Tetracyclines

A patient takes doxycycline with a glass of milk and a multivitamin — two seemingly harmless habits that can render the antibiotic completely useless. Here's the pharmacology behind it.

Tetracyclines, with doxycycline as the most commonly prescribed, are broad-spectrum bacteriostatic antibiotics that inhibit bacterial protein synthesis by binding to the 30S ribosomal subunit. This blocks aminoacyl-tRNA from attaching to the ribosome, halting peptide chain growth. High-yield indications include acne vulgaris, Lyme disease (first-line in adults), Rocky Mountain spotted fever, Chlamydia, community-acquired pneumonia (atypical coverage), and H. pylori combination therapy. Doxycycline is unique among tetracyclines because it can be taken with food (except dairy) and does not require renal dose adjustment — it is excreted primarily through the GI tract. The critical drug-food interaction: divalent and trivalent cations (calcium, magnesium, iron, aluminum-containing antacids) chelate tetracyclines in the gut, forming insoluble complexes that are never absorbed. Separate administration by at least 2 hours. Tetracyclines cause photosensitivity — teach patients to use sunscreen and wear protective clothing throughout therapy. They are absolutely contraindicated in pregnancy (category D) and in children under 8 years because they permanently stain developing teeth yellow-brown and impair bone growth by depositing in calcifying tissues.

Key Distinctions

Don't confuse bacteriostatic (tetracyclines inhibit growth) with bactericidal (kills bacteria, e.g., penicillins, aminoglycosides) — tetracyclines slow the infection while the immune system clears it. Students mix up 30S subunit drugs (tetracyclines, aminoglycosides) with 50S subunit drugs (macrolides, clindamycin) — note that aminoglycosides also target 30S but are bactericidal, so subunit target alone doesn't determine bacteriostatic vs bactericidal. The dairy restriction is about chelation blocking absorption, not about stomach upset — doxycycline can be taken with non-dairy food.

Clinical Pearl

Teeth, sun, and calcium — the tetracycline trio. No kids under 8, no sun without protection, no dairy or antacids within 2 hours of a dose.