Taking psyllium with carbamazepine, which is used to treat seizure disorders, may decrease the absorption and effectiveness of this medication. If you use psyllium while on carbamazepine, your blood levels of this medication should be monitored closely. Consult with your healthcare provider before adding any new supplements to your existing medication regimen.

Combining psyllium with cholestyramine or colestipol may be beneficial in lowering cholesterol levels. Consult with your healthcare practitioner about whether this may be an option for you. However, taking fiber in the form of pectin (from fruit) and oat bran reportedly reduces the body's ability to absorb lovastatin, a cholesterol-lowering medication, and could lead to decreased effectiveness of this medication.

Psyllium may lower lithium levels in the blood, reducing the effectiveness of the medication. Consult your healthcare provider before using psyllium with lithium. If both are used, take them at least one hour apart, preferably two. Lithium levels should be monitored very closely, particularly with any change in intake of fiber.

Fiber supplements may reduce the body's ability to absorb digoxin, a medication used to regulate heart function. Therefore, you should not take fiber supplements at the same time as this medication; it would be best to take them either two hours before or after your dose of digoxin.

While fiber supplements may help to regulate blood sugar levels, they may also interfere with the absorption of your antidiabetic medications, specifically glyburide and metformin. Therefore, you should not take fiber supplements at the same time as your antidiabetic medications. Your blood sugar levels should be monitored closely.

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

Similarly, increasing dietary fiber led to decreased 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 the conditions of these patients. If you are currently taking these medications, it would be best not to increase your fiber intake, but if you do, consult with your healthcare provider so that the blood levels of these medications can be monitored closely.

In general, taking psyllium or other fiber supplements with any medications may reduce or delay the absorption of these medications. It is best to refrain from taking these supplements at the same time as these other medications; take them at least 1 to 2 hours apart.