If you are currently being treated with any of the following medications, you should not use fiber supplements without first talking to your health care provider.

Antidepressant medications (Tricyclic antidepressants) -- Dietary fiber has been reported to lower the blood levels and effectiveness of tricyclic antidepressant medications, such as amitriptyline (Elavil), doxepin (Sinequan), and imipramine (Tofranil), in several patients. Reduced dietary fiber intake increased the blood levels and improved symptoms in these patients. Individuals currently taking tricyclic medications should consult a health care provider before increasing fiber intake.

Diabetes medications -- While fiber supplements may help to regulate blood sugar levels, they may also interfere with the absorption of anti-diabetic medications, specifically glyburide (Diabeta) and metformin (Glucophage). Therefore, fiber supplements should not be taken at the same time as these medications.

Carbamazepine -- Taking soluble fiber such as psyllium with carbamazepine (Tegretol), a medication used to treat seizure disorders, may decrease the absorption and effectiveness of carbamazepine. A health care provider should close monitor blood levels of anyone taking both soluble fiber and carbamazepine.

Cholesterol-lowering medications -- Combining psyllium or other soluble fibers with cholestyramine (Questran) or colestipol (Colestid), two types of cholesterol-lowering medications known as bile acid sequestrants, may be beneficial in lowering cholesterol levels. Individuals taking these medications should consult a health care provider to determine whether psyllium is safe and appropriate.

Fiber in the form of pectin (from fruit) and oat bran reportedly reduces the body's ability to absorb cholesterol-lowering medications known as "statins," including lovastatin (Mevacor) and atorvastatin (Lipitor), and could lead to decreased effectiveness of these medications.

Digoxin -- Fiber supplements may reduce the body's ability to absorb digoxin (Lanoxin), a medication used to regulate heart function. Therefore, fiber supplements should not be taken at the same time as this medication.

Lithium -- Clinical reports suggest that psyllium or other soluble fibers may lower lithium levels in the blood, reducing the effectiveness of this medication. Lithium levels should be monitored very closely by a health care provider, particularly if there is any significant change in fiber intake.

Penicillin -- In one clinical study, the fiber supplement guar gum reduced blood levels of penicillin. Therefore, it would be best not to take penicillin at the same time as fiber supplements.