• Bile acid sequestrants could delay or reduce the absorption of certain drugs in the gastrointestinal tract (e.g. digoxin, verapamil, levothyroxine, tetracycline).
• To minimize possible interference with absorption, other medications should be taken at least 1 hour before, or 4–6 hours after, taking a bile acid sequestrant.
• These drugs may also interfere with normal fat absorption and thus may alter the absorption of fat soluble vitamins (vitamins A, D, E, and K) (for further information, see Cautions and contraindications).
• Bile acid sequestants and warfarin:
• If concurrent use with colestyramine is necessary, close monitoring of the international normalized ratio (INR) should be considered because:
• Colestyramine can reduce the anticoagulant effects of warfarin by interfering with warfarin absorption (directly and via the enterohepatic circulation pathway). This interaction is established, but the magnitude and clinical importance is uncertain. Giving colestyramine 3 to 6 hours after warfarin has been shown to minimize the effects of this interaction [Baxter, 2006].
• Colestyramine may have some direct hypoprothrombinaemic effects by reducing the absorption of vitamin K [Baxter, 2006].
• Isolated and unexplained reports of paradoxical increases in the effects of warfarin have also been documented [Baxter, 2006].
• Colesevelam and colestipol: although these drugs have not been shown to have an effect on the bioavailability of warfarin, the manufacturers advise that anticoagulant therapy should be monitored closely. Specific clinical interaction studies with colesevelam and vitamin K have not been performed.

[ABPI Medicines Compendium, 2005a; ABPI Medicines Compendium, 2007a; ABPI Medicines Compendium, 2008a; ABPI Medicines Compendium, 2008d]