In a study with 4 healthy male volunteers, administration of carbamazepine (200 mg po) with ispaghula husk (3.5 g) reduced the bioavailability of the drug by 1.22 mg/mL and delayed achievement of maximum plasma concentrations (Etman 1995). The decrease in carbamazepine absorption due to fiber consumption could lead to subclinical concentrations of the drug. However, there were no adverse effects reported with the use of psyllium hydrophilic mucilloid (3.4 g bid) in a patient taking carbamazepine (1000 mg/day) (Ettinger et al. 1992). If psyllium is used to ease constipation associated with carbamazepine therapy, the patient should be monitored carefully to assure that plasma concentrations of the drug remain within therapeutic range.

Preclinical and clinical data suggest that psyllium powder can be combined with bile acid sequestrants for the treatment hyperlipidemias. In a study of cholesterol-fed hamsters, various levels of cholestyramine resin were administered with or without psyllium powder (Turley et al. 1996). Although high-dose cholestyramine (3% of diet) was most effective in lowering blood and liver cholesterol, the combination of cholestyramine and psyllium was almost as effective, especially at higher psyllium doses (4% of diet). The combination therapy was more effective in promoting fecal bile acid excretion and inhibiting intestinal cholesterol absorption than the resin alone.

Clinically, a randomized, double-blind controlled trial of 121 patients with moderate hypercholesterolemia examined the effects of colestipol (5 g tid), psyllium (5 g tid), and a combination of colestipol (2.5 g tid) and psyllium (2.5 g tid) for 10 weeks (Spence et al. 1995). Combination therapy was tolerated better than colestipol monotherapy and was as effective as either agent alone.

A study with six healthy male volunteers ages 28 to 40 investigated the effects of psyllium on lithium sulfate treatment (12 mEq in 100 mL of water/day) (Toutoungi et al. 1990). Concomitant administration of lithium and psyllium reduced lithium absorption approximately 14%. A single case report of an interaction between psyllium and lithium supports these findings (Perlman 1990). In spite of increasing oral doses, blood lithium levels remained within sub-therapeutic range until psyllium (1 tsp bid) was discontinued. Withdrawal of psyllium caused a prompt increase in blood lithium levels. To minimize the effects on absorption, psyllium should be taken at least one hour after taking lithium. Lithium levels should be monitored frequently whenever psyllium is introduced or withdrawn.

Etman MA. Effect of a bulk forming laxative on the bioavailability of carbamazepine in man. Drug Dev Ind Pharm. 1995;21(16):1901-1906.

Ettinger AB, Shinnar S, Sinnett MJ, Moshe SL. Carbamazepine-induced constipation. J Epilepsy. 1992;5(3):191-193.

Perlman BB. Interaction between lithium salts and ispaghula husk [letter]. Lancet. 1990;355:416.

Spence JD, Huff MW, Heidenheim P et al. Combination therapy with colestipol and psyllium mucilloid in patients with hyperlipidemia. Ann Intern Med. 1995;123:493-499.

Toutoungi M, Schulz P, Widmer J, et al. Probable interaction of psyllium and lithium. Therapie. 1990;45(4):358-360.

Turley SD, Daggy BP, Dietschy JM. Effect of feeding psyllium and cholestyramine in combination on low density lipoprotein metabolism and fecal bile acid excretion in hamsters with dietary-induced hypercholesterolemia. J Cardiovasc Pharmacol. 1996;27:71-79.