Drug-Induced Constipation (PHLE High-Yield Target)

Opioids (e.g., Morphine, Oxycodone, Codeine)

Stimulate μ-opioid receptors in the enteric nervous system → decreases peristalsis, inhibits fluid secretion, increases fluid absorption. (Does NOT develop tolerance!)

Anticholinergics (e.g., Atropine, Hyoscyamine, Diphenhydramine)

Blocks muscarinic receptors on intestinal smooth muscle → decreases intestinal secretions and motility.

Tricyclic Antidepressants (TCAs) (e.g., Amitriptyline)

Strong anticholinergic side effects.

Calcium Channel Blockers (CCBs) (especially Verapamil)

Inhibits calcium influx into gastrointestinal smooth muscle cells → causes muscular relaxation and slowed transit.

Cation-Containing Agents

• Aluminum Hydroxide

• Calcium Carbonate

• Ferrous Sulfate (Iron)

Aluminum Hydroxide

binds intestinal fluid and reduces contractions.

Calcium Carbonate

relaxes colonic smooth muscle.

Ferrous Sulfate (Iron)

Oral iron supplements cause direct mucosal irritation and decrease fluid secretion.

Diuretics (e.g., Furosemide, Hydrochlorothiazide)

Causes systemic volume depletion/dehydration → increases colonic fluid absorption.