Prochlorperazine (brand names Stemetil, Compazine) : This acts upon the chemo-receptor trigger zone in the brain, ie the part of the brain responsible for stimulating vomiting. By blocking this chemo-receptor trigger zone, nerve impulses are prevented from being sent down to the stomach to cause vomiting. Prochlorperazine is used to treat severe nausea and vomiting, vertigo and balance disorders. The initial dose in the acute attack is 20mg followed by 10mg after 2 hours. The normal, 'preventative' dose is 5 or 10mg three times per day. Some medics advise taking it with food as it can cause slight stomach irritation. Alternatively, it is also available as a buccal preparation, UK brand name 'BUCCASTEM', (3mg) which should be placed high up on the gum between the cheek to slowly dissolve. This anti-emetic is also used following anaesthetics and, in larger doses, as an anti-psychotic drug used to treat schizophrenia. It can have a sedative effect and is useful in helping to treat anxiety. Since the drug can induce symptoms resembling Parkinsons Disease, it is not usually prescribed long-term.

Metoclopramide (brand names Maxolon, Reglan, Emex, Maxeran, Primperan, Metamide) : This acts in the same way as Prochlorperazine but it also acts directly upon the stomach by stimulating stomach muscles to keep everything moving downwards which is beneficial for gastric emptying disorders and the stasis which occurs during attacks of migraine and vertigo. Unfortunately this can also, very occasionally, over-stimulate the gut and lead to diarrhoea. Metoclopramide has a wide range of uses including : gastro-oespophageal reflux, peptic ulcer, dyspepsia, nausea associated with IBS, post-anaesthesia N&V, chemotherapy (high-dose Maxolon), drug-induced vomiting including the N&V occurring as a side-effect of analgesics; gastritis, gastroenteritis, hepatic and biliary disprders, chronic renal failure, cardiac disease, migraine and alcoholism.

Metoclopramide and Prochlorperazine are available as tablets, syrup, suppositories and injection. The usual dose is 10mg at 8 hour intervals. For some people below 60kg, the dose of metoclopramide is reduced by half. Metoclopramide is used with care for the treatment of children and young adults (especially young women) under 20 since side-effects are more common. Metoclopramide is never used after abdominal surgery since a muscle relaxant is likely to have been given and it can take a few days for the gut to begin working again. Both metoclopramide and prochlorperazine can induce movement disorders - the most serious of which are involuntary movements of the limbs, face and eyes and, sometimes, these can be irreversible.

Domperidone (brand name Motilium) : This is another type of anti-emetic and anti-nausea drug which acts at the chemoreceptor trigger zone to block vomiting signals. It has the advantage over metoclopramide that it is less likely to cause acute dystonic reactions with facial and skeletal muscle spasms and oculogyric crises (eye movement disorders). In the UK, it is has recently been made available over the counter from Pharmacies, without a prescription being required.

Other anti-emetics include the phenothiazines which are also known as antipsychotics and 'major tranquillisers and include : Chlorpromazine, Droperidol, Haloperidol, Methotrimeprazine, Perphenazine and Trifluoperazine which are similar to Prochlorperazine in their action and side-effects and are often used also in the short-term treatment of Anxiety Disorders. More powerful anti-emetic drugs, such as Granisetron, Ondansetron and Nabilone, are prescribed for the nausea and vomiting induced by cytotoxic chemotherapy or radiotherapy. 

Anti-histamines can be beneficial, particularly for travel sickness - click bus for details>

Informally, amongst emetophobes, metoclopramide is considered to be the most effective anti-emetic (although there is a very low risk of acute dystonic reactions with facial and skeletal muscle spasms and oculogyric crises (eye movement disorders), especially in girls, young women and the elderly. These usually occur shortly after starting treatment but subside within 24 hours of stopping the drug. Domperidone and then prochlorperazine closely follow Metoclopramide in terms of effectiveness.

NOTE: I am often asked whether anti-emetic drugs are effective in cases of stomach infection. Unfortunately, I have been unable to trace a definitive, conclusive (and, hopefully, reassuring(!)) answer.  It's not a question the medical profession seem to take very seriously…..!