Cholestyramine 4 gm Powder BID PRN: Medication Overview
Cholestyramine 4 gm powder BID PRN is a bile acid sequestrant prescribed as-needed (PRN) at a dose of 4 grams twice daily, though this PRN dosing is unconventional since cholestyramine requires regular scheduled dosing to be effective for its primary indications. 1
Mechanism of Action
Cholestyramine is a positively charged, non-absorbable resin that binds bile acids in the intestinal lumen, preventing their reabsorption and forcing fecal excretion. 2 This depletes the hepatic cholesterol pool, triggering upregulation of LDL receptors on liver cells and increasing clearance of LDL particles from circulation. 2
FDA-Approved Indications
Cholestyramine is FDA-approved as adjunctive therapy to diet for reducing elevated serum cholesterol in patients with primary hypercholesterolemia who do not respond adequately to diet alone. 1 It may be useful for lowering LDL cholesterol in patients with concurrent hypertriglyceridemia, but is not indicated when hypertriglyceridemia is the primary concern. 1
Standard Dosing (Not PRN)
The typical dosing is 8-16 grams per day divided into 2 doses, not as-needed. 2 Each 9-gram packet of powder contains 4 grams of active cholestyramine resin. 1 The medication can be administered once daily as a single dose or divided twice daily without reference to meals, though traditionally given with meals. 3
Expected LDL-C Reduction by Dose:
- 8 grams daily: 16-22% LDL-C reduction 4
- 16 grams daily: 23-27% LDL-C reduction 4
- 24 grams daily: 27-28% LDL-C reduction 4
Critical Administration Requirements
All other medications must be taken at least 1 hour before or 4 hours after cholestyramine to avoid impaired absorption. 2, 4 This is particularly critical for:
- Thyroid hormone replacement therapy 2, 4
- Warfarin (monitor INR frequently during initiation) 2
- Oral contraceptives 2
- Cyclosporine 2
- Phenytoin 2
- Sulfonylureas 2
Fat-soluble vitamins (A, D, E, K) should be administered at least 4 hours before cholestyramine due to risk of deficiency with long-term use. 4
Common Adverse Effects
The most frequent side effects are gastrointestinal: 2
- Constipation
- Dyspepsia
- Nausea
- Abdominal bloating and pain
- Flatulence
Approximately 11% of patients find cholestyramine intolerable due to unpalatability or side effects, with up to 45% of treatment failures related to medication intolerance. 2
Contraindications
Cholestyramine is contraindicated in patients with complete biliary obstruction or history of serious hypersensitivity to the medication. 2, 1
Off-Label Uses
Beyond cholesterol lowering, cholestyramine is used for:
- Bile acid diarrhea: 69.8% success rate as first-line therapy 2
- Pruritus associated with cholestatic disease 5
- Diarrhea from bile acid malabsorption 5
Critical Pitfall: PRN Dosing
The "PRN" designation in your prescription is problematic because cholestyramine requires regular, scheduled dosing to achieve therapeutic effect. 1 The majority of lipid-lowering effect occurs within 14 days of consistent therapy. 6 For bile acid diarrhea, response is typically rapid but still requires regular dosing. 2 PRN dosing would not provide adequate bile acid sequestration for either indication.
Pregnancy and Lactation
Cholestyramine is considered safe to use during pregnancy and lactation as it is not systemically absorbed. 2
Monitoring Requirements
Serum cholesterol and triglyceride levels should be monitored periodically, with a favorable trend expected within the first month of therapy. 1 Triglycerides may increase by 25-29% during treatment. 6 For patients on warfarin, monitor INR frequently during initiation and periodically thereafter. 2