Which two types of medications have case reports shown an association of decreased effectiveness with the intake of dietary fiber?

• A. Antidepressants and lipid-lowering agents
• B. Antihypertensives and antiemetics
• C. Antibiotics and antidiabetics
• D. Anticoagulants and diuretics

Answer: (C)

Dietary fiber can bind certain drugs in the gut, forming complexes that slow or block absorption and lower the amount reaching the bloodstream. When a medication’s absorption is reduced, its effectiveness can drop. This interaction is most consistently described for antibiotics and some oral antidiabetic drugs, where adequate gut absorption is crucial for therapeutic effect. Case reports have noted decreased effectiveness when fiber is taken with antibiotics such as tetracyclines or fluoroquinolones, and with certain oral antidiabetics like metformin, due to reduced bioavailability. The other drug classes listed have less consistent or strong evidence of this kind of fiber-related absorption issue, making antibiotics and antidiabetics the best fit for the described association.