Laxatives and Anti-diarrhoeal agents
Constipation is a common problem in palliative medicine for a variety of reasons, particular as a side effects of opioids. Often laxatives need to be prescribed to avoid the discomfort that develops with worsening constipation. In general there are two main classes of laxatives (those that soften stool and those that increase peristalsis) and two main routes of administration (the oral route and the rectal route).
Diarrhoea is a less common problem but it can be equally distressing. It may be due to infection, medications (e.g. laxatives) or a medical condition (e.g. pancreas cancer). When not due to an infection, various medications (e.g. loperamide) can be used to try to reduce diarrhoea.
Laxatives should generally be given when a patient is constipated or when a patient is expected to become constipated (e.g. on commencing opioids), although it is reasonable to attempt dietary and lifestyle changes first (e.g. increase fibre, increase exercise, increase fluid intake).
There are various mechanisms by which laxatives work, including:
- Softening laxatives - reduce the hardness of stool making it easier to pass
- Osmotic laxatives - non-absorbable sugars that draw water into faeces
- Stimulant laxatives - directly stimulate colonic nerve endings increasing peristalsis
Given the different mechanisms, there is some theoretical rational behind which laxative to prescribe. In patients who lack the energy to push out soft stool, stimulants make logical sense, whereas in patients who are unable to pass hard stool, softening laxatives make sense. In practice, a combination of softening and stimulant laxatives are usually required, especially when using opioids.
Laxatives are typically given orally, however in patients with severe constipation, it may important to give them rectally, especially when there is faecal impaction presenting as soiling and smearing of faeces or overflow diarrhoea. In rare cases, physical manual disimpaction may be required. Regular rectal stimulants also tend to be an effective means of ensuring patients with spinal cord compression do not develop constipation.
Methylnaltrexone is a special class of drug that is a peripheral opioid antagonist that can be used to treat refractory opioid-induced constipation. It can cause acute abdominal pain and vomiting as side effects.
|Table: Common and important laxatives|
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