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

Antidepressant Medications, Tricyclics
Dietary fiber has been shown to lower the blood levels and effectiveness of tricyclic antidepressant medications such as amitriptyline, doxepin, and imipramine in three patients. Reduced dietary fiber intake increased the blood levels and improved symptoms in these patients. Individuals currently taking tricyclic medications should consult a healthcare 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 and metformin. Therefore, fiber supplements should not be taken at the same time as this medication.

Carbamazepine
Taking soluble fiber such as psyllium with carbamazepine, a medication used to treat seizure disorders, may decrease the absorption and effectiveness of carbamazepine. If taking both soluble fiber and carbamazepine, blood levels of carbamazepine should be monitored closely by a healthcare provider.

Cholesterol-lowering Medications
Combining psyllium or other soluble fibers with cholestyramine or colestipol (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 healthcare practitioner to determine whether psyllium is safe and appropriate.

On the other hand, taking 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" (such as lovastatin and atorvastatin), and could lead to decreased effectiveness of this class of medications.

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

Lithium
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 healthcare provider, particularly if there is any significant change in fiber intake.

Penicillin
In one 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.