Osmotic and Bulk laxatives
Increasing water in the gut
Laxatives are the type of constipation treatments that act directly to treat the symptom but not the underlying cause. Two types of laxatives work by increasing the water content of stools, thereby giving it greater volume, softer consistency, and quicker passage through the gastrointestinal tract. Although their effects are similar, the mode of action is quick different.
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The first is known as the bulk laxative, examples of which include psyllium fiber and methylcellulose, which when in the gut, expand after absorbing some water. These constipation treatments bulk up the stool, hence their name. THe other kind of laxative in this class is the osmotic water-binding laxative, examples of which include lactitol and Macrogol. The word osmotic means that it draws water in from the surroundins. So while these agents do not technically bulk up themselves, they do counteract the dehydrating effect of the colon, and improve the natural stool consistency by drawing water into the lumen (or interior of the gastrointestinal space).
Bulking agents
Bulking agents as constipation treatments are most likely to be high fiber agents. The most popular and widely used of these are psyllium, methylcellulose and polycarbophil. In fact, if you approach your primary care giver with a constipation problem, most likely the first line therapy will be intense high fiber treatment. For example, metamucil is a bulking agent that can be bought at a number of places. Because many people report problems with bloating and gas when fiber treatment is first started, it is often suggested that treatment should start with small doses. After 2-3 days, if things have not changed, the dosage should increase until the maximally recommended dose.
Osmotic laxatives
The other kind of laxative constipation treatments that increase water retention in the gut are the osmotic laxatives. The osmotic, water-binding agents can also be bought over the counter like fiber agents, although some osmotic agents are available by prescription only. Popular ones include magnesium-containing agents, like milk of magnesia (or technically magnesium hydroxide), and citrate of magnesia. Some types of sugars that are poorly absorbed by the gut also function as water-binding laxatives. These include sorbitol and a prescription-only lactulose.
Finally, a polymer known as polyethylene glycol or PEG for short is a very popular agent for constipation treatments. It is available in commercial outlets under the name Macrogol. Polyethylene glycol has been used in medical settings for many years as a cleansing prep before colon-based diagnostic procedures. Scientifically controlled studies comparing patients who took osmotic water-binding agents, versus those who did not, conclusively prove that some of these agents are successful at treating constipation. The only agent that has not been fully evaluated is the one containing magnesium.
PEG and long-term use
The osmotic agent polyethylene glycol has been tested on patients with chronic constipation in a clinical trial for long-term use. Tests showed that patients tolerated it well for up to 6 months. For those interested, the success rate of patients taking PEG compared to those taking a placebo was about 55% to 11%, a significant number indicating efficacy of treatment. There are some side effects associated with osmotic agents as there are with traditional bulking agents. Side effects include bloating, gas, cramping, diarrhea and feelings of nausea. For children and those who have kidney problems, osmotic agents can cause an imbalance in electrolytes. This is because the action of osmosis changes not only water content, but the content of salt in the body. Patients with kidney problems might have trouble adapting to electrolytic effects from these kinds of constipation treatments.
Sources:
Eoff and Lembo, Optimal Treatment of Chronic Constipation in Managed Care, Journal of Managed Care Pharmacy, 2008; 14(9a):Supplement