Laxatives

Unlike the acid drugs, "laxatives" is not one drug with one mechanism — it is a family of mechanisms, and knowing which lever a drug pulls tells you its speed, its use, and its danger. There are five main levers. Bulk-forming agents (psyllium, methylcellulose) are soluble fiber that absorb water into the stool, increasing its mass so the bowel wall stretches and pushes it along — the most physiologic, but slow (12–72 h) and useless for urgent relief. Osmotic agents (PEG 3350, lactulose, magnesium hydroxide/citrate) are poorly absorbed solutes that hold water in the lumen by osmosis, softening and loosening stool.

Stimulant laxatives (bisacodyl, senna) directly stimulate the myenteric nerve plexus, increasing motility and intestinal secretion — fast (oral 6–12 h; suppository within an hour) but the ones most associated with cramping and, with chronic overuse, dependence. Stool softeners / emollients (docusate) are surfactants that let water and fat penetrate hard stool — gentle, but the evidence that docusate actually works is weak (near placebo). And lubricants (mineral oil) simply coat the stool for easier passage, at the cost of a real aspiration-pneumonia risk. Newer prescription agents (lubiprostone, linaclotide, the peripheral opioid-blockers for opioid-induced constipation) add secretory mechanisms on top.

The single most important safety concept in this class is a red flag, not a mechanism: never give a laxative before ruling out a bowel obstruction. If the bowel is mechanically blocked — by a tumor, a volvulus, an ileus, a stuck fecal mass, or an inflamed appendix — then stimulating peristalsis or drawing water and gas into the lumen raises pressure against the blockage and can perforate the bowel, a surgical emergency. So laxatives are contraindicated in known or suspected obstruction, ileus, acute abdomen, appendicitis, or any undiagnosed abdominal pain, especially when paired with nausea, vomiting, and distension.

This is why the nurse assesses the bowel before treating it: bowel sounds, the pattern and last bowel movement, distension, and whether pain is crampy-and-relieved-by-passing-stool (constipation) versus severe, constant, or accompanied by vomiting (possible obstruction). Constipation is treated conservatively first — fluids, fiber, and activity — then with the gentlest effective agent, saving stimulants for short-term use. Getting this order wrong is how a "simple" constipation order becomes a perforation.

Most laxative harm is simply too much of the intended effect: drawing water and speeding transit can cause cramping and diarrhea, and with overuse or high doses, fluid and electrolyte depletion — dehydration, hypokalemia, and hyponatremia — which is dangerous in the elderly and those on diuretics or digoxin. Two mechanism-specific hazards deserve singling out. Magnesium-based osmotics (milk of magnesia, mag citrate) are excreted by the kidney, so in renal impairment the absorbed magnesium builds up to hypermagnesemia (hypotension, bradycardia, depressed reflexes, respiratory depression) — avoid them in renal disease. Bulk-forming fiber taken without enough water can swell and obstruct the esophagus or bowel — so it is always given with a full glass of fluid and avoided in patients who can’t swallow safely.

A few agent-specific points complete the picture. Mineral oil can be silently aspirated into the lungs (lipoid pneumonia) — avoid it in anyone with swallowing or aspiration risk, and don’t combine it with docusate (which increases its absorption). Chronic stimulant overuse is associated with dependence. And unlike the others, three laxatives carry agent-specific boxed warnings: oral sodium phosphate bowel preps can cause acute phosphate nephropathy (irreversible kidney injury), and linaclotide (contraindicated < 2 years) and plecanatide (< 6 years) caused fatal dehydration in juvenile animals — these are not class-wide warnings. One bright spot beyond constipation: lactulose treats hepatic encephalopathy — gut bacteria acidify the colon and convert absorbable ammonia (NH₃) into trapped ammonium (NH₄⁺), which is then excreted, lowering blood ammonia; it is titrated to 2–3 soft stools per day.