Which of the following is NOT advisable when taking tetracyclines?

Taking tetracyclines in combination with dairy products is not advisable due to the potential for reduced absorption of the medication. Tetracyclines can chelate (bind to) divalent and trivalent metal ions such as calcium, magnesium, and iron, which are abundant in dairy products. This interaction can significantly decrease the bioavailability of the antibiotic, leading to diminished therapeutic effectiveness. Therefore, it is recommended that patients avoid consuming dairy products or calcium-rich foods close to the time of taking tetracyclines.

In contrast, taking tetracyclines on an empty stomach can enhance absorption, although some individuals may experience gastrointestinal discomfort. Additionally, taking tetracyclines with cations (like those found in antacids and supplements containing iron, magnesium, or calcium) can also impede the absorption of the drug just like the interaction with dairy products. Lastly, while tetracyclines can be used to treat certain infections, their use as a first-line treatment for sexually transmitted infections (STIs) is typically limited, but they may still be considered in specific cases.