common use is ipecacuanha, a plant extract, which is dispensed as ipecacuanha emetic mixture, and vomiting should occur in 15–30 minutes. It may be used as a first-aid treatment for overdose provided that:

Ipecacuanha can be used up to 1 hour after ingestion of poison and longer for some substances, such as tricyclic antidepressants and salicylates, when gastric emptying is delayed. It is not as effective as a stomach washout, but is particularly useful in children, when the upset caused by the process of lavage should be avoided if possible, and in removing such objects as berries, which cannot be washed out of the stomach. In general, the use of emetics in poisoning is decreasing because there is little evidence that, even if used soon after ingestion of poison, they usefully reduce absorption.

Apomorphine stimulates dopamine receptors in the CTZ. It is closely related to morphine but has none of its analgesic effects. It has, however, a very powerful emetic action and also produces some cerebral depression. It was formerly used as an emetic but because of its depressant action it should not be used in treating patients who have taken an overdose. At present its use is confined to patients with resistant Parkinson’s disease (see p. 260).