Which of the following should be avoided when taking Tetracyclines?

Dairy products should be avoided when taking Tetracyclines because they can significantly interfere with the absorption of the antibiotic. Tetracycline and its derivatives (such as doxycycline and minocycline) can form chelates with calcium, magnesium, and iron found in dairy. This chelation decreases the bioavailability of the drug, rendering it less effective. Therefore, it is crucial for patients taking Tetracyclines to not consume dairy products during the course of treatment, ideally waiting a few hours before or after taking the medication to ensure optimal absorption.

In contrast, caffeine, fiber supplements, and most fruits do not have the same impact on the effectiveness of Tetracyclines. While it's generally advisable to maintain a healthy diet and consider interactions, dairy products are the primary concern when it comes to Tetracycline absorption.