Can Isoniazid (INH) cause constipation?

Isoniazid and Constipation

Isoniazid (INH) can cause constipation as a side effect, though it is not among its most commonly reported adverse effects. This is evidenced by a documented case report of INH-induced pellagra where constipation was a presenting symptom 1.

Gastrointestinal Effects of Isoniazid

Isoniazid's primary gastrointestinal side effects typically include:

• Nausea
• Vomiting
• Abdominal pain
• Loss of appetite

However, constipation can occur in some patients, particularly those receiving Isoniazid Preventive Therapy (IPT) 1.

Mechanism of Constipation with Isoniazid

The constipation associated with isoniazid may be related to:

1. Niacin metabolism interference: Isoniazid interferes with the absorption of niacin 1, which can affect gastrointestinal motility.

2. Intestinal absorption effects: Studies show that isoniazid's intestinal absorption can be affected by co-administered substances 2, which may influence gut function.

3. Food interactions: Isoniazid bioavailability is reduced when taken with food 3, which may indirectly affect bowel habits.

Risk Factors for Isoniazid-Induced Constipation

Patients at higher risk for developing constipation while on isoniazid include:

• Patients with HIV on Isoniazid Preventive Therapy 1
• Patients with primarily maize-based diets (due to potential niacin deficiency) 1
• Elderly patients (generally more susceptible to medication-induced constipation)
• Patients taking other medications that may cause constipation

Management Recommendations

If a patient develops constipation while on isoniazid:

1. Continue isoniazid if the constipation is mild and the drug is necessary for TB treatment or prevention.

2. Dietary modifications:

   • Increase fiber intake
   • Ensure adequate hydration
   • Consider adding soluble fiber supplementation such as ispaghula 4

3. Medication timing: Take isoniazid 30-60 minutes before food or 2 hours after food to optimize absorption 5.

4. Pyridoxine supplementation: All patients taking isoniazid should receive pyridoxine (vitamin B6) to reduce the risk of peripheral neuropathy 5. Standard dosing is 10-25 mg/day 5.

5. Monitor for other symptoms: Watch for other signs of niacin deficiency (pellagra) which may accompany constipation, such as dermatitis and neurological symptoms 1.

Important Considerations

• The British Thoracic Society guidelines note that isoniazid should be taken 30-60 minutes before food or 2 hours after food 5, which may help minimize gastrointestinal effects.

• Patients should be monitored for hepatotoxicity, which is a more common and serious side effect of isoniazid than constipation 5.

• If constipation is severe or accompanied by other concerning symptoms, consider evaluating for pellagra, especially in patients with risk factors 1.

Conclusion

While constipation is not among the most commonly reported adverse effects of isoniazid in major guidelines, case reports indicate it can occur. Management should focus on dietary modifications, proper medication timing, and appropriate supplementation with pyridoxine while continuing isoniazid therapy if medically necessary.