How should I prescribe senna for an adult patient with functional constipation who has no bowel obstruction, inflammatory bowel disease, acute abdominal pain, or severe electrolyte imbalance?

How to Prescribe Senna for Functional Constipation

Start senna at 8.6-17.2 mg (1-2 tablets of 8.6 mg) once daily at bedtime, and titrate upward based on response over 1-2 weeks, with a goal of achieving one spontaneous bowel movement every 1-2 days. 1

Initial Dosing Strategy

• Begin with a low dose of 8.6-17.2 mg (1-2 tablets) taken once daily at bedtime rather than the higher doses used in clinical trials (1 gram daily), as 83% of trial participants reduced their dose due to abdominal cramping and diarrhea. 1

• The FDA-approved dosing for adults is 2-3 teaspoons (10-15 mL) once daily for liquid formulations, with a maximum of 3 teaspoons (15 mL) twice daily. 2

• Most commercially available senna tablets contain 8-9 mg of sennosides per tablet, making the practical starting dose 1-2 tablets at bedtime. 1

Dose Titration and Monitoring

• Increase the dose by 8.6-17.2 mg (1-2 tablets) every 1-2 weeks if the patient does not achieve one non-forced bowel movement every 1-2 days. 1, 3

• Monitor for dose-dependent side effects including abdominal cramping, pain, and diarrhea, which indicate the need for dose reduction. 1

• The recommended maximum is 4 tablets twice per day (approximately 68.8 mg daily), though most patients respond to lower doses. 1

Duration of Therapy

• While clinical trials evaluated senna for only 4 weeks, longer-term use is appropriate for chronic constipation, though periodic reassessment is essential given limited long-term safety data. 1

• Senna is primarily recommended for short-term use or rescue therapy, but can be continued long-term if needed with monitoring. 1

• Consider attempting dose reduction or weaning after 6 months of stable bowel function (one bowel movement daily without soiling or impaction). 4

Mechanism and Clinical Considerations

• Senna requires gut bacteria to convert sennosides A and B into active metabolites (rheinanthrone and rhein), which stimulate colonic peristalsis and chloride secretion. 1

• Antibiotic use may reduce senna's efficacy by affecting the colonic bacteria needed for drug activation. 1, 3

• Over 90% of sennosides are excreted in feces, making senna safe in chronic kidney disease patients without risk of accumulation. 5

Combination Therapy Algorithm

• If senna monotherapy at optimized doses fails after 1-2 weeks, add polyethylene glycol (PEG) 17 grams once daily rather than further escalating senna. 1, 3

• Start with either senna OR PEG as monotherapy; avoid starting both agents simultaneously at full dose, which increases risk of diarrhea and electrolyte disturbances. 5, 3

• For refractory cases not responding to senna plus PEG, consider bisacodyl 5-10 mg for short-term rescue therapy or prescription secretagogues (lubiprostone, linaclotide, plecanatide). 1, 3

Absolute Contraindications

• Do not prescribe senna in patients with intestinal obstruction, ileus, severe dehydration, or acute inflammatory bowel conditions. 1, 3

• Rule out mechanical obstruction with digital rectal exam and clinical assessment before initiating therapy. 6

• Avoid in pregnant women due to weak genotoxic effects in animal studies, though evidence is controversial. 1

Common Pitfalls to Avoid

• Starting at the trial dose of 1 gram daily (10-12 times higher than standard tablets) causes excessive cramping and diarrhea in most patients. 1, 6

• Failing to advise patients taking senna at night to change soiled undergarments promptly can lead to rare but significant perineal blistering from prolonged stool-to-skin contact, particularly at higher doses (>60 mg/day). 7

• Relying on senna alone without addressing inadequate fluid intake or considering osmotic laxatives for patients requiring long-term therapy. 6

• Forgetting that senna works best when taken at bedtime, with onset of action typically 6-12 hours after oral administration. 1

Special Populations

• In patients on chronic opioid therapy, initiate senna prophylactically at 8.6-17.2 mg daily and increase proportionally when opioid doses are escalated. 6

• For elderly patients with constipation unresponsive to senna, add PEG 17 grams daily rather than escalating senna further, as PEG has superior safety for long-term use in this population. 6

• In cancer patients receiving chemotherapy (especially vinca alkaloids), start prophylactic senna 2 tablets every morning immediately to prevent constipation. 6