Constipation is still one of the common bowel habit problems that affects people across the globe. Approximately 15% of Americans suffer with chronic constipation – this is is constipation that persists for several months. Acute constipation is usually short lived lasting a few days to weeks. Laxatives are the first choice of treatment for constipation when dietary and lifestyle remedies do not yield the desired results.
What is a laxative?
A laxative is a substance that promotes bowel movements. There are different types of laxatives that work in different ways but all laxatives facilitate the expulsion of stool. While these drugs are largely safe to use over a short period of time, it is sometimes open to abuse due to the ease of availability. Laxatives are not only used for the treatment of constipation. It can also be used for preventing constipation in situations where constipation is likely to arise or where constipation can complicate other existing medical conditions.
Constipation is defined as having less than three bowel movements in a week, where there is difficulty passing stool. There is usually considerable straining during a bowel movement and the stool is hard. Constipation is classified as primary when it occurs on its own with no underlying cause. This is usually due to slow movement through the bowels (slow bowel motility) and not due to any disease. It is classified as secondary when constipation occurs as a symptom of some underlying medical condition.
How do laxatives work?
Different types of laxatives work in different ways. These mechanisms are further discussed under each type of laxative. However, it is important to understand the mechanism of constipation to then understand how the laxatives may help counteract it.
When the fluid intestinal contents enter the colon, it is gradually transformed into a soft solid mass that is known as stool. This occurs when the colon gradually reabsorbs water to form this mass. When there is sufficient bulk due to the accumulation of stool in the colon, the colon walls stretch and this is felt as an urge to defecate.
The muscles in the colon wall contracts. It pushes the stool into the rectum which then fills up and the rectal walls stretch. At this point a person has a strong urge to defecate. The involuntary anal sphincter relaxes and opens in preparation for defecation.
Once a person finds an appropriate setting, they can voluntarily relax the other sphincter and stool is pushed out through the anus. However, this process does not occur as smoothly when constipation is present. In addition the stool may remain in the colon for long periods of time and the stool hardens.
Types of Laxatives
There are various types of laxatives and the choice of laxative depends on multiple factors such as:
- how long the constipation has been present,
- any underlying condition causing constipation,
- anatomic obstruction within the bowels, and
- pre-existing medical conditions.
There are also combination laxatives which may utilize more than one type of laxative simultaneously.
These laxatives absorb water since it contains soluble fiber. It helps to bulk up the stool which in turn assist with movement through the bowels (peristalsis). It is used in the long term prevention and treatment of constipation. Bulk-producing laxatives should not be used in people with a bowel obstruction.
Examples of bulk-producing laxatives include:
Osmotic laxatives contain substances that draws out fluid into the bowels and limits water reabsorption in the colon. These substances should be used with caution as there is a risk of dehydration if used in large amounts. People with kidney impairment should avoid these laxatives unless it is prescribed by a medical doctor.
Examples of osmotic laxatives include:
- Milk of magnesia
- Sodium phosphate
- Magnesium citrate
As the name suggests, lubricant laxatives provide lubrication for the stool to move through the bowels with greater ease. These lubricants cannot be absorbed and therefore remain in the bowels. It also prevents absorption of water from the bowels. Lubricant laxatives are used for acute and subacute constipation.
Examples of lubricant laxatives include:
- Mineral oil
- Magnesium hydroxide
Saline laxatives work in a similar way to osmotic laxatives. It helps the stool to retain fluid which bulks up the stool and this promotes movement through the bowels. Saline laxatives are mainly used for acute constipation provided there is no bowel blockage. It is also used prior to surgery or diagnostic procedures where it is necessary to flush the bowels.
Examples of saline laxatives include:
- Magnesium citrate
- Sodium acid phosphate
Stimulant laxatives work by stimulating the nerves in the wall of the bowels. This nerve stimulation increases peristalsis within the bowels thereby pushing out the stool. Some types of stimulant laxatives may also help the stool retain water.
Examples of stimulant laxatives include:
- Sodium picosulfate
Stool softeners are one of the first choices of constipation treatment for acute and subacute constipation. It works as a surfactant which reduces the oil-water tension. This allows for water and fats to be incorporated into the stool thereby softening the stool.
Examples of stool softeners include:
Prokinetic drugs are not used only for constipation. These drugs stimulate movement through the bowels by promoting persistaltic activity. These substances may also facilitate retention of fluid in the stools. Prokinetic agents are considered when other types of laxatives fail to act in chronic primary constipation.
Examples of prokinetic drugs include:
Side Effects of Laxatives
There may be individual side effects of certain types of laxatives. Broadly, laxatives may cause the following side effects:
- Abdominal cramping
- Nausea and sometimes vomiting
- Excessive belching
- Rectal irritation
Apart from the type of laxative, the amount of the laxative used and the duration of use can also determine the side effects. It is therefore important to consult with a medical professional before using a laxative although many of these drugs are available over-the-counter. Furthermore laxative use should be intergated alongside dietary and lifestyle management to ease constipation.