Table 3 presents a classification of commonly used oral laxatives.
|TABLE 3 Oral Laxatives|
|1. Anthraquinone Laxatives|
|senna (eg. Senokot)|
|cascara (eg. together with aloe in Nature’s Remedy)|
|danthron (eg. Modane)|
|2. Diphenylmethane Laxatives|
|phenolphthalein (eg. Ex-lax, Feen-a-mint)|
|bisacodyl (eg. Dulcolax)|
|3. Castor oil|
|magnesium hydroxide (eg. Milk of Magnesia)|
|magnesium citrate (eg. Citro-Mag)|
|dioctyl sodium sulfosuccinate (eg. Colace)|
|dioctyl calcium sulfosuccinate (eg. Surfak)|
|psyllium preparations (eg. Metamucil)|
|Lactulose (eg. Chronulac)|
Stimulant laxatives are thought to act on the intramural nervous plexus or intestinal smooth muscle. There is some reason to be concerned that prolonged use of any of the stimulant laxatives may damage the myenteric plexus, thereby impairing bowel function. However, at this point there is no direct evidence to implicate the diphenylmethane group. The anthraquinone and diphenylmethane laxatives act in six to ten hours. Their action is confined to the colon, whereas castor oil acts on both small and large bowel. Castor oil produces one to two semifluid stools within two to six hours and is appropriate only for bowel preparation. Although not a consideration in geriatric practice, the active principles of the anthraquinone laxatives may be excreted in breast milk in sufficient quantities to affect nursing infants. Bisacodyl is enteric coated because it can cause gastric irritation. Patients should therefore be advised to swallow bisacodyl tablets whole.
In general, stimulant and saline laxatives should be reserved for bowel preparation for certain X-ray procedures, administration after some treatments for intestinal parasites and for acute constipation during periods of administration of constipating drugs, during periods of enforced bedrest and following barium X-ray examinations. Stimulant and saline laxatives may cause cramping.
Saline laxatives are slowly and incompletely absorbed and retain water in the lumen of the gastrointestinal tract, producing reflex peristalsis by distending the gut wall. Dehydration can occur if hypertonic solutions of saline cathartics are administered repeatedly. Magnesium salts should be given with caution to patients with impaired renal function (which includes all elderly to some degree) because magnesium may accumulate and produce intoxication. Saline laxatives other than magnesium hydroxide, which is slower, begin acting within three hours. Magnesium citrate should be reserved for bowel preparation because of its potency.
Emollient laxatives are surface active agents which by reducing surface tension are thought to facilitate penetration of the fecal mass by water and fats. They produce stool softening within 24-48 hours. Giving them prior to meals may facilitate mixing.
Bulk-forming laxatives begin to act in 12-24 hours but the full effect may not be felt for two to three days. Although they are free of systemic effects, intestinal obstruction has been reported. Adequate hydration should insure against this possibility. Esophageal obstruction has occurred when they have been swallowed dry. In a 1952 study by Cass and Wolf comparing psyllium preparations and methylcellulose in bedridden or partially ambulatory chronic care patients, psyllium preparations produced clearly superior results. The main action of psyllium preparations is through bulk formation, perhaps leading to reflex peristalsis. Transient bloating occurs in some patients at the beginning of treatment. Psyllium preparations can be mixed with cooked cereal as well as with liquids. Administration immediately before, during or immediately after meals may improve mixing with gut contents.
The usual recommended daily dose of bran is 12-20 g (approximately six to nine tablespoons). Unprocessed bran, apparently because of its superior water holding effect, provides greater laxative effect than processed bran. Cereals such as 100% Bran or Allbran have about half as much bran as the same weight of unprocessed bran. Unprocessed bran can be eaten as a breakfast cereal, sprinkled over cereal, fruit or soup, added to cereals, sauces and meat loaves or baked into bread or muffins. Patients should be advised to expect bloating for a few weeks. In some patients bloating or flatulence may be intolerable.
Lactulose is currently being heavily promoted for the treatment of chronic constipation. Lactulose has an osmotic action and in addition stimulates peristalsis by lowering colonic pH. Its role in the treatment of constipation is not yet established. For the present it should probably be regarded as an alternative to stimulant or saline laxatives and not as a substitute for bulk-forming or emollient laxatives. Lactulose costs two to four times as much per dose as the commonly used stimulant and saline laxatives.
Preparations containing the lubricant laxative mineral oil, including the ever-popular Magnolax and Agarol, should probably be abandoned. Mineral oil reduces absorption of fat soluble vitamins, carries a risk of chronic lipid pneumonitis, may leak through the anus causing itching and is a suspected carcinogen. Admittedly, anal leakage and lipid pneumonitis would seem unlikely when mineral oil is in a stable emulsion as with Agarol and Magnolax. In the interest of dispelling a tenacious myth, it should be noted that licorice has no laxative properties.
Glycerin and biscodyl are probably the most commonly prescribed suppositories. Glycerin is mild and cheap. Bisacodyl reflexly stimulates the whole colon and may cause cramping, rectal burning and proctitis. Suppositories usually act in about 30 minutes but in some individuals may require two to three hours to produce a response.
The most frequently administered enemas are probably tap water and hypertonic phosphate. Fleet is the best known brand of hypertonic phosphate enema.