Osmotic laxatives: Are they SAFE?

Dr John P Wright

Updated: 2 July 2008

Broadly speaking laxatives are used in two situations. The first is for constipation. Here the patient feels uncomfortable and has not passed stool for a while. The interpretation of discomfort and the time between each stool passage is highly subjective but suffice it to say that both patient and physician feel it is disturbed. Traditionally in this situation a bowel of dried fruit or similar was consumed and Mafekeng was relieved. The use of stimulatory laxatives such as fruit and senna has however declined as physicians have realized the damage caused by the long term use of such laxatives. Into this fraught situation dietary advice often falls on deaf ears as the patient demands a rapid solution to the ills of the modern low fiber diet.

In this context the clinician has two approaches. The first is to relent and to recommend colonic stimulants such as the traditional laxatives fruit, senna or busacodyl. These stimulatory agents are associated with colonic damage such as melanosis coli. The other option is to suggest osmotic agents. These are generally viewed as safe and non-damaging to the colonic physiology. First popularized for colonic clearance prior to endoscopic examinations but now widely used for the seriously constipated. Two varieties are commonly used. These contain either PEG (polyethylene glycol) or sodium phosphate.

PEG containing solutions have been used for colonic cleansing since 1980. They have an unpleasant chemical taste which is compounded by the need for large volumes of isotonic solution to lavage the colon. Typically 4 liters are drunk over four hours. The reward for this Herculean task is a spotlessly clean colon with undamaged colonic mucosa. This of critical importance if biopsies need to be taken during colonoscopy to detect mucosal disease. Another advantage is that fluid balance is not affected so that hypertensive, cardiac or renal patients are not adversely effected. The patient is not dehydrated and other than a sore anal area escapes unscathed prior to the indignities of the colonic examination.

The alternative cleansing agent is the small volume sodium phosphate agents. These have the obvious advantage of being small volume. Normally a few doses are taken over 24 hours. This produces the required diarrhoea to clear the colon but the fluid used to wash the colon out is from the patient’s own fluid space. This may therefore dehydrate the patient and thereby compromise the fluid dynamics particularly in the aged or infirm. The colonic mucosa may exhibit the changes of a hypertonic solution and render the diagnosis of mild colitis difficult. Of greater importance is the increased incidence of renal failure after sodium phosphate ingestion. A commonly used small volume laxative involves taking 45 to 60 mg of sodium phosphate which has been associated with acute renal failure. Some preparations have 15 mg sodium phosphate but the “safe” dose has not yet been established. The renal failure is apparently caused by acute phosphate nephropathy associated with dehydration and greatly increased serum phosphate levels. These deleterious effects can be reduced by making sure that the patient has no significant cardiovascular problems and that adequate hydration is maintained during the preparation.

An alternative small volume laxative is magnesium citrate. This should also not be used in the presence of known renal impairment but does not of itself induce renal damage outside of the dehydration associated with all small volume osmotic laxatives. In summary osmotic laxatives are efficient and other than when using sodium phosphate are generally safe.

In patients with simple constipation the smaller volume PEG laxatives on the market are a useful simple solution without the fear of colonic damage. They are particularly useful in the young and elderly. They do not “feed” the colonic bacteria and thereby increase gas production which is so common after fiber supplementation.

Most important is the realization that constipation is essentially a lifestyle condition which should be resolved with dietary means and not laxatives. In this regard the difference between fruit and medicinal laxatives is simply the dose and not the method of action.