In a study with 4 healthy male volunteers, the bioavailability of carbamazepine was reduced by 1.22 mg/mL when carbamazepine (200 mg po) was coadministered with ispaghula husk (3.5 g) (Etman 1995). Coadminstration delayed achievement of maximum carbamazepine plasma concentrations. The decrease in carbamazepine absorption due to fiber consumption could lead to subclinical concentrations of the drug. However, there were no reports of adverse effects associated with the use of psyllium hydrophilic mucilloid (3.4 g bid) for constipation in a patient on carbamazepine therapy (1 g/day) (Ettinger et al. 1992).

Preclinical and clinical data suggests that psyllium powder can be combined with bile acid sequestrants for the treatment of hyperlipidemias. In a study of cholesterol-fed hamsters, the combination of cholestyramine and psyllium (4% of diet) was almost as effective as cholestyramine monotherapy (3% of diet) in lowering blood and liver cholesterol levels (Turley et al. 1996). However, the combination therapy was more effective in promoting fecal bile acid excretion and inhibiting intestinal cholesterol absorption than the resin alone. In a randomized, double-blind controlled trial involving 121 patients with moderate hypercholesterolemia, treatment with a combination of colestipol (2.5 g tid) and psyllium (5 g tid) for 10 weeks was found to be better tolerated than colestipol monotherapy and as effective as either agent alone (Spence et al. 1995).

In one study, administration of digoxin tablets (0.75 mg) with a high fiber meal (5 g crude fiber) markedly decreased the absorption and urinary excretion of the drug in 12 healthy volunteers (Brown et al. 1978). However, in a double-blind study with 10 healthy volunteers, guar gum did not significantly alter urinary digoxin concentrations at 24 hours in spite of an initial reduction in drug absorption (Huupponen et al. 1984). Digoxin absorption is not affected significantly when the drug is taken as a capsule rather than as a tablet (Johnson et al. 1987). Although administration of two capsules of digoxin (0.4 mg/day) with breakfast supplemented with bran fiber (11 g) reduced serum digoxin levels by 6 to 7% compared to controls, the differences were not deemed to be clinically significant. The effects of high fiber intake on the absorption of digoxin from capsules were less than those reported for tablets.

Coadministration of glibenclamide (2.5 mg glyburide) and glucomannan (3.9 mg), a dietary fiber, reduced drug absorption and plasma levels in 9 healthy volunteers (Shima et al. 1983). However, mean plasma glucose levels were lower in the group treated with both glibenclamide and fiber compared to glibenclamide alone.

Psyllium mucilage administered to six healthy male volunteers reduced lithium sulfate (12 mEq in 100 mL water/day) absorption by approximately 14% (Toutoungi et al. 1990). Clinically, a single case of a potential interaction between psyllium and lithium has been reported (Perlman 1990). In spite of increasing oral doses of lithium, blood levels remained sub-therapeutic until psyllium (1 tsp. bid) was discontinued; blood lithium levels increased promptly when the psyllium was withdrawn. Individuals taking lithium should wait at least one hour before taking psyllium mucilage. Lithium levels should be frequently monitored whenever psyllium is introduced or withdrawn.

In one study, patients taking lovastatin (80 mg/day) with pectin (15 g/day) or oat bran (50 to 100 g/day) and a lipid-lowering diet experienced a dramatic increase in LDL-cholesterol (Richter et al. 1991). Discontinuation of the pectin and oat bran caused LDL-cholesterol values to fall to pretreatment levels. Both forms of fiber may reduce lovastatin absorption. Concentrated intake of soluble fiber should be avoided while on lovastatin therapy because it may reduce the efficacy of the drug.

Coadministration of guar gum with metformin (1700 mg) reduced absorption rates for the drug (Gin et al. 1989). Fiber therapy may diminish the hypoglycemic effects of metformin.

In a double-blind study with 10 healthy volunteers, coadministration of guar gum with penicillin significantly reduced serum concentrations of the drug (Huupponen et al. 1984).

An increase in dietary fiber intake caused three patients to become refractory to previously effective treatment with tricyclic antidepressants (Stewart 1992). Commencement of a high-fiber diet decreased serum levels of the drugs. Reduced dietary fiber increased serum concentrations and improved the clinical outcome.

Brown DD, Juhl RP, Warner SL. Decreased bioavailability of digoxin due to hypocholesterolemic interventions. Circ. 1978;58(1):164-172.

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.

Gin H, Orgerie MB, Aubertin J. The influence of guar gum on absorption of metformin from the gut in healthy volunteers. Horm Metab Res. 1989;21(2):81-83.

Huupponen R, Seppala P, Iisalo E. Effect of guar gum, a fibre preparation, on digoxin and penicillin absorption in man. Eur J Clin Pharmacol. 1984;26(2):279-281.

Johnson BF, Rodin SM, Hoch K, Shakar V. The effect of dietary fiber on the bioavailability of digoxin in capsules. J Clin Pharmacol. 1987;27(7):487-490.

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

Richter WO, Jacob BG, Schwandt P. Interaction between fibre and lovastatin. Lancet. 1991;338;706.

Shima K, Tanaka A, Ikegami H, et al. Effect of dietary fiber, glucomannan, on absorption of sulfonylurea in man. Horm Metab Res. 1983;15(1):1-3.

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.

Stewart DE. High-fiber diet and serum tricyclic antidepressant levels. J.Clin. Psychopharmacol. 1992;12:438-440.

Toutoungi M, Schulz P, Widmer J, Tissot R. Probable interaction entre le psyllium et le lithium. Therapie. 1990;45(4):358.

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.