Laxatives and cathartics

Laxatives and cathartics are medications that increase the passage of stool.

Laxatives ease the passage of fully formed fecal matter from the rectum; while cathartics have a stronger effect, and cause the evacuation of the entire colon, usually in the form of watery, unformed stool.

Some medications can function both as a laxative and a cathartic, depending on the dosage.

Both types of medications are used to treat constipation, which is partly characterized by a decrease in stool passage frequency; small, hard stools; or difficulty with initiating bowel movements.

Normal stool frequency is usually at least 3 times per week for someone on a typical Western diet.

Now, the small and large intestines are where most of the absorption happens in the GI tract.

The small intestine contains smooth muscles that perform peristalsis, which is a series of coordinated wave-like muscle contractions that help push the food bolus through the GI tract.

Lining the luminal surface of the intestine is a layer called the mucosa, which absorbs nutrients or secretes different molecules, like ions and water, into the lumen.

The undigested component of the food bolus eventually reaches the large intestine and becomes feces or stool.

The large intestine mainly absorbs excess water from the stool and that helps condense it into a more solid form.

However, stool should still be 70-80% water by weight, so if the feces becomes too dry, it could condense into a large, hard mass that’s difficult to pass.

So instead of peristalsis, which only pushes the food bolus in one direction, the colon undergoes segmental contraction, which pushes the feces in both directions within the haustra to constantly mix it with water; kind of like how a cement truck keeps churning to keep the cement from drying.

Now, constipation can occur due to a poor diet or a malfunction within the GI tract itself, although, up to 60 percent of chronic constipation does not have a clear cause.

One of the most important dietary factors is the lack of fiber, which is the part of food that resists digestion and reaches the colon relatively unchanged.

Fiber usually comes from plants like fruits and vegetables.

They get incorporated into the stool where they absorb water, causing the stool to swell and bulk up, which prevents it from hardening, while also making it easier to pass.

Another cause of constipation is due to decreased motility of the GI tract, which could be caused by disorders like irritable bowel syndrome, or medications like opioids.

In either case, it takes longer for the digested food to travel through the GI tract, so more water is absorbed, and the stool hardens.

When this hardened stool forms a large mass that’s difficult to pass, it’s called a fecal impaction, and it could lead to large bowel obstructions.

A large colonic carcinoma or an inflamed and swollen diverticulum can cause a blockage in the colon, which could lead to constipation.

Finally, problems like anal fissures and hemorrhoids can make defecation so painful that the person will voluntarily avoid passing stool.

The longer the stool stays in the colon, the harder it will become, ultimately making the situation worse.

Medications that are used to manage constipation include: bulk forming agents, stimulant laxatives, osmotic laxatives, and stool softeners.

Let’s start with bulk-forming agents, which include methylcellulose and psyllium.

These fibrous compounds are composed of polysaccharide polymers from plants.

In addition, bulk-forming agents also include synthetic fibers, such as polycarbophil.

Now these medications are taken perorally and can’t be digested by the enzymes in our GI tract.

So these fibers end up getting incorporated into the stool, where they draw in more water, making the stool swell up into a soft, bulky mass, kind of like a stinky sponge.

This increase in size stimulates intestinal motility and the softer stool is also easier to pass.

Bulking agents are great for long-term treatment of constipation but they can also be used to treat acute diarrhea.

Except bloating and flatulence, they don’t have many side effects; but it’s important to note that they should be taken with fluids to avoid fecal impaction.

Finally, they are contraindicated in those with intestinal obstruction as the increased stool mass could worsen the blockage.

Next are the stool softeners, which are also known as emollient laxatives.

The main representative of this group is docusate, which can be taken peroral or per rectum, as a suppository.

Normally water and lipids don’t mix, so the fats in stool could prevent water from entering.

Docusate is a surfactant, which means it has a hydrophilic head that’s water soluble, and a hydrophobic tail that’s water insoluble, so it sticks out into the lipids.

This disrupts the normal surface tension between water and fat, which allows the water to penetrate the stool and make it softer.

Although docusate is widely used, it’s shown to have low efficacy for the treatment of constipation.

However, it can be used to prevent constipation and minimize straining during defecation in hospitalized individuals.

As far as side effects go, docusate can cause diarrhea because it increases intestinal fluid and electrolyte secretion.

Next are the osmotic laxatives, which include saline laxatives, such as magnesium sulfate, magnesium hydroxide, magnesium citrate, and sodium phosphate,; and indigestible alcohols and sugars, like polyethylene glycol, polyethylene glycol-electrolyte solution, and lactulose.

Both magnesium cations and phosphate anions are not well absorbed, so they draw more water out of the cells in the intestinal wall via osmosis and increase the amount of water in the lumen.

This increases intestinal motility, which pushes the stool through the GI tract and also helps mix the stool with water.

Besides treating constipation, magnesium citrate and sodium phosphate can be given rectally to cleanse the bowels before procedures like colonoscopies or surgeries, kind of like using a drain cleaner to clean dirty pipes.

The downside of these medications is that they can cause diarrhea and fluid loss; therefore, individuals treated with saline laxatives should increase fluid intake to prevent dehydration.

In addition, magnesium- and phosphate- containing medications should be avoided in small children and individuals with renal impairment, cardiac conditions, or pre-existing electrolyte disbalance since these individuals are associated with an increased risk of hypermagnesemia and hyperphosphatemia.

Moreover, hypermagnesemia complications include heart block, neuromuscular block, and central nervous system depression; while hyperphosphatemia can lead to acute renal failure due to tubular deposition of calcium phosphate; but also metabolic acidosis, hypocalcemia, tetany, and even death.