ANTI-EMETICS

The theory –  anti-emetics can be divided into 3 groups:

1. Anti-emetics acting mainly on the vomiting center in the mid-brain. These are anti cholinergic drugs (which can cause dry mouth, drowsiness, blurred vision). Antihistamines are also anti-cholinergic.

This group includes:

•  Scopolamine

•  Cyclizine

•  Promethazine (sedating)

2. Some  anti-emetics act mainly on the chemoreceptor trigger zone (CTZ). (The CTZ is in the medulla, close to the vomiting center.) The CTZ has dopamine receptors, therefore these are anti-doparninergic drugs (which can have Parkinsonian side effects of stiffness and dystonic movements).

This group includes:

•  Prochlorperazine

•  Thiethylperazine

•  Haloperidol

•  Chlorpromazine

•  Methotrimeprazine

3. Gastro-kinetic anti-emetics which act on the CTZ and also increase gastric emptying and gut peristalsis.

This group includes:

• Metoclopramide

The neurophysiology of vomiting is complex, and has been investigated mainly in the cat. A human is not a cat and the pathways may be different. For example, in the cat motion sickness is known to be mediated via the CTZ whereas in humans anti-cholinergics are more effective than phenothiazines in preventing motion sickness, suggesting motion in humans acts directly on the vomiting center.

Useful drugs:

1. Prochlorperazine 5mg 3 times a day is useful to prevent nausea when starting morphine. It has few side effects and causes little or no drowsiness.

2. Thiethylperazine is a phenothiazine that is similar in clinical usage to prochlorperazine, and is less sedating than chlorpromazine. It is a useful drug in preventing and controlling nausea. The usual oral dose is 10mg 2 to 3 times a day. It is available in suppository form (usual dose, 10mg every 12 hours).

3. Haloperidol is a strong anti-emetic. It is a narrow-spectrum  anti-emetic, with a powerful antagonist action at the dopamine receptors of the CTZ, but with no action at cholinergic or histamine receptors. It is the strong  anti-emetic of first choice, causing little drowsiness. The usual starting dose is 1.5mg at bedtime. The dose can be increased to 5mg 2 times a day, or for short periods to 5mg 4 times a day, but extrapyramidal effects (stiffness initially) are common. It can be used in a continuous subcutaneous infusion.

4. Chlorpromazine is a strong  anti-emetic but can cause drowsiness and dry mouth. It is indicated for the nausea of uremia, if the patient also has hiccups, and is used if anxiety is contributing to the nausea, or if the patient also needs some sedation. The oral dose is usually 10mg to 25mg 3 times a day.

5. Cyclizine 50mg every 8 hours either orally or by intramuscular injection is a very useful and logical addition to drugs that act at the CTZ, since it has a different site action, acting on the cholinergic or histamine receptors of the vomiting center. It is therefore indicated for nausea due to intestinal causes or due to brain metastases causing raised intracranial pressure. It is also particularly indicated if there is an element of motion sickness, e.g., nausea that is worse on turning over in bed. It causes some drowsiness and dry mouth. Cyclizine can be used alone in a continuous subcutaneous infusion up to a concentration of 15mg/ml. Mixed with morphine, however, there is a risk of precipitation if the concentration of each drug exceeds 10mg/ml.

6. Metoclopramide is particularly useful if there is poor gastric emptying, e.g., hepatomegaly compressing the stomach and causing heartburn, wind and fullness as well as nausea. Metoclopramide is particularly effective in high doses for the nausea of cytotoxic therapy. It acts at the CTZ but also causes gastric emptying, by relaxing the pylorus, tightening the lower esophageal sphincter, and causing peristalsis of the stomach and of all the small bowel. It is useful if there is reflux esophagitis or hiccups due to gastric distention. It should be avoided if there is colicky pain. It can be used in continuous subcutaneous infusions, and has been used in high dosage (up to 240mg per day) to overcome gastric stasis.

7. Methotrimeprazine is a powerful  anti-emetic and sedative. It is used in a continuous subcutaneous infusion (50mg to 200mg over 24 hours) for severe nausea, but can cause marked drowsiness and dry mouth. In ambulant patients it can also cause postural hypotension. It is also particularly useful for terminal agitation and restlessness. (see Subcutaneous infusions, Terminal Phase)

The author and publisher have taken precautions to ensure that the information in this book is error-free. However, readers must be guided by their own personal and professional standards of good practice in evaluating and applying recommendations made herein. The contents of this book represent the views and experience of the author, and not necessarily those of the publisher.